DIPLOMA THESIS Tests of semiconductor microstrip detectors of ...

Charles University in Prague

Faculty of Mathematics and Physics

DIPLOMA THESIS

Pavel Reznıcek

Tests of semiconductor microstrip detectors

of ATLAS detector

Institute of Particle and Nuclear Physics

Supervisor: Dr. Zdenek DolezalStudy programme: Physics

Study field: Nuclear and Subnuclear Physics

I would like to thank to my supervisor Dr. Zdenek Dolezal for his leading of mydiploma thesis, for inspiring discussions and ideas. For help in building of the test setup Iwould like to thank all the members of the VdG accelerator lab at Institute of Particle andNuclear Physics MFF UK, especially to Dr. Peter Kodys who together with my supervisorintroduced me into the problematics of the tests of ATLAS microstrip detectors.

Many thanks belong to Bettina Mikulec and Rainer Wallny for their consultationsof radioactive source tests analysis and patience with testing my software during measure-ments at CERN. Concerning computer simulations I’m very grateful to Szymon Gadom-ski, the author of the simulation software, who instructed me how to use the software,and to Grant Gorfine for his help with Geant4 simulations. For the provided beam testsdata and discussions of features of the simulation I want to thank to Marcel Vos andJose Enrique Garcia Navarro. For help with experiencing the standard and beam moduletests I wish to thank to Gareth F. Moorhead, Monica D’Onofrio, Mariane Mangin Brinet,Mauro Donega, Lars Eklund, Peter W. Phillips, the author of the standard software usedfor module tests, and many other ATLAS SCT developers I met during my diploma thesistraining.

Finally I’m very grateful to many members and students of Institute of Particle andNuclear Physics MFF UK for their suggestive questions.

Prohlasuji, ze jsem svou diplomovou praci napsal samostantne a vyhradne s pouzitımcitovanych pramenu. Souhlasım se zapujcovanım prace.

I declare that I wrote my diploma thesis independently and exclusively with the useof the cited sources. I agree with lending the thesis.

Prague, 17th April 2003 Pavel Reznıcek

2

Contents

1 Introduction 5

2 ATLAS detector system 6

3 Semiconductor detectors 93.1 Comparison to other detectors . . . . . . . . . . . . . . . . . . . . . . . . . 93.2 Silicon properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.3 Drift and diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.4 The P-N junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.5 Reverse current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.6 Interactions of particles in silicon . . . . . . . . . . . . . . . . . . . . . . . 143.7 Microstrip detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.8 Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.9 Radiation damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Detector modules 204.1 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2 Read out system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.3 Standard QA tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.4 Beam tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5 Simulations 285.1 Geant4 simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.2 SCT digitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.3 Beam tests simulation and digitization . . . . . . . . . . . . . . . . . . . . 32

6 Source tests 386.1 Radioactive β−source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386.2 Measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3 Analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.4 Measurement results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.5 Source tests simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

7 Conclusion 51

References 53

3

Nazev prace: Testovanı polovodicovych stripovych detektoru pro detektor ATLAS

Autor: Pavel Reznıcek

Katedra (ustav): Ustav casticove a jaderne fyziky

Vedoucı diplomove prace: RNDr. Zdenek Dolezal, Dr.

e-mail vedoucıho: [email protected]

Abstrakt: Cılem teto diplomove prace bylo provadenı a vyvoj testu detekcnıch moduluvnitrnıho detektoru ATLAS. V praci jsem se zameril na testy pomocı radioaktivnıhoβ− zarice. V Praze byla vyvinuta aparatura a napsan software pro merenı a analyzudat. Smyslem techto testu bylo doplnenı merenı provadenych na svazku z SPS v CERNu,tedy proverenı skutecnych detektcnıch vlastnostı modulu. Vysledky testu se zaricema na svazku byly porovnany a byl nalezen vztah mezi merenymi signaly. Merenı ukazala,ze pomer strednı energie, kterou zaricem emitovany elektron ztratı pri pruchodu mod-ulem, ku signalu zjistenemu z dat testu na svazku je 1.109±0.070. Pro uplne pochopenıtohoto rozdılu byla provedena simulace obou typu testu. Simulaci merenı na svazkujsem pouzil k overenı simulacnıho softwaru. Vysledkem je dobra shoda trendu uhlovych ajinych zavislostı s daty, ovsem v absolutnıch hodnotach dochazı k podhodnocenı merenehosignalu. Proto jsem provedl pouze relativnı srovnanı simulovanych odezev modulu z oboutypu testu. Simulace predpovıda pomer strednıch signalu z testu na svazku vuci testumse zaricem: 1.117±0.020. Tyto testy se tedy, vzhledem k dobre definovanemu vztahujejich vysledku vuci vysledkum merenı na svazku, staly vhodnym nastrojem pro overenıdetekcnıch vlastnostı modulu.

Klıcova slova: Kremikove stripove detektory, Testy zaricem, Testy na svazku, Geant4,SCT, ATLAS

Title: Tests of semiconductor microstrip detectors of ATLAS detector

Author: Pavel Reznıcek

Department: Institute of Particle and Nuclear Physics

Supervisor: Dr. Zdenek Dolezal

Supervisor’s e-mail address: [email protected]

Abstract: The setup of system for testing silicon microstrip detectors with 90Sr sourceof electrons was developed. The aim of the measurements was to determine the median sig-nal of particle passing through prototype modules for the ATLAS semiconductor tracker.Comparison to beam tests results was performed to check the consistence of the sourcetests results. The ratio of signals measured in beam tests to the source tests signal isabout 1.109±0.070. To fully understand the results computer simulation of the setupswas performed. The beam tests simulation, used to validate the simulation software,resulted in good description of trends of observed characteristics but in underestimationof the signal. The source test simulation confirmed the relation of the beam tests resultsto the source tests: the ratio of simulated median signal of beam tests to source tests wasabout 1.117±0.020. The defined relation of beam and source tests measurements madethe radioactive source tests usable for the signal determination.

Keywords: Silicon microstrip detectors, Source tests, Beam tests, Geant4, SCT, ATLAS

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1 Introduction

The semiconductor detectors in high energy physics are mostly used for precise mea-surement of particles’ tracks. If placed in a magnetic field the detectors provide highaccuracy momentum measurement. According to relatively high energy loss (hundredsof eV/µm) of charged particle passing through the semiconductor, low energy neededto release free charge carriers (several eV for creation of electron-hole pair) and possibilityof creation of fine structures (in the order of µm) of various properties, the semiconduc-tor detectors can measure the position with an accuracy of several µm. This makes thedetectors be able to detect secondary vertexes of decays of very short time living particles.

In the Center of European Nuclear Research (CERN) new hadron collider (LHC)is being built. One of 4 detector systems at the LHC will be the ATLAS, describedin section 2. One part of it will be a semiconductor tracker consisting of silicon stripsdetector modules.

This diploma thesis deals with tests and simulations of the SCT modules. The firstparts of this thesis describe general properties and usability of semiconductors as detec-tors, while the rest concerns SCT modules only. The standard quality assurance (QA)procedure described in section 4.3 consists of detector tests and tests of readout electron-ics. One of the main characteristics of SCT modules is the signal to noise ratio (S/N).While the QA tests are able to find the noise, signal can be determined by using realparticles only. For this purpose SCT modules were tested in beam on SPS at CERN (seesection 4.4). Because of the high cost and unavailability of beam tests, method usingthe β− radioactive source for signal measurement has been developed and results of sev-eral modules shown in section 6.4 were compared to the results of ATLAS simulation anddigitization software described in sections 5.1 and 5.2. The simulation and digitizationsoftware has been validated on the beam-tests data in section 5.3, that were analyzedby other SCT groups [10]. Because of very high luminosity of the LHC, SCT moduleswill operate in high radiation environment. So the tests were focused on measurementof properties of modules irradiated to dose equivalent to the dose after 10 years of oper-ation of the ATLAS detector system. Most of the tests were done on irradiated forwardmodules, however several were performed on unirradiated as well.

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2 ATLAS detector system

The ATLAS (A Toroidal LHC ApparatuS) detector system [1] is general-purpose de-tector which is designed to exploit the full discovery potential of Large Hadron Col-lider (LHC). The LHC properties (energy of interacting protons 7 TeV, expected lumi-nosity 1034 cm−2s−1) offer a large range of physics opportunities. The major ATLASinterest is the origin of mass at the electroweak scale based on spontaneous symmetry-breaking. One of the possible manifestation of spontaneous symmetry-breaking mecha-nism is the existence of standard model Higgs boson or of a family of Higgs particles.Alternative manifestation could involve a strongly interacting Higgs system. Other goalare the searches for heavy W - and Z-like objects. Considering their leptonic decays, highresolution lepton measurements and charge identification are needed even in the rangeof few TeV. For supersymmetric particles searches hermecity and missing transverse en-ergy ET capability of the detector is necessary. Another class of signatures of a newphysics like the composition of the fundamental fermions can be provided by very hightransverse momentum pT jet measurements. An important chapter of the LHC will bea high rate b- and t-quark factory. The main emphasis in B-physics will be on precisemeasurement of CP violation, determination of the angles in CKM unitary matrix andgeneral spectroscopy of states with b-quarks.

The set of ATLAS physics goals demonstrates that sensitivity to a variety of final statessignatures is required. The basic design considerations lead to the following ATLAS de-tector systems: electromagnetic calorimetry for electron and photon identification andmeasurement, hermetic jet and missing ET calorimetry, tracking for lepton momentummeasurement, for b-quark tagging, for electron and photon identification and for tau andheavy-flavour vertexing. The other features are stand-alone, precision muon momentummeasurements, large acceptance in η-coverage and triggering and measurements of parti-cles at low-pT thresholds. The ATLAS detector system is shown in figure 1 and describedbelow.

The ATLAS magnet system consists of a solenoid and air-core toroids. The 2 Tsolenoid is positioned in front of the barrel electromagnetic (e.m.) calorimeter. In orderto avoid degrading the e.m. calorimeter performance the thickness of the solenoid had tobe minimized. The superconducting coil is integrated into vacuum vessel of the calorimeterbarrel cryostat to eliminate the material and space of independent vessel walls. The di-mensions of the solenoid are 1.22 m in radius and 5.3 m in length. The superconductingtoroid magnet system consists of 26 m long barrel part with outer diameter 19.5 m andinner bore of 9.4 m, and of 2 end-caps with length 5.6 m and bores of 1.26 m. Magneticinduction varies from 3 Tm−1 to 8 Tm−1. The curved trajectories of charged particlesin the magnetic field allow momentum measurement using the inner detector and muonchambers tracking data.

The calorimetry of ATLAS consists of an inner barrel cylinder and end-caps usingliquid argon (LAr) technology, that is intrinsically radiation resistant, and hadronic scin-tillator tile calorimeter surrounding the LAr one in full length. The barrel part of the liquidcalorimetry includes a presampler detector for correction to the influence of solenoid coilof the thickness of 0.83·X0 at normal incidence. The minimal thickness of the e.m. barrelcalorimeter is 26·X0, while in case of end-caps calorimeters the minimal thickness is 27·X0.The hadronic scintillator tile calorimeter is based on a sampling technique with plasticscintillator plates (tiles) placed in plane perpendicular to the beam axis and embeddedin iron absorber and read out by wavelength shifting fibers. The outer radius of the wholecalorimetry system is 4.23 m and total length is 6.7 m. This high performance systemmust be capable of reconstructing the energy of electrons, protons and jets as well as

6

measuring missing ET .The muon detector system involves 3 layers of chambers in the barrel part and 3 or

4 layers in the end-cap part. In the barrel region 2 muon chamber planes are attachedto the magnetic toroids and the third one is in the mid-plane to measure the sagitta.In the forward region the chambers are placed at the front and back faces of the toroidcryostats, with a third layer against the cavern wall to maximize the lever arm of the point-angle measurement. Every chamber consists of detectors for the precision measurementand for the triggering. In the barrel part, 2 multilayers of drift tubes are used for pre-cision measurement while in the end-cap part cathode strip chambers are used in addi-tion. For triggering resistive plates are used in the barrel region and thin gap chambersin the end-cap region. The basic measurement in each muon chamber is a tracks segment,providing a vector for robust pattern recognition and momentum determination.

Figure 1: The ATLAS detector system.

The inner detector system [2] shown figure 2 and covering range of pseudorapidity|η| < 2.5 is composed of 3 different detectors: semiconductor pixel detector, semiconductorstrip detector and transition radiation tracker. The nearest one to the beam pipe isthe pixel detector. It is designed to provide a very high-granularity and high-precisionset of measurements as close to the interaction point as possible. The system consistsof 3 layers in the barrel part and 4 disks in the end-cap part, and offers 140 milliondetector elements, each 50 µm in the Rφ direction and 300 µm in the z. The maximalradius of barrel layer is 14 cm and of forward disk is 20 cm. The total length is 2.2 m.The furthest part of the inner detector system is the transition radiation tracker basedon straw tubes. Electron identification capability is added by employing xenon gas todetect transition radiation photons created in radiator between the straws and by using2 independent thresholds for tracking hits and transition radiation hits. The techniqueallows typically 36 measurements to be made on every track. The diameter of every strawis 4 mm and drift-time measurements give a spatial resolution of 170 µm. The maximum

7

straw length is 150 cm. In the barrel region straws are parallel to the beam direction andperpendicular in the end-cap region. The middle part of the inner detector is the siliconstrip detector - semiconductor tracker (SCT) - consisting of 4 barrel layers and 9 forwardwheels. The SCT system is designed to provide 4 precision measurements per trackin the intermediate radial range and contributing to the momentum, impact parameterand vertex position measurement. The maximum radius of the barrel layer is 52 cmand 56 cm of the forward wheel. The system requires very high dimensional stability,cold operation of the detectors and evacuation of heat generated by the electronics anddetector leakage current.

Forward SCT

Barrel SCT

TRT

Pixel Detectors

Figure 2: The inner detector.

The group of VdG accelerator at Institute of Particle and Nuclear Physics (IPNP)of Faculty of mathematics and physics at Charles University in Prague has been involvedin working places where QA tests of 200 SCT forward detector modules will be performed.

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3 Semiconductor detectors

3.1 Comparison to other detectors

Semiconductor detectors are in high energy physics mostly used for precision trackingthat allows detection of secondary vertexes of very fast decaying particles. The advantagesof semiconductor detectors compared to the others being used for tracking are following:

• The gap energy between valence and conduction band is 1.11 eV in silicon and sothe average energy for creation of electron-hole pair (e-h) is 3.6 eV. That is approx-imately 10 times lower compared to the ionization energy of gases used in propor-tional chambers, drift chambers, time projection chambers etc.

• Due to high density of semiconductors, the average energy loss per unit of lengthis also higher according to the energy loss in gases. In case of silicon and minimumionizing particle (MIP) the value is 390 eV/µm while for the gases the loss is 3 ordersof magnitude lower. Consequently the thickness of semiconductor detectors can bevery small which minimizes the multiple Coulomb scattering. Usual thickness isaround 300 µm.

• Another advantage connected to the high density is the reduction of range of ener-getic secondary electrons that leads to good spatial resolution.

• The present advanced technology of silicon detector production allows creationof very fine structures on them (in the dimensions of micrometers). The dimensionsof the structures (usually strips or pixels) then mainly contribute to the resolutionof the semiconductor detectors.

• Since the readout electronics is usually based on semiconductor technology, the de-tectors and electronics can be integrated together. Noise of such a module is thanreduced.

• These detectors are mechanically rigid and so not complicated supporting structuresare needed.

• High mobility of the charge carriers results in high rate of reading and lower deadtime. Typical width at half maximum (FWHM) of the read out pulse is 20 ns.

But the semiconductor detectors have beside their high cost also one disadvantagecompared to the gaseous detectors. It is the absence of multiplication of the amountof primary generated charge carriers and so the signal is only a function of the detectorthickness.

3.2 Silicon properties

Silicon is an element of IV group of the group of elements and has 4 electrons on the va-lence shell. All the conductivity is realized by electrons excited from the valence bandinto the conducting one. Such an excitation leads to a generation of hole - empty statethat left after the electron excitation and that behaves as a positively charged particle.In the silicon without impurities the densities of electrons and holes are the same. By re-placing some of the silicon atoms by atoms from the III or V groups the p- or n-typematerials are obtained. Elements from the III group (acceptors) have 3 valence electronsand easily attach an electron from silicon atoms. Elements from the V group (donors)

9

have one very weakly bound electron that can be easily excited to the conduction band.The ”binding” energy of electrons in n-type and of holes in p-type silicon semiconductoris approximately 45 meV. Very heavily doped semiconductors are marked n+ or p+ re-spectively. In both n- and p-type semiconductors there are the other type carriers as well,due to thermal excitations, called minority carriers.

The density of intrinsic charge carriers is [4]:

n(T ) =∫ ∞

Eg

De(E, T )fe(E, T )dE (1)

where De(E) is the state density [3]:

De(E) =1

2π2

(

2me

h2

)3/2

(E − Eg)1/2 (2)

and fe(E) the Fermi-Dirac function for system of fermions:

fe(E) =1

eE−EF

kT + 1(3)

The used symbols are the energy of electrons E, the Fermi level EF , the gap energy Eg,the temperature T , the Boltzmann constant k, the Planck constant h and the effectiveelectron mass me connected to the second derivative of energy as a function of momentum.Application of equations (2) and (3) in (1) and use of similar relations for the densityof holes p(T ) results in:

n(T ) = 2(

mekT

2πh2

)3/2

eEF−Eg

kT (4)

and

p(T ) = 2(

mhkT

2πh2

)3/2

e−EFkT (5)

In Si without any impurity both densities are equal (ni) and do not depend on the Fermilevel:

n(T )p(T ) = n2i = 4

(

kT

2πh2

)3

(memh)3/2e

−EgkT (6)

In doped silicon of densities of NA acceptors and ND donors the relation (6) still holdssince compared to intrinsic semiconductor it is the Fermi level EFe that changes only.The extrinsic carrier densities follow equations coming from zero net charge density [5]:

n = nieEFe−EF

kT =1

2

[

ND − NA +√

(ND − NA)2 + 4n2i

]

≈ ND (7)

and

p = nieEF−EFe

kT =1

2

[

NA − ND +√

(ND − NA)2 + 4n2i

]

≈ NA (8)

where the approximations are valid when ND NA, ni , (n p) and NA ND, ni ,(p n) respectively.

Properties of silicon material are written in table 1. Particle passing through the de-tector ionizes the Si atoms and so effectively creates the e-h pairs. For typical thicknessof silicon detector 300 µm the number of generated e-h pairs by MIP passing perpendicu-lar through the detector (see section 3.6) is 3.2·104 which is 4 orders magnitude lower thanthe total number of free carriers in intrinsic silicon of a surface of 1 cm2 and the thicknessmentioned above. In doped material the S/N ratio would be even smaller. One way toincrease the ratio, is to cool the semiconductor. Another way is to deplete the detectorof free carriers through a reverse biases P-N junction. The second way is the principleof operation of a silicon radiation detectors.

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Atomic number 14Atomic weight 28.08Atomic density 4.99·1022 cm−3

Density 2.33 g/cm3

Dielectric constant 11.6Gap energy 1.11 eV

Effective states density in conduction band 2.80·1019 cm−3

Effective states density in valence band 1.04·1019 cm−3

Electron mobility 1350 cm2V−1s−1

Hole mobility 480 cm2V−1s−1

Electron Hall mobility 1670 cm2V−1s−1

Hole Hall mobility 370 cm2V−1s−1

Electron diffusion constant 34.6 cm2s−1

Hole diffusion constant 12.3 cm2s−1

Intrinsic carrier density 1.45·1010 cm−3

Breakdown field 30 V/µmDiamond type lattice spacing 0.5431 nm

Mean energy for e-h pair creation 3.63 eVFano factor 0.115

Table 1: The physical properties of silicon at room temperature.

3.3 Drift and diffusion

Drift of charge carriers is their movement under external field ~E. The speed ~v of sucha movement is proportional to the external field:

~v = ∓µ~E (9)

where the coefficient µ of the proportionality is mobility of electrons and holes respectively.Movement of charge carriers under magnetic field ~B results in change of the movement

direction by Lorentz angle ϑL:tanϑL = µHB (10)

The coefficient µH is Hall mobility.For silicon with inhomogeneous carriers density the mean movement of the carriers

of charge q is, due to thermal fluctuations, nonzero and follows the opposite directionof the density n gradient:

~F = −D~∇n (11)

This equation expresses the proportionality of flow ~F to the density gradient ~∇n usingthe diffusion coefficient D. This coefficient is related to the mobility by Einstein equation:

D =kT

qµ (12)

coming from the zero value of sum of drift and diffusion flows.

3.4 The P-N junction

As mentioned above, reverse biased P-N junction reduces the number of free carriers.Due to gradient of electrons’ and holes’ densities in the junction of n- and p-type semi-conductors, the free charge carriers diffuse and recombine. The result is net positive and

11

negative charge in the n- and p-type materials respectively. This region of net chargecalled depletion region causes built-in potential barrier that, assuming NA, ND ni, canbe calculated from [5]:

VD =EFA − EFN

q=

kT

qln(

NAND

n2i

)

(13)

with EFA and EFN being the Fermi levels in n- and p-type crystals respectively.The depletion region can be widened by applying reversed potential Vbias on the P-N

junction. The barrier height would than be VB = Vbias+VD. The electric field distributioncan be obtained by solving a one-dimensional Poisson equation:

d2V

dx2= ∓qN

ε(14)

Let WA and WD be the width of depletion layers where uniform net charge densities are NA

and ND in the p and n regions respectively. Considering the neutrality of the crystal(NAWA = NDWD) the solution of the Poisson equation is [5] (see figure 3):

WA =

√

2εVB

qNA(1 + NA/ND)≈√

2εVB

qND

· ND

NA

(15)

and

WD =

√

2εVB

qND(1 + ND/NA)≈√

2εVB

qND(16)

where ε is the permitivity of silicon. Choosing the material so that NA ND (seethe approximation), the depletion region is wide on the n-side and shallow on the p-side.

Since there is a voltage dependent charge increment dQ = qNdW that appears on ei-ther side of the junction as a result of the widening of the depletion region on that side dW ,caused by an increase of the barrier voltage dVB, then it is possible to define junctioncapacitance [5]:

Cj =dQ

dVB=

dQ

dW· dW

dVB=

√

qεNAND

2(NA + ND)VB≈√

qεND

2VB(17)

The capacitance decreases with rising bias voltage until depletion layer reaches the backof the crystal. Such a VB is called depletion voltage Vdep.

3.5 Reverse current

The depletion region is free of majority carriers, but under equilibrium conditions e-hpairs are generated continuously anywhere within the volume of the crystal. In oppositeto non-biased detector, the created carriers have little chance to recombine. The pairsare separated and electrons and holes drift under the influence of the electric field. Thiscurrent is called leakage or reverse current. Depending on where the e-h pair is generatedthere are 2 components: a generation current of density jgen caused by charge generatedwithin the depletion region and a diffusion current jdiff coming from charge generatedin the neutral silicon and diffusing to the depletion region.

Assuming very low charge densities n, p ni in the depletion zone of width W andeffective life time τ0 of minority carriers [5]:

jgen =1

2qni(T )

τ0W (Vbias) (18)

12

Acceptor ion

hole

Donor ion

electron

P N

x [mm]-0.15 -0.1 -0.05 0 0.05 0.1 0.15

Char

ge de

nsity

[C/m

]

-0.0015

-0.001

-0.0005

0

0.0005

0.001

x [mm]-0.15 -0.1 -0.05 0 0.05 0.1 0.15

Elec

tric f

ield [

V/m]

-1

-0.8

-0.6

-0.4

-0.2

0

x [mm]-0.15 -0.1 -0.05 0 0.05 0.1 0.15

Poten

tial [V

]

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Figure 3: The charge density, electric field (intensity) and potential in the P-N junction.

13

The temperature dependence is only through ni(T ). Considering equation (6) ni(T )increases by factor of 2 with a temperature increase of 8 K. The current is proportionalto

√Vbias, when Vbias is lower than depletion voltage, and constant above it.

Pairs e-h generated in the neutral region in the proximity of depletion one havea chance to diffuse into it before recombination. Denoting by τe and τh the lifetimesof electrons and holes in the n- and p-type region respectively, the width of the layerfrom which carriers would diffuse is [5]:

L =√

Dτ (19)

where D is the diffusion coefficient for proper free carriers of density n. The diffusioncurrent can be than calculated from:

jdiff = qn

τL (20)

3.6 Interactions of particles in silicon

There are two mechanisms of energy loss of charged particles in solids: the ionizationand the bremsstrahlung. Important part of the ionization process is the release of highenergy electrons (δ-electrons) that increase the mean energy loss. Another importanteffect is the Coulomb scattering resulting in beam divergence after passing through thedetector.

The mean energy loss due to ionization of particle of charge z, mass M and velocity(in units of speed of light c) β =

√1 − γ−2, is described by Bethe-Bloch formula:

−(

dE

dx

)

ion=∫ Tmax

Tmin

TnedσRuth

dTdT = 2 · 2πα2h2z2

meβ2

Z

AρNA · ln

(

Tmax

Tmin

)

(21)

where T is the energy loss, ne is the density of electrons of mass me in material of atomicnumber Z, atomic weight A and density ρ, σRuth is the Rutherford scattering cross-section,α is the fine structure constant and NA is the Avogadro constant. The minimum energyloss Tmin is equal to the ionization potential I0 ≈ 16 ·Z0.9 [6], while the maximum energyloss is:

Tmax =2mec

2β2γ2

1 + 2γ me

M+ (me

M)2

(22)

The factor of 2 in relation (21) accounts for such effects as atomic excitation. Modificationof the formula (21) for fast electrons was found to be [9]:

−(

dE

dx

)

ion,e−=

2πα2h2z2

meβ2

Z

AρNA

[

ln(

m2ec

4β2γ

2I2(1 − β2)

)

− ln 2(

2

γ− 1

γ2

)

+1

γ2+

1

8

(

1 − 1

γ

)2]

(23)The statistical fluctuations around the mean energy loss in a layer of thickness δx are

described by Landau, Vavilov or Gaussian theory, depending on ratio κ, that is propor-tional to the ratio of mean energy loss to the Tmax:

κ =ξ

Tmax

=2πα2h2NAz2Zρ

meβ2A· δx

Tmax

(24)

The assumptions on Landau theory are that the ratio ξ/I0 1 and that the typical energyloss is small compared to Tmax and is large compared to the binding energy of the mosttightly bound electrons. The Landau distribution function is shown in figure 4. The firstrestriction is removed in the Vavilov theory. According to the assumptions, Landau

14

Energy loss [keV]50 100 150 200 250 300

Nu

mb

er o

f ev

ents

0

50

100

150

200

250

300

350

400

Figure 4: Simulated energy loss of 180 GeV negative pions using Geant4 and distributionfunction of Landau theory (solid curve).

theory can be used when κ < 0.01, while the Vavilov theory is used when 0.01 < κ < 10.In the region of κ > 10 which describes non-relativistic particle energy loss, Gaussiandistribution can be applied, assuming a large number of collisions involving the loss of mostof the incident particle energy,

The tail in the region of high energy loss in Landau distribution is caused by highenergetic electrons (δ-electrons) released by the incoming particle and resulting in sig-nificantly higher average energy loss than the most probable value. Since the rangeof the δ-electrons is in the order of 10 µm (40 µm for 100 keV electron), they can causedisplacement of the measured track position. The number of δ-electrons of energy higherthan Tδ is [8]:

dNδ

dx=

2πα2h2z2

meβ2

Z

AρNA · δx

Tδ

(25)

A δ-electron of kinetic energy Tδ is produced at an angle θδ determined by relation [8]:

cos θδ =Te

pe

· pmax

Tmax

(26)

where pmax and pe are momenta corresponding to the kinetic energies.Another mechanism of energy loss important for e∓ is the electromagnetic radiation

(bremsstrahlung) described by formula [9]:

−(

dE

dx

)

brem,e−=

α2h2γZ(Z + 1)NAρ

137meA

[

4 ln(2γ) − 4

3

]

(27)

The relative influence of ionization and bremsstrahlung in solids is described by criticalenergy [8]:

Ec =610

Z + 1.24· MeV (28)

Assuming thickness of the layer being passed through δx X0, with X0 = 9.36 cmbeing the radiation length in silicon, the mean number of radiated photons with energies

15

between Eγmin and Eγmax is [8]:

Nγ =δx

X0

[

4

3ln(

Eγmax

Eγmin

)

− 4(Eγmax − Eγmin)

3mec2γ+

(Eγmax − Eγmin)2

2(mec2γ)2

]

(29)

The coulomb scattering at small angles [8] of particle of momentum p passing througha thin layer is described by RMS of Gaussian distribution of deflection angles:

P (Θ) =1

√

2πΘ2RMS

· exp(

Θ2

Θ2RMS

)

dΘ (30)

ΘRMS =z · 21MeV

βcp

√

δx

X0(31)

3.7 Microstrip detectors

Schematic diagram of n-type microstrip detector is shown in figure 5. The mainpart of the depletion region is in the weakly doped n-type material (see formula (16)).Particle traversing through the detector creates e-h pairs along its path. The numberof the pairs is proportional to the energy loss described in section 3.6. Since the detectorof thickness d is reverse biased the generated carriers drift along the electric field E(x) [7]towards the strips and backplane:

E(z) = −Vbias + Vdep

d+

2Vdep

d

z

d(32)

The carriers diffuse in the direction ~x perpendicular to the electric field. The distributionof number of holes and electrons after drift to the strips and backplane respectively followsthe Gaussian law:

dN =1

√

2π[2Dt(z) + δ2]· exp

[

− x2

4Dt(z) + 2δ2

]

N(z)dzdx (33)

where dN is the charge in the element dx, at distance x from the track, and com-ing from the charge N(z)dz generated in the element dz of the track at distance zfrom the strips. N(z) is the linear density of the generated charge. The other usedsymbols are diffusion coefficient D, width of the track δ and the time of charge carriersdrift t(z) from the place of generation to the strips and backplane. The drift time can becalculated combining definition (9) and relation (32):

th(z) =d2

2Vdepµh

ln[

(Vbias + Vdep)d

(Vbias + Vdep)d − 2Vdepz

]

(34)

for holes and:

te(z) =d2

2Vdepµe

ln[

(Vbias + Vdep)d − 2Vdepz

(Vbias − Vdep)d

]

(35)

for electrons respectively. The product of D · t(z) is independent of µ and so is the widthof the distribution.

When measuring the amplitude of the signal on each strip (analog readout), partialreconstruction of the distribution is possible, which results in much better localizationprecision compared to strip pitch p:

∆x ≈ p

S/N(36)

16

When binary readout is used, signal on the strips is compared to a given threshold andthe RMS of the measured and real track position ∆x2 can be calculated, assuming nocharge loss, from formula:

∆x2 =1

p

∫ +p/2

−p/2x2dx =

p2

12(37)

The charge division between the strips can be realized in resistive or capacitive way.The resistive one leads to noise generation. The capacitive one is naturally realizedby interstrip capacitance, but there is a non-linearity and charge loss due to strip-by-stripand strip-ground capacitance.

As mentioned in section 3.1 one of the advantages of semiconductor detectors is the in-tegration of the sensitive material and the read out electronics. There are 2 possible waysof such integration: direct connection (see left part of figure 5) where reverse current flowsthrough the electronics or capacitive connection (see right part of figure 5) where only cur-rent changes are detected by the electronics. The capacitor C can be easily implementedon the detector using a layer of SiO2 as well as the bias resistor R using polysilicon.

n+

Al

p+

p+

Al Al

n

SiO2

+ -

+ -

+ -

+ -

+ -

+ -

+ -

z

d

dx x

Front-endelectronics

particle

Front-endelectronics

RC

Figure 5: The slice of n-type microstrip detector with DC-readout (left) and AC-readout(right).

3.8 Noise

Referring to section 3.1 there is no multiplication of the amount of generated chargecarriers. In events with tracks crossing the detector between 2 strips or at large angles,due to charge sharing, only a fraction of total charge is collected on each strip. Todistinguish the real signal, low noise is essential. The electronics and detector itselfcontribute to the noise in different ways:

17

• The main contribution comes from the capacitance of the strip being read out to itsneighbours and to the backplane. It causes a signal loss and acts as a load capac-itance C of the preamplifier. For conventional charge sensitive amplifier the elec-tronics noise is calculated as equivalent noise charge (ENC) from the formula:

ENCload = A + B · C (38)

where A and B are constants depending on the preamplifier.

• Another contribution is the equivalent noise referred to the input of the amplifierfrom the leakage current I and is given by:

ENCleak =e

q

√

qITp

4(39)

where e is natural logarithm base, q is the electron charge. and Tp is the peakingtime equal to the integration time of a CR-RC shaper. The peaking time differsfrom the integration time for other types of shapers.

• Bias resistor R contribute to the noise as well by following formula:

ENCbias =e

q

√

TpkT

2R(40)

with k being the Boltzmann constant and T temperature.

The error in measurement of the signal caused by all these contributions can be ob-tained as their sum in squares:

ENC =√

ENC2load + ENC2

leak + ENC2bias (41)

3.9 Radiation damage

As ATLAS will operate in high radiation environment, changes to the properties parti-cles with the nuclei in the lattice may lead to permanent material changes due to followingprocesses:

• Displacement of lattice atoms leading to interstitials and vacancies

• Nuclear interactions

• Secondary processes from energetic displacement lattice atoms leading to possibledefect clusters

Most of the primary defects are mobile at room temperature and will therefore par-tially anneal. However, there are also stable defects: combination of vacancy and oxy-gen (A-center), a vacancy phosphorus complex (E-center) and 2 vacancies next to eachother (divacancy). Although the primary interaction of radiation with silicon is stronglyparticle-type and -energy dependent, due to smoothing out by secondary interactions andconsidering non-defect-producing interactions with electrons, it is possible to use scalingby the non-ionizing energy loss (NIEL) of 1MeV neutrons.

The defects can act as trapping centers reducing signal and as recombination centersleading to an increase of the leakage current. They can change the resistivity of undepleted

18

regions and charge density in the depleted region, thus requiring an increase of depletionvoltage. In case of n-type detector, long term radiation leads to effective type inversion.

Damage in electronics has different effect as induced change in doping concentration isnot important due to much higher doping densities than in detectors. The most importanteffects are the damage on silicon oxide layers in metal-oxide-semiconductor field effecttransistors (MOSFET) and the decrease of minority carriers lifetime in case of bipolarand junction field effect transistors (JFET). The effects lead to decrease of amplificationcharacteristics of transistors and increase of noise.

19

4 Detector modules

4.1 Construction

Every SCT module consists of 2 or 4 silicon strips detector wafers connected by fan-insto a hybrid with 12 readout chips. The detectors are glued on a mechanical basement.The typical surface of the sensitive wafers is 6×6 cm2. There are 4 types of the SCTmodules (see figure 6): a barrel module and 3 forward modules differing in the numberof detectors and their geometry. In modules with 4 silicon wafers, the detector wafers arebonded to 2 pairs and so providing effectively 2 sensitive wafers of length approximately12 cm.

Figure 6: The barrel module (top left), forward outer module (bottom left), forwardmiddle module (bottom right), forward inner module (top right).

All the silicon wafers are single sided p-in-n detectors, 285 µm thick and containing 768Al strips 23 µm wide. Every module has 2 parallel detector planes and thus 1536 readoutchannels. The 2 planes are rotated by an angle of 40 mrad to provide 2D track positionmeasurement by combining the hit strips numbers. The strip pitch of barrel detectorsis constant: 80 µm. Thus taking into account the cylindrical coordinate system (R,Φ,z)with z direction parallel to the beam pipe, the point resolution is 23 µm (see formula (37)).Combining the 2 points from both detector planes gives precision of 16 µm in the RΦ-direction and 580 µm in the z-direction. The forward detectors differ from the barrel onesby non-parallel strips converging to one point close to the beam line for easy extractionof the (R,Φ,z) coordinates of the measured tracks. The strip pitch varies from 54 µmup to 95 µm and consequently the point resolution is position dependent. While the planeof the barrel detectors is parallel to the beam direction, the forward modules’ detectorplanes are perpendicular to it and so the last mentioned precision is not in the z-directionbut in the R-direction for forward modules.

20

Readout buffer

Com

pre

ssio

nF

orm

at

Contr

ol

128 strips x 132 cellspipeline

Data Out

Calibrationpulse

ThresholdDiscriminatorComparator

Preamplifier

Shaper

Front-end electronics

Detector

Str

ips

Figure 7: The FE and readout electronics.

The ATLAS SCT readout electronics is responsible for supplying the hits informationto the ATLAS 2nd level trigger and data acquisition system. To ensure low noise opera-tion, front-end (FE) electronics is mounted immediately at the strips’ electrodes. Thereare 12 readout chips on hybrid of every module and every chip reads out 128 channels(strips). The chips on the first and second module side are marked M0 S1 S2 S3 S4 E5and M8 S9 S10 S11 S12 E13 respectively (see figure 8). The ATLAS SCT uses binary

DetectorsMechanicalbasement

Bonds

Chip S3

Chip S4

Chip E5

Chip S2

Chip S1

Chip M0

Fan-ins

Hybrid

Figure 8: Module description.

readout (signal on strips is compared to a given threshold) to reduce the amount of datato be transmitted and stored. The schematics diagram [11] of FE architecture is shownin figure 7. The data from the strips are every 25 ns (LHC bunch crossing rate) storedinto chips’ pipelines and are held there for the duration of the level 1 trigger (L1) la-tency waiting for the decision to transmit the data or discard it. The average triggerrate of the FE electronics operation is 100 kHz. If the data are to be read out, theyare compressed and transmitted out using optical fibers. To suppress noisy hits (clustersof channels where read signal is greater than set threshold), certain type of data readoutand compression based on special timing pattern recognition can be applied. Importantfeature of the electronics is the calibration circuit allowing to associate threshold on dis-criminator to an appropriate charge at the input of the preamplifier. To obtain the best

21

possible uniformity of the calibration process, thresholds can be adjusted individuallyfor every channel. This process is called trimming. The chips and hybrid constructionallows to bypass non-functional chips and there is a redundant optical connection to fixpossible failure of the standard one.

4.2 Read out system

The schematic diagram of the readout system for QA procedures for a single moduleis shown in figure 9. The hardware is based on Versa Module Eurocard (VME) modulesof the following functionality:

PC

ROOTV

ME

co

ntr

olle

r

SC

TLV

3

SC

TH

V

Mu

STA

RD

CL

OA

C

SL

OG

PP

R

VME crate

Clean room

Slo

w c

on

tro

l syste

mC

oo

ling

Module box

Supportcard

Figure 9: The readout system for a single module.

• SCTLV3 module [12] provides low voltages (digital 4.0 V, analog 3.5 V) to the read-out electronics and assures monitoring of the temperature and power consumption.

• SCTHV module [13] provides bias voltage for the detectors and monitors the leakagecurrent.

• MuSTARD module [14] reads out and stores the data from the hybrid

• CLOAC module [15] and SLOG module [16] generate command sequences like trig-ger, calibration and reset signals. The latest ones resend configuration to the chipsto correct possible loss of threshold and other settings. The CLOAC module allowsuse of external triggers (for example from a scintillator in beam tests) and can fan-out the command sequences that were sent to the readout electronics. The latermentioned feature can be used to trigger a laser.

• VME controller assures communication of the modules with personal computer.

• The PPR together with the support card are passive components connecting datalinks from the hybrid and the VME modules.

The data acquisition (DAQ) software [18] is based on ROOT [17] - a C++ interpreterwith additional classes for easy data manipulation and visualization. The software con-tains a buttons control panel, ROOT interactive window and a basic information panelsshowing data control system (DCS) monitoring and the occupancy of the strips afterapplied a burst of triggers, results of performed scans etc.

22

As the silicon modules have to be tested in a clean environment, clean room was builtfor this purpose at IPNP [28]. Typical readout system for QA allows to test up to 6modules in parallel. Since the modules have to be cooled during the tests and humidityreduction by flowing a dry air on the modules is needed to prevent shorts at the detector,the tested devices are placed into special boxes. The monitoring of the environmentconditions, data backup and solving of accidents like power failure, is assured by slow-control system [26].

4.3 Standard QA tests

The standard QA procedure includes tests of the detectors and functionality testsof the readout electronics. The quality of the detector wafers is checked by measure-ment of the leakage current as a function of bias voltage. Example of this IV-curve isin figure 10. Accounting the dependence of depletion layer width on the bias voltage,the relation (18) for generation current is valid up to 300 V, where the avalanche effectsstart to modify the shape of the IV-curve, and the depletion voltage is around 60 V. Ap-plying in the relation (18) τ0 = 1 ms [4] gives the generation current of 2 µA. This valueis consistent in the order with the measured one. Precise comparison is complicated dueto τ0 dependence on the temperature and number of impurities in the silicon. The usedvalue of 1 ms is a very rough approximation for weakly doped n-type silicon. In additionthe diffusion current jdiff was neglected.

0 50 100 150 200 250 300 350

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Bias [V]

Le

aka

ge

cu

rre

nt

[A

]m

0 100 200 300 400 5000

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

Bias [V]

Le

aka

ge

cu

rre

nt

[A

]m

Figure 10: The IV-curve of unirradiated (left) and irradiated (right) module. Monitoredtemperatures on the hybrids were around 25 Celsius degrees on the unirradiated moduleand 0 Celsius degrees on the irradiated one.

The tests of the readout electronics involve bypass and redundancy tests, but the mostimportant part is the calibration process including trimming and noise occupancy mea-surement. Another issue is the long-term stability test lasting 24 hours. Since the SCTreadout is binary, it is not possible to find the amplitude of the signal directly by onemeasurement, but integral of the spectrum of the signal can be obtained by scanningthe threshold to which the signal is being compared. If the signal is generated by chargedparticle passing through the detector, one obtains integral of the Landau curve, whilefor signal coming from the calibration circuit, the result is an error function, becausethe charge provided by calibration circuit has a narrow Gaussian distribution. The realmeasured sigma of the Gaussian function is higher and is equal to the noise (see sec-

tion 3.8) assuming Gaussian distribution of the noise with given sigma equal to ENC andzero mean. Example of such an error function is shown in figure 11. The noise occupancymeasurement (see figure 12) is a simple high statistics threshold scan with no calibra-tion charge applied. The trimming (see figure 13) is done for selected calibration charge

23

Amplitude [fC]-2 -1 0 1 2 3 4

Sp

ectr

um

0

0.2

0.4

0.6

0.8

1

Threshold [fC]1 1.5 2 2.5 3 3.5 4 4.5 5

Nu

mb

er o

f h

its

0

200

400

600

800

1000

Figure 11: The threshold scan (right) of single channel with 0.2 fC step and with zero-width calibration charge of 3 fC - see dashed curve in figure (left). For every set thresh-old, the calibration pulse was applied and signal was read out 1000 times. The his-togram (right) shows the number of events when the read out signal was greater thanthe threshold. The dashed curve corresponds to ideal readout with no noise, while the fullone is the smeared by Gaussian distribution of the noise (left).

by scanning the threshold and tuning the readout chips’ settings so that the thresholdof 50% efficiency (median) is uniform over all channels as much as possible. The calibra-tion process consists of scanning the calibration charge and calculating the appropriatemedian of threshold scan for every setting of the charge. Example of such a dependencecalled response curve is shown in figure 14. The derivative of the response curve deter-mines gain of the FE preamplifiers. The aim of these tests is to check whether the modulematches the specifications (see section 4.4) on the rate of noisy hits and the purposeof the calibration is to find the threshold of around 1 fC, where the efficiency should behigh enough.

Channel number

100 200 300 400 500 600 700

Th

resh

old

[m

V]

0

20

40

60

80

100

120

140

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

Threshold [mV]

0 20 40 60 80 100 120 140

No

ise

occ

up

ancy

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

4.88E-006

Figure 12: The noise occupancy scan of KB-105 module. The left figure shows the oc-cupancy separately for every channel, while the on the right average noise occupancyof the 6 chips (1 side of the module) is shown. The marked point is the noise occupancyat 1 fC threshold.

The radiation damage influences properties of both the sensitive wafers and the read-out electronics. The latter mentioned can be seen in an increase of the leakage current

24

and consequently the noise occupancy, depletion voltage increases as well. From the beamtests (see section 4.4) decrease of the amount of collected charge is obvious. In the elec-tronics the radiation affects noise and gain. The irradiation of the modules is performedat CERN PS by 24 GeV protons to the dose of 3·1014 protons/cm2 during approximately2 weeks. The typical characteristics for both the irradiated and unirradiated modulesof 4 detector wafers are summed in table 2. Note the higher noise occupancy, leakagecurrent and bias voltage needed for operation of irradiated modules compared to unirra-diated ones. In spite of the fact that the median collected charge in irradiated detectorsis lower and so is the efficiency, the average size of strips clusters (see section 4.4) isthe same as for the unirradiated ones. This is due to stronger charge sharing in the ir-radiated detectors. The lower gain of the FE electronics confirms the effects of radiationon the electronics described above.

Channel number100 200 300 400 500 600 700

Pu

lse

amp

litu

de

[mV

]

40

60

80

100

120

140

160

Channel number100 200 300 400 500 600 700

Pu

lse

amp

litu

de

[mV

]

40

60

80

100

120

140

160

Figure 13: The points of 50 % efficiency from threshold scan of all channels of KB-105module before (left) and after trimming (right) showing better uniformity of the measuredsignal amplitudes after trimming.

Calibration charge [fC]

0 1 2 3 4 5 6 7 8

Gai

n [

mV

/fC

]

0

10

20

30

40

50

60

70

80

90

100


0 1 2 3 4 5 6 7 8

Med

ian

[m

V]

0

100

200

300

400

500

600


0 1 2 3 4 5 6 7 8

EN

C

0

500

1000

1500

2000

2500

Figure 14: The results of M0 chip calibration of module KB-105: response curve (left),FE electronics gain (center) and ENC (right).

4.4 Beam tests

To verify the efficiency of the modules in detecting particles, prototypes of the moduleswere tested in beam of SPS at CERN. The schematic diagram of the beam tests setup isshown in figure 15. The modules were placed in a light-tight box with integrated coolingsystem. To find the efficiency of the tested devices, positions of the particles’ tracks must

25

Quantity Unirradiated modules Irradiated modules

Efficiency [%] 99.5 99.0Noise occupancy 10−6 .. 10−5 10−4 .. 10−3

Cluster size 1.27 1.26Median [fC] 3.4 2.7

Leakage current [µA] 1 2000FE gain [mV/fC] 50 35

Used bias voltage [V] 150 350

Table 2: The characteristics of SCT modules: unirradiated modules, irradiated modules.The presented values are given at 1 fC threshold and cluster size at incidence angle 16 de-grees. The values are typical as they vary from module to module. The leakage currentscorrespond to measurements with monitored temperatures on the hybrids around 0 Cel-sius degrees.

be known. This information is provided by 4 silicon strip detectors of strips pitch 50 µmand analog readout. The precision of this telescopes is up to 5 µm (see formula 36). Thereare 3 important characteristics of the measurements:

• Efficiency - the number of events when there is a cluster of neighbouring stripswith read signal greater than the threshold and the track given by the telescopes isnot too distant (<150 µm) from the position of the center of such a cluster of strips.

• Median - the threshold where the efficiency reaches 50% (see section 4.3).

• Noise occupancy - probability that there will be a signal on a strip greater thanthe threshold and the position of the strip is far from the track in the detectordetermined by the telescopes.

• Cluster size - an average width of the hit strips clusters that are assumed to be causedby the particle passing through the detector (see the definition of the efficiency).

Light-tight box

B

Tested modulesAnaloguetelescopes

Beam

Scintillatorsin coincidence Analogue

telescopes

&

trigger signal

Figure 15: The beam tests setup.

There are 3 possible effects that lead to the existence of 2- and more-strips clusters:

• δ-electrons with range sufficient to reach also the strips neighbouring to the nearestone.

26

• Charge sharing between strips due to possible non-perpendicular incidence angle anddue to diffusion of the generated charge carriers as they always drift to the neareststrip (see figure 17).

• Cross-talk between strips due to their capacitive coupling.

The region of interest is at around 1 fC threshold where efficiency has to be sufficientlyhigh (>99%) and noise occupancy low (<5·10−4) as defined in the Technical Design Re-port (TDR) [2] to provide the expected reconstruction capability. Another importantcharacteristics are the dependence of the median (and consequently the efficiency) andcluster size on the incidence angle and change of the response in magnetic field of 1.56 T.The precise information about the track positions allowed to study the characteristicsat the edges of the detectors and to measure the median and cluster size dependenceon the relative position (η) of the track position with respect to the position of the strips.Results of the beam tests can be found in [10].

27

5 Simulations

To understand the results of beam tests and their difference to the source tests, simula-tion of SCT modules functionality was performed. The whole simulation is divided into 2parts: Geant4 [22] simulation and digitization under standard ATLAS SCT software [23].

5.1 Geant4 simulation

Geant4 is used to calculate the energy loss of particle passing through the module.The output is a particle track divided into several segments with deposited energy in them.This information is than used by the digitization software described in section 5.2.

The Geant4 simulation accounts following processes of particle’s interactions in matter.For electrons ionization including δ-electrons production, bremsstrahlung and multiplescattering is applied. For positrons annihilation is used in addition and for muons, pairproduction is considered as well. Photons interact through photo-electric effect, Comptonscattering and conversion to electron-positron pair. For hadrons ionization and multiplescattering is used only.

The ionization is divided into 2 parts: ionization process with local energy depositionand creation of δ-electrons. There is an energy cut Tcut in Geant4 given by minimaldistance, that particle of energy E and mass m has to pass in material in order its energyloss not to be involved into local energy deposition. So the local energy deposition is givenby [22]:

−[

dE(Z, E, Tcut)

dx

]

ion= ne

∫ Tcut

0

dσion(Z, E, T )

dTdT (42)

where σion(E, T ) is the cross-section of particle’s interaction with electrons in the material.The cross-section of release of δ-electrons with kinetic energy T greater than Tcut comesfrom the cross-section σion(Z, E, T ):

σδ(Z, E, Tcut) =∫ Tmax

Tcut

dσion(Z, E, T )

dTdT (43)

Compared to equations (21) (23) Geant4 ionization involves density effect correction thattakes into account material polarization, when high energetic particle passes throughit. The second correction is the shell one that involves lower probability of particle’sinteraction with electrons on inner atomic shells (K,L,. . . ).

Analogically the bremsstrahlung is divided into 2 parts: local energy deposition andphoton radiation if the photon has sufficient energy Eγ (greater than Eγcut) to passin the material the predefined minimal distance. The local energy deposition is givenby equation:

−[

dE(Z, E, Eγcut)

dx

]

brem= ne

∫ Eγcut

0

dσbrem(Z, E, Eγ)

dEγdEγ (44)

And the cross-section for radiation of photons with energy greater than Eγcut can becalculated from:

σγ(Z, E, Eγcut) =∫ E−mc2

Eγcut

dσbrem(Z, E, Eγ)

dEγ

dEγ (45)

To calculate energy loss at distance ∆x in the material, Geant4 slices the track into Nsegments ∆xi:

∆E =N∑

i=1

(

dE

dx

)

i∆xi (46)

28

The directions of track segments include multiple scattering given by equation (30) andalso the lengths ∆xi are corrected to the multiple scattering.

The geometry used for the simulation is written directly in Geant4 and is shownin figure 16. While the detector wafers were implemented in the geometry, the hybridwith readout electronics was not because both in the beam and source tests, that weresimulated, the particles weren’t crossing the area of the hybrid.

Figure 16: Geometry of the Geant4 simulation of the beam tests. The geometry for sourcetests differs only in different source (point) and in one case in aluminum plate betweenthe module and scintillator.

5.2 SCT digitization

The digitization software simulates the drift and diffusion of the generated chargecarriers inside silicon and the function of the readout electronics. The software is describedin [24] and the used physical processes in [25]. The purpose of the digitization is to providerelatively fast and acceptable response of the SCT modules according to the geometricaland electrical settings used in the beam tests.

The software reads the output of the Geant4 simulation described in section 5.1.The segments of the track can be linearly divided to get vernier segmentation. For everysuch segment its position, direction and energy loss in it is known. As the average energyneeded to create e-h pair is well known, it is possible to associate an appropriate generatedcharge to every segment. The holes drift to the strips for time given by (34). The electronsdrift to the backplane of the detector and so their contribution to the signal on the stripsis not taken into account. Current induction on the strips coming from the movementof the holes is neglected and so the signal is assumed to appear precisely when the holesreach the side of the detector with the strips. This approximation can be used as the fieldis the strongest near the strips and so the high drift velocity of the holes causes the mostsignificant induction on the strips. The field approximation by formula (32) in the detec-tor volume is not valid near the strips. Using simple numeric method to solve the Poison

29

"Surface drift"to the nearest strip

z

d

particle

Strip

B

qL

Drift t

ime

t (z

+d

/2)

h

Pulse shapeAmplifierNoiseThreshold

Arrival positionon the surface:

gaussian distribution(diffusion)

d-electronGeant4

simulation

h=0 h=1

Figure 17: The principal scheme of the digitization.

equation results in field shown in figure 18. The arrival position of the holes on the surfaceof the detector is shifted from the segment position due to diffusion by random distance

z [mm]0

0.050.1

0.150.2

0.25x [mm]0

0.010.02

0.030.04

0.050.06

0.070.08

E(x

) [k

V/m

m]

-1.5

-1

-0.5

0

0.5

1

1.5

z [mm]0

0.050.1

0.150.2

0.25x [mm]

00.01

0.020.03

0.040.05

0.060.07

0.08

E(z

) [k

V/m

m]

-1.8-1.6-1.4-1.2

-1-0.8-0.6-0.4-0.2

-0

Figure 18: Electric field in microstrip detector. The coordinates x and z match the selec-tion in figure 5. The field coordinate in the direction along strips is zero. The strips arelocated at x = 0 mm and x = 0.08 mm in the plane z = 0 mm.

with Gaussian distribution given by (33). Accounting the electric field in the detectorshown in figure 18, such a surface charge is than supposed to be collected on the near-est strip. The process is symbolically shown in figure 17. The digitization than simulatesthe readout electronics: the noise, shaping of the signal on the strips, comparing the pulseheight at given time (see figure 19) to a set of thresholds and finally producing the digits- the map of strips with the signal greater than the threshold. The software containsseveral free parameters mostly concerning the electronics like gain and offset correlationof the amplifiers or peaking time of the shaper, but there are also physical parameters like

30

Time [ns]0 10 20 30 40 50 60 70 80 90 100

Sig

nal

[fC

]

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Time [ns]0 10 20 30 40 50 60 70 80 90 100

Lo

gic

al p

uls

e

0

0.2

0.4

0.6

0.8

1

Figure 19: The pulse shape (top) and output of the comparator (bottom) (see figure 7)when the threshold was set to 2 fC (dashed line).

temperature, ENC or magnetic field. To simulate the response of the modules in magneticfield, the field is not applied in Geant4 because its influence on the momentum directionof the primary particle is not measurable (for 180 GeV pions in magnetic field of 1.56 Tthe Lorentz angle - the particle movement direction deviation due to magnetic field dur-ing going through the detector - is approximately 0.7 µrad). Because the magnetic fieldconsiderably influences the movement of the generated free charge carriers inside the de-tector, the magnetic field is applied in the digitization software. The parameter thatdefines the magnetic field in the software is the Lorentz angle.

The standard outputs of the digitization for every scan-point (determined by thresholdand time of readout) are the same as in beam tests:

• Efficiency - the maximum distance of the nearest cluster of strips is 50 µm. The 3times wider limit in the beam tests analysis is used because of the inaccuracyof the telescopes determining the track position, while in the digitization the trackis known absolutely from the Geant4 simulation. Using the dependence of efficiencyon threshold, median can be calculated.

• Noise occupancy - the only discrepancy from the beam tests analysis is analogi-cally to the efficiency definition the limit on the minimum distance of the noisy hitfrom the track in the detector.

• Cluster size - see definition in section 4.4. Statistical error of cluster size is calculatedas well.

It was checked that the used limits (that are set in the software as default) provide,accounting the statistical errors of the efficiency and the noise occupancy calculation,the same results as with the limits used in beam tests.

31

The existing version of the digitization of beam tests written by Szymon Gadomski [25]does not write out the binary information (digits) from the strips, but calculate straight-way the efficiency etc. described above, so I made a slight modification to the softwareto assure this feature. The map of digits is useful as it is possible to use it as real mea-sured data in analyzing software of beam and source tests. Therefore this allows to testthe analyzing software and to perform comparison to the real data at the basic level.

5.3 Beam tests simulation and digitization

To validate the simulations software, beam tests configuration was set in Geant4 ge-ometry. The validation was performed on simulation of a single barrel module, becausedue to constant strip pitch the response simulation is simpler compared to forward mod-ules. The important characteristics that the simulation should be able to reproduce arethe noise occupancy (simply determined by the free parameter ENC), the dependenceof the cluster size and the efficiency (respectively the median) on bias voltage, incidence

Geant4Particle π−

Particle’s kinetic energy 180 GeVMinimal step length 10 µm

Minimal energy of secondary particles (δ-e−) in silicon 31.5 keVMinimal energy deposited in scintillator 0 keV

Total number of simulated events 1000Length of the track segments 80 µm

DigitizationModule type Barrel

Depletion voltage 70 VTemperature 0 oC

Length of the track segments 80 µm (10 µm)Number of simulated charges’ drifts per segment 10 (1)

ENC 1500 e−

Number of bad channels 0Surface drift time on the whole strip pitch 10 ns

Maximal distance from efficient hit to the track position 50 µmMinimal distance of noise hit from the track position 150 µm

Peaking time 21 nsCross factor to neighbouring strips 0.10

Cross factor to backplane 0.07FE electronics gain spread (RMS) 0.061

FE electronics offset spread 680 e−

Offset and gain correlations -0.60

Table 3: The configuration of Geant4 and the digitization software used for the beamtests simulation. The values in brackets were used for 16 degrees incidence angle toobtain vernier division of charges’ positions with respect to the strips.

angle and magnetic field. Very detailed validation can be performed by the η-dependences(see section 4.4) of the efficiency and cluster size. The parameter η scales the track posi-tion between strips to the strip pitch and so it has values between 0 and 1 and is chosen

32

so that the chips are located at η=0 and η=1. The simulation was compared to the resultsof beam tests in 2001.

Angle [deg]-15 -10 -5 0 5 10 15

Med

ian

[fC

]

2.2

2.4

2.6

2.8

3

3.2

3.4

Angle [deg]-15 -10 -5 0 5 10 15

Med

ian

[fC

]

2.6

2.8

3

3.2

3.4

3.6

3.8

Angle [deg]-15 -10 -5 0 5 10 15

Med

ian

[fC

]

2.6

2.8

3

3.2

3.4

3.6

3.8

Angle [deg]-15 -10 -5 0 5 10 15

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

Angle [deg]-15 -10 -5 0 5 10 15

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

Angle [deg]-15 -10 -5 0 5 10 15

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

Figure 20: The angular dependence of the median (top) and cluster size (bottom). Sim-ulation is shown in the left figures and the beam tests results in the right. Open circlescorrespond to results with magnetic field of 1.56 T and full circles were obtained withoutmagnetic field. The geometrical model is drawn by dashed curves.

In table 3 there is the configuration of Geant4 and the digitization used for the simula-tion of the beam tests. The angular and magnetic field dependence of median and clustersize at 1 fC threshold is shown in figure 20. The simulation apparently underestimatesthe median by approximately 0.4 fC and overestimates the cluster sizes. The shifts of ex-tremes of the curves due to magnetic field are in good agreement with the beam tests data,because the shift is equal to the Lorentz angle - the parameter by which is the magneticfield defined in the digitization software. The charge generated by particle passing throughthe detector at an incidence angle α is proportional to the path length in the sensitivematerial and so it is inversely proportional to cos α. The median decreases with the in-cidence angle due to increasing charge sharing between strips. Assuming the charge tobe collected at the nearest strip, a simple geometrical model for median Qmed and clustersize ncs dependence on the incidence angle can be used. The results are superimposedto the figures 20:

Qmed =Q⊥

cos α

(

1 − d tanα

4p

)

(47)

ncs = 1 +d tanα

p

(

1 − 2 cos α · Qthr

Q⊥

)

(48)

where d is the detector thickness, p is the strip pitch, Qthr is the threshold at whichcluster size is calculated and Q⊥ is the median at perpendicular incidence (3.9 fC). Thesecalculated angular dependences are as can be seen at the figures more sensitive to incidence

33

angle compared to the beam tests data. Better agreement of the calculated medians sizescan be reached by lowering the detector thickness from 285 µm to approximately 250µm.

Bias [V]100 120 140 160 180 200 220 240 260

Med

ian

[fC

]

2.7

2.75

2.8

2.85

2.9

2.95

3

3.05

3.1

Bias [V]100 120 140 160 180 200 220 240 260

Med

ian

[fC

]

2.9

3

3.1

3.2

3.3

3.4

3.5

3.6

Figure 21: The bias voltage dependence of the median. Simulation is shown in the leftfigure and beam tests results in the right.

The bias dependence is drawn in figure 21. The comparison of beam tests and simula-tion shows differences in the absolute values (the median is underestimated as mentionedabove), but the trends of the curves are in good agreement. The drop of the median withlow bias voltage is caused by 3 effects:

• If the bias is lower than the depletion voltage (approximately 70 V for unirradiatedmodules - see table 3), then the sensitive area (depletion region) is narrower thanthe detector thickness and so the total collected charge is lower, because only the freecharge generated by passing particle in the depleted region can be collected.

• The drift time of holes towards the strips can cause drop of median measurementif the drift time is large compared to the width of shaper pulse (see figures 7,19).In that case signals from holes generated near the strips and near the backplane aretoo distant in the time to be composed in the optimal way - the pulse is wider butwith lower amplitude (the total charge is constant). The overall signal at the outputof the shaper is given by convolution of the raw signal on the strips and the shaperpulse. The bias dependence of drift time through the whole thickness of the detectoris shown in figure 22.

• At low bias voltages due to long drift time the charge diffuses into larger area (referformula (33)) and so is shared between strips. The dependence of the diffusion sigmaof the distribution (33) for holes drifting from the backplane towards the strips isshown in figure 22.

The 2 latter effects are involved in the digitization, while the first one is not, becauseit does not match real working conditions of the modules and because beam tests with biasvoltage lower than the depletion one have been never done.

The η-plots of efficiency and cluster size at incidence angle 16o are shown in figure 23.The comparison of the simulation with the beam tests follows the conclusions of angularand bias dependence simulation above - the absolute values differ, but the trends arein good agreement. But the η-plots for perpendicular (see figure 24) incidence differin the trend as well. The region of efficiency drop and high cluster size is much narrowercompared to beam tests results. There are 3 causes of this discrepancy:

• The points in graph of beam tests results are averaged over a 6 µm wide interval.

34

Bias voltage [V]80 100 120 140 160 180 200 220 240

Dri

ft t

ime

[ns]

0

5

10

15

20

25

30

35

Bias voltage [V]80 100 120 140 160 180 200 220 240

Dif

fusi

on

sig

ma

[mm

]

0.004

0.005

0.006

0.007

0.008

0.009

Figure 22: The drift time of holes through the whole thickness of the detector (left) andthe diffusion sigma of Gaussian distribution of holes drifting from the backplane towardsthe strips.

Relative interstrip position0 0.2 0.4 0.6 0.8 1

Eff

icie

ncy

0.2

0.4

0.6

0.8

1


Clu

ster

siz

e

1

1.2

1.4

1.6

1.8

2


Clu

ster

siz

e

1

1.2

1.4

1.6

1.8

2


Clu

ster

siz

e

1

1.2

1.4

1.6

1.8

2

Figure 23: The interstrip position dependence of the efficiency (top) at thresholds 1.5 fC,2.0 fC, 2.5 fC and 3.0 fC and cluster size (bottom) at 1 fC threshold. Simulation is shownin the left figures and beam tests results in the right. The incidence angle is 16 degrees.

35


Eff

icie

ncy

0.2

0.4

0.6

0.8

1


Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8


Eff

icie

ncy

0.2

0.4

0.6

0.8

1


Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8


Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8


Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Figure 24: The interstrip position dependence of the efficiency (left) at thresholds 1.5 fC,2.0 fC, 2.5 fC and 3.0 fC and cluster size (right) at 1 fC threshold. Beam tests results arelocated at the bottom, simulation with correction to track position uncertainties is shownin the middle and pure simulation on the top. The particles incident perpendicularly.

36

• The precision of track position measurement is determined by the telescopes (seesection 4.4) and is up to 5 µm. This adds uncertainty in the interstrip positiondetermination.

• Due to multiple scattering of beam particles in the modules and additional material(PVC covers of the modules in their boxes) the real track of the beam particlein a tested module differs from the track given by the telescopes. The distributionof the deviations is approximately Gaussian with sigma around 6 µm.

Application of these uncertainties of the interstrip position in the η-plots for perpendicularincidence is shown in figure 24. The shape of the modified curve is then in better agreementwith the beam tests data. The above described uncertainties in the track determinationin beam tests can be also responsible for the higher pedestal of the cluster size η-plotsmeasured in beam tests.

37

6 Source tests

6.1 Radioactive β−source

β− radioactivity consists of neutron decay: n → p νe e− inside a nucleus. Assumingzero mass of the antineutrino, the spectrum of electron energies can be derived using Fermitheory [20] (in the equation (49) the speed of light and Planck constant are assumed tobe unity):

dw =1

2mn

|Mfi|2 ·d3~pp

(2π)32Ep

· d3~pe

(2π)32Ee

· d3~pνe

(2π)32Eνe

· (2π)4δ(4)(pn − pp − pe − pνe) (49)

dw

dTe= C · (Te + mec

2)(Tmax − Te)2√

T 2e + 2Temec2 (50)

where C is a normalization factor, p is the momentum of the electron of mass me, T isits kinetic energy and E = T + mec

2. The half-width Γ of the decay is proportionalto the Fermi constant GF and maximal possible kinetic energy ∆ of the emitted electron:

Γ ≈ ∆5G2F (51)

The real spectrum differs from the formula (50) due to influence of the electromagneticfield of the decaying nucleus and electrons in the atomic shell on the emitted electron.The correction is applied through Fermi functions F (Z, T ) for 90Sr and 90Y that can bereasonably approximated by formula [21]:

F (Z, T ) =2παZβ−1

1 − e−2παZβ−1(52)

with Z being the atomic number and βc the velocity of electron:

β(T ) =

√

1 −(

mec2

mec2 + T

)2

(53)

Kinetic energy [MeV]0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

Nu

mb

er o

f ev

ents

0

200

400

600

800

1000

Figure 25: The spectrum of the radioactive source.

38

For the measurement of detector’s charge collection capability the tests using radioac-tive β− source 90Sr was developed. The decay chain is following: 90Sr → 90Y νe e−

and 90Y → 90Zr νe e−. The half-life of 90Sr is T1/2Sr= 28.5 years, while the half-life

of 90Y is T1/2Y= 68 hours. Because T1/2Sr

T1/2Ythe numbers of decays of 90Sr and 90Y

are equal. The normalization factors for the elements are: CSr = 258 and CY = 1.The maximum energy of emitted e− coming from the decay of 90Sr is 0.546 MeV [8] andso accounting the spectrum of the e− energies, most of the electrons will hardly passthrough both detector wafers 285 µm thick, as the mean energy of electrons with rangeequal to 2× the detector thickness is around 500 keV, which can be found using the re-lation (23). Since the maximum energy of the e− from 90Y decay is 2.283 MeV, onlya minority of these electrons will stop in the silicon. The combined spectrum of the ra-dioactive source is shown in figure 25.

6.2 Measurement setup

The measurement setup for β− source tests is shown in figure 26. The readout istriggered by signal from photomultiplier to which a scintillator is connected. As the read-out data have to go through the pipelines [11] (132×25 ns) of the chips, it is importantto set proper delay between the trigger signal and readout of the data from the endsof the pipelines. The photons created in the scintillator are transported by an opticalfiber to a photomultiplier. The amplitude of pulse from the photomultiplier is comparedto a threshold on discriminator and the output logical pulse is sent to an external-triggerinput of the CLOAC VME module of the DAQ system (see section 4.2). The module andscintillator are placed in a light-tight volume to prevent triggers caused by light photons.The threshold on the discriminator is set so that there are reasonable high rate of realtriggers and low rate of fake ones.

CLO

AC

VME crate

External trigger input

Aluminum plate

Hybrid

1. detector plane

2. detector plane

90 90Sr ( Y) source

Scintillator

Photomultiplier Discriminator

Figure 26: The setup of the source tests.

For source tests special software for DAQ and analysis was written by myself. The soft-ware is based on the DAQ system for standard QA tests. The main features of the DAQpart are assured by macros for:

39

• adjusting the optional timing (delay between the external trigger pulse and readoutof the data),

• sending burst of external triggers that is done at each scan-point,

• performing a scan (for example scan of threshold),

• data storage and reading of already stored data to be analyzed.

The rest of the software assures the analysis and visualization:

• The analysis macro made for the timing scan finds and sets the optional timingdetermined by maximum efficiency.

• Analysis is also made for every scan-point (usually determined by the value of thresh-old) where the most important characteristics and some plots are shown:

– Efficiency

– Noise occupancy

– Cluster size

– Histogram of multiplicity (total number of channels with read signal greaterthan the threshold) that is closely related to the noise occupancy

– Histogram of cluster widths (mean value is the cluster size)

– Monitored values of bias voltage, leakage current, set threshold, number of an-alyzed events and some other parameters

– Reconstructed profile of the beam of electrons that passed through both the de-tector planes

– Impact point position dependence of the efficiency and the cluster sizes, whichcan be used for adjusting the requested geometry of the source test, becauseboth the characteristics strongly depend on the impact direction of the elec-trons

– Check of correct connection between strips and channels of the readout elec-tronics, using the rate of special types of hit-patterns (see section 6.3)

• Finally analysis of the whole scan is performed and the dependence of efficiency,noise occupancy and cluster size on the scanned variable (usually threshold) is plot-ted. The results are:

– Median

– Cluster size at 1 fC threshold

– Noise occupancy at 1 fC threshold

– Visualization of the monitored values of bias voltage, leakage current, temper-ature and power consumption of the readout electronics

– A simple differentiation of the multiplicity by threshold to get the spectrumand Gaussian noise is performed and shown

– Rate of hits of cluster sizes 1,2,. . . is drawn to compare the median of singlewidth clusters with 2- or more-wide clusters

The software became a part of the standard DAQ and analysis software for the SCTmodule tests [27] and was used for all the measurements in Prague and CERN.

40

6.3 Analysis methods

The most important results from source tests are the efficiency (or median), clustersize, noise occupancy and checking of the connection (bonding) between strips and chips’channels. This characteristics can be calculated separately for both detector planes orfor some group of strips (for example the ones connected to the same chip). The methodsof analysis are described below:

• Efficiency - because there are no devices except the tested one able of precision po-sition measurement of particle passing through the detector the efficiency cannot becalculated in the same way as in beam tests analysis. The simplest way is to markthe event to be efficient if there is at least one hit on the detector plane. But thisabsolute method is sensitive to fake triggers from the scintillator (or photomulti-plier) that lead to efficiency drop. This problem can be partially solved by selectinga part of the beam area and only events with hit inside this region are assumed to beefficient. This is valid until higher noise occupancy of tested module (at low thresh-olds) or high rate of fake triggers is reached. To avoid this, another method basedon comparison of both detector planes of the module is used. Let n2 be the numberof events with a hit inside the selected part of the beam area at the second detectorplane. Because the angle of rotation of the planes is very small (40 mrad), if particlepasses through the module, the hit strips numbers on both planes should be close(accounting the geometry of the module, the difference in the strips’ numbers is notmore than 60). Let’s get the n2 mentioned events and select from them the n1both

ones where the hit strip numbers on both planes are close enough. Typical usedmaximal difference that corresponded to the beam profile coming from the setupof the source and the scintillator, was 15. Than the efficiency of the first detec-tor plane can be defined as the ratio n1both/n2. The error on this efficiency comesfrom binomial distribution:

Probp(n, k) =

(

n2

k

)

· pk · (1 − p)n2−k (54)

Selecting a confidence level CL (95%), the high and low errors are determinedby probabilities phigh and plow so that:

Probphigh(n2, k ≥ n1both) = 1 − CL/2 (55)

respectivelyProbplow

(n2, k ≤ n1both) = 1 − CL/2 (56)

To speed up the error calculation (finding the values phigh and plow) Gaussian ap-proximation of binomial distribution is used:

Probp(n, k) ≈ 1√

2πnp(1 − p)· exp

[

− (k − np − 0.5)2

2np(1 − p)

]

(57)

The efficiency of the second plane is obtained analogically with the first plane asreference.

There are 2 consequences of this method that have to be taken into account: higherstatistical errors compared to the absolute method due to lower statistics espe-cially at higher thresholds and some artificial effects at low thresholds connectedto high noise occupancy. This effects are shown in figure 27. At very low thresholds(< 0.7 fC) the noise occupancy is so high that no matter if there was a fake trigger

41

Threshold [fC]0 1 2 3 4 5 6

Eff

icie

ncy

[%

]

0

20

40

60

80

100

Figure 27: The efficiency dependence on the threshold.

there will nearly always be hits inside the selected part of the beam area on bothsides of the module with close strips numbers. With rising threshold the probabilityto find the close noisy hits decreases while the probability to find noisy hit at leastat one side of the module inside the selected part of the beam area is still high.The result is efficiency drop in the surrounding of 0.8 fC. At higher thresholds alsothe latter mentioned probability decreases and so the fake triggers from scintillatorresult in events with empty occupancy in the selected parts of beam area on both de-tector planes and these events are, as described above, excluded from the efficiencycalculation.

Threshold [fC]1 2 3 4 5 6

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Figure 28: The cluster size dependence on threshold. Dotted curve shows the values notcorrected to noise occupancy.

• Cluster size - to calculate the average size of the clusters it is especially at lowthresholds needed to distinguish noisy hits from the real ones caused by electron

42

passing through the detector. A simple calculation that averages over all clustersinside a selected part of the beam area leads to underestimation of the cluster sizesat low thresholds as the noisy clusters have usually the width of 1 (the probabilityto find 2 neighbouring noisy hits is proportional to the noise occupancy squared).So to avoid this underestimation the largest efficient cluster inside the beam regionis considered to be caused by passing electron. Because this method would leadto overestimation of the cluster size, corrections to noise occupancy are applied.Let ni be the number of clusters of width i than this number is effectively decreasedby probability that there were 1 to i-1 noisy hits in the cluster. Consequentlyni must be also effectively increased by the probability that there were k noisy hitsin clusters of width i+k. Example of the graph of cluster size versus threshold isshown in figure 28.

• Noise occupancy - the noise occupancy is defined as the number of channels, whereread noisy signal was greater than the threshold, divided by the total numberof channels. To avoid counting in the hits coming from the electron passing throughthe detector, the channel clusters that are assumed to be caused by the electron arenot counted in the occupancy. The total number of channels is lowered appropri-ately as well. Example of the graph of noise occupancy versus threshold is shownin figure 29. The errors follow a binomial distribution approximated by the Gaussianone of the summed noise occupancy over all events.

Threshold [fC]0 1 2 3 4 5 6

No

ise

occ

up

ancy

10-6

10-5

10-4

10-3

10-2

10-1

Figure 29: The threshold scan of noise occupancy.

• Bonding checking - the connection between strips and chips’ channels can be checkedusing the cluster sizes. If the connection is wrong than for example instead of clustersof width 2 (”001100”) one would observe hit patterns of 2 hits surrounding 2 channels(”010010”) with no hit. Example and principles are shown in figure 30. As itis important to have as many clusters of width greater than 1 as possible, it isfeasible by proper geometry, when electrons pass through the detector at an angleaccording to the z-axis (see figure 5) in the x-direction (across the strips). To checkthe bonding, histogram of readout digit-map patterns of types (see figure 30): ”101”,”1001”, ”10001”,. . . is drawn.

43

110 0000000 1 10 0000000

Signal Signal

De

tecto

r

Bond-pad

Strip

Fa

n-in

Readout patterns at channels

Readout electronics Readout electronics

Figure 30: The principle of checking for correct bonding. Correct bonding scheme andappropriate pattern is shown in the left part, while the center figure shows readout patternwhen the connections between strips and readout channels is shifted by one pad at the toprow. The right figure shows photography of the connection between detector and fan-inpads.

For the signal determination alternative method can be used. It is based on the deriva-tive of multiplicity by threshold. The result is the spectrum of deposited energy andthe Gaussian spectrum of the noise. The result of this method is the most probable de-posited energy instead of the median one calculated by the analysis above and measuredin beam tests. The most probable and median deposited energy are different due to asym-metry of the spectrum of energy loss (for example see the Landau distribution in figure 4).Because the derivative is approximated by difference of 2 close values of the neighbouringscan-points, high statistics and fine step of the threshold in the scan is needed. The re-sults of this method developed by CERN SCT group are consistent with the ones comingfrom the analysis described above. Due to not well defined relation between the medianand most probable energy loss and accounting the fact that the results of beam tests arethe medians, this method is not very suitable for the beam and source tests comparison.

6.4 Measurement results

The measurements were done in Prague and in CERN. In Prague 2 unirradiated for-ward modules were tested, but the tests were focused on the development of the DAQ andanalysis software, and so only single result of 1 unirradiated module is presented. Becauseof preparation of Endcap Module Final Design Review the emphasis was on tests of ir-radiated modules. Due to formal difficulties in transporting such modules from CERN,the measurements could not be done in Prague. In CERN foremost irradiated forwardmodules were tested. Several of them were tested in CERN SPS beam as well and socomparison of source and beam tests was possible. The overall results are summed in ta-ble 4, irradiated modules are marked by asterisk. For all the measurements my softwaredescribed in section 6.2 was used. The tests took place in August and in November and

44

December 2002. In both periods members of the Prague SCT group attended the mea-surements in CERN. I have taken part in the tests of K5-503∗ module and the first 7 testscans of K5-504∗ module described in the table. All the used beam tests results comefrom preliminary analysis of May and August beam tests that can be found in the SCTbeam tests WWW page [19].

Because of multiple scattering of electrons inside the detectors, all the presented re-sults, that were also used for comparison with the beam tests data, are the mediansfrom the detector plane closer to the radioactive source, where the incidence directionsof electrons’ momenta were well defined. So the compared data were not so strongly influ-enced by the multiple scattering as were the results from the further detector plane, wherethe impact directions were hardly defined due to multiple scattering in the plane nearerto the source. The detector plane further from the source has effectively the same functionas the aluminum plate or scintillator threshold: a minimum energy cut on the electronsthat passed through the whole module and additional material if present. The difficul-ties in results from the further plane can be well understood from the angular dependenceof medians and cluster sizes, when the multiple scattering leads to longer paths of the elec-trons in the detector and so higher medians, but the multiple scattering suppresses the me-dians due to charge sharing. This fact is reflected in the data by systematically highermeasured cluster sizes in the detector plane further from the source, while the mediansfrom these detector planes were in part of the measurements higher and in the rest lowerthan the ones measured in the nearer to source detector planes. The relation probablydepended on the used geometry, because the same relation of the 2 medians was ob-served for every group of consequent measurements. The differences between the resultsfrom both detector planes can be also seen on the position dependence of the clustersize (see figure 31), where a U-shape structure corresponding to the angular distributionof incident electrons is observable in the nearer plane and quite homogenous dependenceon the further one.

Important aim of the signal measurement is its dependence on the bias voltage1.Graphs of this measurements with irradiated K5-504∗, and unirradiated K5-303 moduleare shown in figure 32. It can be seen that even at the bias of 500 V, which is the limitthat ATLAS SCT power supplies can provide, further rise of the bias would probablylead to higher measured signal on the irradiated module, while on the unirradiated onethe almost plateau of this dependence is reached at 150 V. These facts are in agreementwith the beam tests 2001 results of the module prototypes. The measurement at 100 Von the K5-503∗ module resulted in significantly higher cluster size at 1 fC thresholdcompared to the configurations with higher bias voltage. This fact confirms charge sharingincrease due to diffusion with decreasing electric field in the detector (see figure 22).

The results of K5-503∗, K5-304∗ irradiated modules showed low sensitivity of the me-dians measured in the source tests setups on minor geometrical changes like the dis-tance of the source from the module, positioning of the source so that the electronspassed through the mechanical basement, or placement of 1 mm thick aluminum platebetween the module and the scintillator. The later change had the same effect as in-creasing the threshold on the pulse from scintillator: effective set of minimum energycut on the electrons that passed through the whole module. While the median hasn’tsignificantly changed, the cluster size especially on the further detector plane decreasedwith the minimum energy cut application. The cluster sizes on that plane of K5-503∗

module are in between 1.58 and 1.61 in the basic configuration and 1.35 when aluminumplate was used. This shows that the large cluster sizes are as expected caused by low

1The bias voltage presented in the tables was corrected to voltage drop caused by leakage currenton serial resistor of 11 kΩ

45

Module Bias Chips Median Noise Cluster Comment[V] [fC] occupancy size

Prague August:

K4-203 100 S0,S1,S2 3.3±0.3 (2.3±0.1)·10−3 1.27±0.04

CERN August:

K5-303 100 S2,S3 2.56±0.07 (3.8±0.5)·10−4 1.53±0.05

K5-303 150 S2,S3 3.51±0.06 (5.7±0.3)·10−4 1.38±0.02

K5-303 200 S2,S3 3.56±0.13 (3.7±0.4)·10−4 1.39±0.04

K5-303 249 S2,S3 3.60±0.08 (4.8±0.5)·10−4 1.39±0.04

K5-303 300 S2,S3 3.54±0.14 (3.4±0.4)·10−4 1.38±0.04

K5-303 350 S2,S3 3.60±0.07 (3.4±0.5)·10−4 1.39±0.04

K5-310∗ 488 S2,S3 2.31±0.07 (3.2±0.4)·10−4 1.22±0.04

K5-308∗ 312 S2,S3 2.28±0.08 (1.8±0.5)·10−2 1.27±0.04

K5-308∗ 304 S2,S3 2.35±0.07 (1.1±0.5)·10−2 1.27±0.04

K5-308∗ 409 S2,S3 2.59±0.07 (1.4±0.5)·10−2 1.30±0.04

K5-308∗ 489 S2,S3 2.59±0.07 (9.6±0.4)·10−3 1.28±0.04

K5-308∗ 600 S2,S3 2.70±0.08 (1.6±0.5)·10−2 1.37±0.04

K5-308∗ 471 S2,S3 2.32±0.10 (2.0±0.4)·10−3 1.25±0.04

K5-308∗ 478 S10,S11 2.55±0.14 (2.6±0.4)·10−3 1.20±0.04

K5-312∗ 341 S4,E5 2.62±0.10 (9.4±0.3)·10−4 1.37±0.04

K5-312∗ 415 S4,E5 2.79±0.18 (9.6±0.3)·10−4 1.37±0.04

K5-312∗ 490 S4,E5 2.85±0.09 - -

K5-312∗ 489 M0,S1 2.60±0.27 (8.3±0.3)·10−4 1.39±0.04

K5-312∗ 550 S4,E5 2.83±0.07 (8.3±0.4)·10−3 1.37±0.04

K5-312∗ 489 S4,E5 2.84±0.07 (7.4±0.3)·10−4 1.38±0.04

CERN November,December:

K5-503∗ 473 S3,S4 2.62±0.06 (4.3±0.1)·10−3 1.25±0.04 beam throughmechanical basement

K5-503∗ 471 E5 2.81±0.06 (3.7±0.1)·10−3 1.41±0.04

K5-503∗ 470 S4,E5 2.77±0.06 (3.2±0.1)·10−3 1.37±0.04 source shifted 3 cmfurther from module

K5-503∗ 471 E5 2.73±0.05 (2.7±0.1)·10−3 1.33±0.04 chip analog voltageincreased to 3.8 V

K5-503∗ 473 E5 2.95±0.11 (3.0±0.1)·10−3 1.44±0.04 edge detect typereadout mode

K5-503∗ 473 E5 2.85±0.08 (2.6±0.1)·10−3 1.27±0.05 1 mm Al plate betweenmodule and scintillator

K5-504∗ 481 S1 2.90±0.05 (7.1±0.1)·10−3 1.29±0.04

K5-504∗ 432 S1 2.84±0.05 (7.8±0.1)·10−3 1.31±0.04

K5-504∗ 383 S1 2.75±0.05 (8.0±0.1)·10−3 1.29±0.04

K5-504∗ 334 S1 2.52±0.07 (8.5±0.1)·10−3 1.32±0.04

K5-504∗ 334 S1 2.59±0.06 (8.0±0.1)·10−3 1.28±0.04

K5-504∗ 284 S1 2.30±0.08 (5.4±0.1)·10−3 1.26±0.04

K5-504∗ 480 S1 2.85±0.04 (5.3±0.1)·10−3 1.24±0.04 higher thresholdon the scintillator

K5-504∗ 480 S1 2.84±0.11 (2.9±0.1)·10−3 1.21±0.04 edge detect typecompression mode

K5-504∗ 480 S1 2.85±0.06 (2.9±0.1)·10−3 1.26±0.04 edge detect typereadout mode

K5-504∗ 478 M0,S1,S2 2.98±0.08 (5.0±0.1)·10−3 1.25±0.04 source shifted 3 cmfurther from module

KB-100∗ 468 S3 2.71±0.04 (3.8±0.1)·10−3 1.24±0.04

KB-100∗ 420 S3 2.54±0.02 (3.9±0.1)·10−3 1.23±0.04

KB-100∗ 372 S3 2.47±0.04 (3.3±0.1)·10−3 1.21±0.04

KB-100∗ 468 S3 2.73±0.03 (3.2±0.1)·10−3 1.28±0.04 lower thresholdon the scintillator

Table 4: The source tests results. The hit chips match the description in section 4.1.The noise occupancy and cluster size are taken at 1 fC threshold.

46

Channel number100 150 200 250 300 350 400 450 500

Clu

ster

siz

e

1

1.5

2

2.5

3

3.5

4

Channel number250 300 350 400 450 500 550 600 650

Clu

ster

siz

e

1

1.5

2

2.5

3

3.5

4

Channel number100 150 200 250 300 350 400 450 500

Nu

mb

er o

f h

its

20

40

60

80

100

120

140

160

Channel number250 300 350 400 450 500 550 600 650

Nu

mb

er o

f h

its

20

40

60

80

100

120

140

160

Figure 31: The position dependence of the cluster size measured with unirradiated moduleK5-303. The left figures correspond to the detector plane nearer to the source, the rightto the further plane. The top graphs show the cluster size dependence on the channelnumber (impact position). The bottom histograms show the number of hits over allthe channels. The blue histograms account clusters of non-single widths only.

Bias [V]0 100 200 300 400 500 600

Med

ian

[fC

]

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

Bias [V]0 100 200 300 400 500 600

Med

ian

[fC

]

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

Figure 32: The bias dependence of the median measured with unirradiated K5-303 module(open circles) and irradiated K5-504∗ module (full circles).

47

Module Test type Tested Bias [V] Median [fC] Noise occupancychips at 1 fC threshold

K5-310∗ Source S2,S3 488 2.31±0.07 3.2·10−4

August Beam S3 490 3.1 6.0·10−4

K5-308∗ Source S3,S4 480 2.59±0.12 9.6·10−3

Source S2,S3 471 2.32±0.10 2.0·10−3

May Beam S2,S3 478 2.7 2.7·10−3

August Beam S3 478 2.4 2.3·10−3

K5-308∗ Source S10,S11 478 2.55±0.14 2.6·10−3

May Beam S10,S11 478 3.1 5.8·10−3

August Beam S10 478 3.0 4.0·10−3

K5-312∗ Source M0,S1 489 2.6±0.3 8.3·10−4

Source S4,E5 490 2.85±0.11 -Source S4,E5 489 2.84±0.07 7.4·10−4

August Beam S2 490 3.2 4.0·10−4

K5-303 Source S2,S3 150 3.51±0.06 5.7·10−4

May Beam S2,S3 150 3.8 5.0·10−5

August Beam S3 150 3.9 1.4·10−4

Table 5: The comparison of source and beam tests medians. The hit chips match the de-scription in section 4.1 and are chosen from the detector plane nearer to the source.The beam tests results were measured in 2002 and all the source tests in August 2002.

energetic electrons, because, taking into account the shape of Bethe-Bloch curve, theirmean energy loss is higher and so are the angles of multiple scattering as can be concludedfrom formula (31).

The comparison to beam tests is shown in figure 33 and written in table 5. To checkthe stability and errors of beam tests results, there were used more beam tests datafor every module if available. Except measurements of K5-310∗ and beam tests resultsof K5-308∗ from August 2001, the ratios of medians from source and beam tests are similarover the modules. Excluding the 2 mentioned results, the ratio of medians from the beamtests to the source is about 1.109±0.070. The presented value was obtained as an av-erage weighted by inverse of the errors squared and the error is the standard deviationof the ratios from the average value. Due to multiple scattering, the cluster sizes mea-sured in source tests (between 1.2 and 1.6 at 1 fC threshold) are higher than the valuesmeasured in beam tests (around 1.06 at 1 fC threshold for both the irradiated and unir-radiated modules [10]). The comparison of measured noise occupancies of the modulesresults in good agreement in the orders. The values could hardly be in more precise agree-ment, because the noise occupancy is very sensitive to the used configuration especiallycorrect grounding, cable shielding, temperature, calibration,. . . This fact can be also seenon the differences between the pairs of beam tests results.

6.5 Source tests simulation

To fully understand the differences between the results of source and beam tests, sim-ulation of the source tests setup was performed. Because the multiple scattering is moreimportant for electrons than for 180 GeV pions passing through the detector, the stepin Geant4 simulation was lowered down to 20 µm, but the settings in the digitization wasused the same as for the simulation of the η-plots at incidence angle 16o: step 10 µm and

48

1 2 3 4 5

Med

ian

[fC

]

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4

1 2 3 4 5

Med

ian

[fC

]

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4

K5-310

S3

S2,S3

S2,S3

K5-308

S3

S10,S11

S3,S4

K5-308

S10

S2,S3

S2,S3

K5-312

S2

S10,S11

K5-303

S3

M0,S1

S4,E5

S2,S3

Figure 33: The comparison of the beam and source tests medians.

single drifting charge carrier per track segment. 3 setups were simulated: with and with-out the 1 mm thick aluminum plate and with source shifted 3 cm further from the mod-ule. The position of the scintillator, module and source in the basic geometry followedthe real geometry of the measurements: both the distances of the scintillator and sourcefrom the module were approximately 2 cm. In spite of the fact that forward modules weretested, the simulation was performed using barrel module description. The only differencein the digitization software is the strip pitch. Because both the beam and source testsresults showed low sensitivity of the medians on the strip pitch and simulation itself is ableto describe trends of measured dependencies only, there should be no critical influenceon the results of using the barrel module layout instead of the forward one.

The results are summed in table 6. The simulation confirmed the difference of mediansbetween beam and source tests, when the simulation gave the ratio of beam tests to sourcetests medians 1.117±0.020. The value is within the errors in agreement with the measuredratio: 1.109±0.070. The shapes of the efficiency and cluster size versus threshold curves(see figures 34,35) are in good agreement as well. The simulation also confirmed highercluster sizes, measured during source tests, compared to the beam tests results and highercluster sizes at the plane further from the radioactive source compared to the nearer one.The absolute values of cluster sizes and medians were not confronted with the simulationbecause the simulation was not able to reproduce absolute values of beam tests.

Because the simulation software provides the simulated data in the same format asthe real data are stored, it was possible to validate the efficiency, cluster size and noise oc-cupancy calculation methods described in section 6.3. All differences were within the cal-culated errors, but systematical: the cluster sizes from the digitization are approximately3 % lower compared to the calculation of the analysis software using the map of hits,while the efficiency is higher about the same systematical difference.

The trends of the characteristics across the 3 simulated geometries are in agreementwith the measurements:

• Minor change of the median on the nearer detector plane, while the median on planefurther from the source is more sensitive.

• Lower cluster size, especially on the further plane, in the setup with aluminum plate.

49

Bias [V] Plane Median [fC] Noise Cluster Commentoccupancy size

Source tests:200 nearer 2.79±0.08 (1.39±0.18)·10−4 1.38±0.03

further 2.89±0.08 (2.56±0.24)·10−4 1.52±0.03

200 nearer 2.77±0.06 (1.61±0.17)·10−4 1.39±0.02 source shiftedfurther 2.82±0.07 (2.84±0.23)·10−4 1.66±0.03 3 cm away

200 nearer 2.68±0.07 (8.70±0.13)·10−4 1.30±0.02 aluminum platefurther 2.62±0.07 (1.13±0.15)·10−4 1.36±0.03

Beam test:200 nearer 3.07±0.02 (1.07±0.06)·10−4 1.056±0.003

further 3.05±0.02 (1.36±0.06)·10−4 1.074±0.004

Table 6: The source and beam tests simulation results. The noise occupancy and clustersize are taken at 1 fC threshold. The presented errors of beam tests simulation correspondto difference between the first and second detector plane.

Threshold [fC]0 1 2 3 4 5 6 7

Eff

icie

ncy

[%

]

0

20

40

60

80

100

Threshold [fC]0 1 2 3 4 5 6 7

Eff

icie

ncy

[%

]

0

20

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60

80

100

Threshold [fC]0 1 2 3 4 5 6 7

Eff

icie

ncy

[%

]

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20

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60

80

100

Threshold [fC]0 1 2 3 4 5 6 7

Eff

icie

ncy

[%

]

0

20

40

60

80

100

Figure 34: The efficiency dependence on the threshold for both the source (circles) andbeam tests (triangles). Simulation in the basic geometry is shown in the left figure, sourcemeasurement of K5-503∗ module and test beam of K5-312∗ module in the right.

Threshold [fC]0 1 2 3 4 5 6 7

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

Threshold [fC]0 1 2 3 4 5 6 7

Clu

ster

siz

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1.1

1.2

1.3

1.4

1.5

1.6

Threshold [fC]0 1 2 3 4 5 6 7

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

Threshold [fC]0 1 2 3 4 5 6 7

Clu

ster

siz

e

1

1.1

1.2

1.3

1.4

1.5

1.6

Figure 35: The cluster size dependence on the threshold for both the source (circles)and beam tests (circles) and for both the nearer (open marks) and further (full marks)detector plane. Simulation in the basic geometry is shown in the left figure and sourcemeasurement of K5-503∗ module in the right.

50

7 Conclusion

A method using radioactive β− source for detection capability check of ATLAS siliconmicrostrip detectors has been developed. This method provides relatively fast measure-ment of the electrons median energy deposit in 285 µm thick silicon wafers. Comparedto the beam tests, the advantage of source tests is the possibility to build the setup at anyworking place equipped with standard tools needed for QA tests of the SCT modules. Dueto the simplicity of the source tests setup, module response in various geometrical config-urations, like set of incidence angles, can be studied. Great advantage of the source testsis their availability during the whole year without complicated preparation compared tothe beam tests, that take place few times a year. The easy reproduction of electrical andgeometrical setting makes the source tests applicable for repeated tests of modules for ex-ample before and after irradiation or any other intervention to the module functionality.But the simplicity of the setup also leads to the main disadvantages of the source tests.Firstly compared to the beam tests, the track positions of the passing electrons are knownonly with the precision and limitations (depending on the set threshold etc.) of the testedmodule. Secondly, as was described in chapter 6.3, the efficiency calculation methoddoesn’t provide correct values at low thresholds, where the requirements on module ef-ficiency and noise occupancy are defined. And so it is needed to judge on the efficiencyfrom the measured median signal as there must be fixed relation between these 2 char-acteristics. The next disadvantages are connected with the used source: the particlespassing through the detector are not monoenergetic, their initial tracks are not paralleland due to low energy and mass of the electrons, multiple scattering has more significantinfluence compared to the beam tests, which was observed in the source tests data, andconfirmed by simulation, as wider signal clusters.

Analysis and data acquisition software for the source tests was written by myself.Based on the DAQ software used for standard module tests, it assures necessary mea-surement features and calculation and visualization of basic tested module characteristicsas the efficiency, noise occupancy, width of hit clusters, etc. Development of the soft-ware provided me great experience in using ROOT [17] tools for the analysis and DAQpurposes.

Simulation of the module response in both the beam and source tests was performed.The main aim was to describe the different results from the 2 setups. In spite of the factthat the simulation involves both the detector and readout electronics functionality, it wasnot possible to reach agreement with the beam tests data on the level of absolute valuesof measured signal’s height and width. But trends of angular, bias, etc. dependenciesof the resulting signal from the simulated readout electronics follow the data trends. Itried to find mechanism causing the discrepancies firstly by simplifying the model to puregeometry and than in the opposite by involving in the current induction on the stripsfrom the drifting free carriers. But the results stayed close to what the original modelpredicted.

To verify the usability of source tests, it was needed to find the relation of their resultsto the beam tests ones. Having taken into account the difficulties of the simulation soft-ware, I compared the ratio of the simulated median signals from the 2 tests to the ratioobtained from the real data analysis. Both the measurements and simulations resultedin higher signal in the beam tests compared to the source ones, and the measured andsimulated average ratios 1.109±0.070 and 1.117±0.020 are in agreement within the writ-ten standard deviations. The simulation also confirmed lower sensitivity to geometricalsettings of the signal measured at detector plane nearer to the source compared to the fur-ther one, and so to get stable results, it is suggested to test every detector plane separately

51

by proper placement of the source. The desired result is to have defined relation betweenthe source and beam measurements, but the presented values show quite large spreadof the beam-to-source tests ratios of signals. There are several effects that cause devia-tions of the measurement results. Firstly, the calibration process, that is made separatelyfor every chip, has finite precision, that was observed on the beam tests results as differentmeasured signals depending on from which chip were the data analyzed. These standarddeviations are around 0.2 fC [19] and are reflected in the errors of ratios by approximately0.026. Secondly, the sensitivity of the median signal, measured is source tests on the de-tector plane nearer to the source, is lower than in the further-from-source plane, but somedependence within 0.1 fC is still there and is reflected in the error of beam to source testssignals ratio by around 0.013. And at last contribution of efficiency calculation statisticalerrors has to be considered.

The discussion above shows that source tests are able to provide median signal mea-surement in ATLAS SCT detectors almost as precisely as the beam tests. Some of the pre-sented source tests results were involved in the Endcap Module Final Design Review(CERN August 2002). The software used for the source tests became a part of the stan-dard DAQ package used for QA module tests.

52

References

[1] ATLAS Collaboration (1994): ATLAS Technical Proposal for a General-Purpose ppExperiment at the Large Hadron Collider at CERN, CERN, Geneva

[2] ATLAS Inner Detector Community (1997): ATLAS Inner Detector Technical DesignReport, Volume I, CERN, Geneva

[3] Kittel C. (1996): Introduction to Solid State Physics, John Wiley and Sons, NewYork

[4] Lutz G. (1999): Semiconductor Radiation Detectors, Springer, Berlin

[5] Sauli F. (1993): Instrumentation In High Energy Physics, World Scientific PublishingCo. Pte .Ltd., Singapore

[6] CERN Program Library documentation of Geant (detector description and simula-tion tool)http://wwwinfo.cern.ch/asdoc/geantold/H2GEANTPHYS430.html

cited 15.04.2003

[7] Wagner W. et. al. (1998): Characterization of Silicon Microstrip Detectors Usingan Infrared Laser System, Werner-Heisenberg-Institut, MPI-PhE/98-13

[8] Hagiwara K. el. al. (2002): Particle Physics Booklet, CERN, Geneva

[9] Knoll G. F. (1979): Radiation detection and measurement, John Wiley and sons,New York

[10] Barr A. J. et. al. (2001): Beamtests of ATLAS SCT Modules in August and October2001, ATL-INDET-2002-024, CERN, Geneva

[11] Documentation of ATLAS SCT readout electronics: ABCD3T chip specification ver-sion V1.2, 24/7/2000http://scipp.ucsc.edu/groups/atlas/elect-doc/abcd3t_spec.pdf

cited 15.04.2003

[12] Documentation of ATLAS SCT Low Voltage VME module providing power supplyto the readout electronicshttp://www-hep2.fzu.cz/Atlas/WorkingGroups/Projects/MSGC.html

cited 15.04.2003

[13] Documentation of ATLAS SCT High Voltage VME module providing bias voltageon detectorshttp://chall.ifj.edu.pl/~atlas/SCTHV/SCTHV_PAJ.html

cited 15.04.2003

[14] Documentation of Multichannel Semiconductor Tracker ABC(D) Readout VMEmodule used in ATLAS SCT module testshttp://hepwww.rl.ac.uk/atlas-sct-mm/Mustard/

cited 15.04.2003

[15] Documentation of Clock and Control VME module used in ATLAS SCT moduletestshttp://www.hep.ucl.ac.uk/atlas/sct/#CLOAC

cited 15.04.2003

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[16] Documentation of Slow Command Generator VME module used in ATLAS SCTmodule testshttp://hepwww.rl.ac.uk/atlas-sct-mm/Slog/

cited 15.04.2003

[17] Home page of an object-oriented data analysis framework ROOThttp://root.cern.ch

cited 15.04.2003

[18] Description of the ATLAS SCT module tests data acquisition systemhttp://atlas.web.cern.ch/Atlas/GROUPS/INNER_DETECTOR/SCT/testdaq/

cited 15.04.2003

[19] Home page of the ATLAS SCT beam testshttp://atlas.web.cern.ch/Atlas/GROUPS/INNER_DETECTOR/SCT/testbeam/

cited 15.04.2003

[20] Horejsi J. (2003): Fundamentals of Electroweak Theory, Karolinum, Praha

[21] Description of β− spectroscopy experimenthttp://www.ph.unimelb.edu.au/studentresources/undergraduate/

part2labs/pracs/beta_v2.doc

cited 15.04.2003

[22] Home page of a detector description and simulation tool Geant4http://geant4.web.cern.ch/geant4

cited 15.04.2003

[23] Home page of the ATLAS SCT simulationhttp://atlas.web.cern.ch/Atlas/GROUPS/INNER_DETECTOR/SCT/simulation/

cited 15.04.2003

[24] Documentation of the ATLAS SCT beam test digitization software written by Szy-mon Gadomskihttp://gadomski.home.cern.ch/gadomski/SCT_Digitization.ps

cited 15.04.2003

[25] Gadomski S. (2001): Model of the SCT detectors and electronics for the ATLASsimulation using Geant4, ATL-SOFT-2001-005, CERN, Geneva

[26] Broklova Z. (2003): Vyhodnocenı ucinnosti a kvality polovodicovych stripovych de-tektoru pro detektor ATLAS (LHC CERN), Diploma thesis at Faculty of Mathemat-ics and Physics at Charles University, Prague

[27] Eklund L. et. al. (2003): Electrical Tests of SCT Hybrids and Modules,ATL-COM-INDET-2003-004, CERN, Geneva

[28] Home page of Prague SCT group at Institute of Particle and Nuclear Physicsat Charles University in Praguehttp://www-ucjf.troja.mf.cuni.cz/~sct

cited 15.04.2003

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