Gaseous and solid state tracking Scintillators, photo ...

CERN Academic Training 97/98Particle Detectors

Christian Joram I/1

Introduction

Introduction

Gaseous and solid state trackingdevices

Scintillators, photo detectors, scint.fibers, nuclear emulsions, ‘electronicbubble chamber’

Calorimetry

Particle identification

Many topics will remain uncovered...

Design of detector systems Electronics, trigger and data acquisition Non-particle physics (medical, biological etc.)

applications Detector construction Detector operation (monitoring, calibration,

alignment)


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Introduction

Literature on particle detectorsLiterature on particle detectors

Text books C. Grupen, Particle Detectors, Cambridge

University Press, 1996 W. R. Leo, Techniques for Nuclear and Particle

Physics Experiments, 2nd edition, Springer, 1994 R.S. Gilmore, Single particle detection and

measurement, Taylor&Francis, 1992 W. Blum, L. Rolandi, Particle Detection with Drift

Chambers, Springer, 1994 K. Kleinknecht, Detektoren für Teilchenstrahlung,

3rd edition, Teubner, 1992

Review articles Experimental techniques in high energy physics,

T. Ferbel (editor), World Scientific, 1991. Instrumentation in High Energy Physics, F. Sauli

(editor), World Scientific, 1992. Many excellent articles can be found in Ann. Rev.

Nucl. Part. Sci.

Other sources Particle Data Book (Phys. Rev. D, Vol. 54, 1996) R. Bock, A. Vasilescu, Particle Data Briefbook

http://www.cern.ch/Physics/ParticleDetector/BriefBook/

Proceedings of detector conferences (ViennaWCC, Elba, IEEE)


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Introduction

A W+W- decay in DELPHI

e+e- (√s=161.2 GeV) → W+W- → qqqq → 4 hadronic jets_ _

≈ 10

m

≈ 10 m


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Introduction

A W+W- decay in ALEPH

e+e- (√s=181 GeV) → W+W- → qqµνµ → 2 hadronic jets + µ + missing momentum

_


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Introduction

Reconstructed B-mesons in theDELPHI micro vertex detector

τB ≈ 1.6 ps l = cτγ ≈ 500 µm⋅γ

Primary Vertex

Primary Vertex


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Introduction

Particle identification methods


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Introduction

≈ 13

m


Christian Joram I/8

Introduction

A simulated event in ATLAS (CMS)H → 4µ

pp collision at √s = 14 TeVσinel. ≈ 70 mbInterested in processes

with σ ≈ 10−100 fb

≈ 23 overlapping minimum bias events / BC≈ 1900 charged + 1600 neutral particles / BC

L = 1034 cm-2 s-1, bunch

spacing 25 ns


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Introduction

The ‘ideal’ particle detector for highenergy physics experiments

High energy collisions ( )

→ production of a multitude of particles (charged,neutral, photons)

The ‘ideal’ detector should provide….

coverage of full solid angle (no cracks, finesegmentation

detect, track and identify all particles (mass, charge)

measurement of momentum and/or energy fast response, no dead time

practical limitations (technology, space, budget)

Particles are detected via their interaction withmatter.

Many different physical principles are involved(mainly of electromagnetic nature).

Eventually we will observe...

ionizationionization and excitationexcitation of matter.

pppp,ep,,ee


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Charge Coupled Devices (CCD)

Momentum measurement

Multiple scattering

Bethe-Bloch formula / Landau tails

Ionization of gases

Wire chambers

Some derivatives of wire chambers

Drift and diffusion in gases

Drift chambers

Micro gas detectors

Chamber aging

Gas detector simulation

Silicon drift detectors

Radiation hardness

Silicon detectorsstrips/pixels

?

Introduction





the sagitta s is determined by 3 measurements witherror σ(x):

for N equidistant measurements, one obtains(R.L. Gluckstern, NIM 24 (1963) 381)

ex: pT=1GeV/c, L=1m, B=1T, σ(x)=200µm, N=10

L

s

B

y

x

T

TT

T

T

T

p

BLs

BLpp

p

BLL

Bp

qBp

22

83.0

82cos1

3.0sin

3.022sin

2

m)(T3.0)cGeV(

2

23

23.

3.0

8)()()(

BL

px

s

x

s

s

p

p Tmeas

T

T

)4/(720

3.0

)(2

.

NBL

px

p

p Tmeas

T

T (for N ≥ ≈10)

%5.0

.

meas

T

T

p

p

231

2xx

xs

(s ≈ 3.75 cm)



Multiple Scattering

Scattering

An incoming particle with charge z interacts with atarget of nuclear charge Z. The cross-section for thise.m. process is

Average scattering angle Cross-section for infnite !

Multiple ScatteringSufficiently thick material layer → the particle willundergo multiple scattering.

2sin

14

4

22

p

cmzZr

d

d ee

00

Rutherford formula

d/d

L

plane

rplane

RMSRMSplaneplane space

2

120



Multiple Scattering

Approximation

X0 is radiation length of the medium(accuracy ≤ 11% for 10-3 < L/X0 < 100)

The lateral displacement can also be approximated bya Gaussian distribution with width

000 ln038.01

6.13X

L

X

Lz

cp

MeV

P

plane0

Gaussian

sin-4(/2)

20

2

0 2exp

2

1

planeplaneP

0 LLr RMSplane

RMSplane




Back to momentum measurements:contribution from multiple scattering

ex: Ar (X0=110m), L=1m, B=1T

0

0

0

1045.0

3.0

0136.0)(

10136.0sin

LXBBL

X

L

p

p

p

p

X

L

pppp

T

MSMS

T

planeRMS

MS

(p)/p

(p)/p

(p)/p

p

MS

meas.total error

independentof p !

%5.0)(

MS

Tp

p



Momentum Measurement

Momentum measurement in experiments withsolenoid magnet:

polar angle has to be determined from a straight linefit x=x(z).

N equidistant points with error σ(z)

+ multiple scattering contribution….

In practical cases:

In summary:

B

B x

zy

x

yz

sinppT

NBL

px

p

pmeas

1)(2

.

))1(/()1(12)(. NNN

L

zmeas

T

T

p

p

p

p

normally small



Interaction of charged particles

Detection of charged particlesHow do they loose energy in matter ?

Discrete collisions with the atomic electrons of theabsorber material.

Collisions with nuclei not important (me<<mN).

If are big enough ionization.

A photon in a medium has to follow the dispersionrelation

For soft collisions + energy and momentumconservation → real photons:

222

2 022 nck

n

ck

nc

11

cos

cos

nvk

kvkv

density electron :N

ddE

dNE

dx

dE 0

e-

k ,

0,mv

→ Cherenkov photonsif n1

k ,



Interaction of charged particles

Average differential energy loss

Many different approaches (classical, semi-classical) exist...

e.g. Allison+Cobb (Ann.Rev. of Nucl. and Part. Phys, 30 (1988), 253),Photo Absorption Ionization (PAI) modelincludes Cherenkov + transition radiation

PAI model relates dσ/dE to empirical atomicphotoabsorption cross-sections σγ(E)

Ionisation only → Bethe - Bloch formula

1

optical absorptive X-ray

Cherenkovradiation

ionisation transition radiation

regime:

effect:

Re

Im

dxdE

schematically !



Bethe-Bloch formula

dE/dx in [MeV g-1 cm2]

dE/dx depends only on β, independent of m Formula takes into account energy transfers

Bethe-Bloch formula only valid for “heavy”particles (m≥mµ).

Electrons and positrons need specialtreatment (mproj=mtarget), in additionBremsstrahlung!

2

2ln

14 2max

2

222

21

2222

TI

cm

A

ZzcmrN

dx

dE eeeA

eV10 with

potential excitationmean :

00

max

IZII

ITdEI

2121 cm MeV g..dxdE

Z/A does notdiffer muchfor the variouselements,except forHydrogen!

(rough approximation, Ifitted for each element)



solid line: Allison and Cobb, 1980dashed line: Sternheimer (1954)data from 1978 (Lehraus et al.)

Bethe-Bloch formula

dE/dx first falls ∝ 1/β2 (more precise β-5/3),kinematic factor

then minimum at βγ ≈ 4 (minimum ionizing particles,MIP)

then again rising due to ln γ2 term, relativistic rise,attributed to relativistic expansion of transverse E-

field → contributions from more distant collisions.

relativistic rise cancelled at high γ by “densityeffect”, polarization of medium screens more distant

atoms. Parameterized by δ (material dependent) →Fermi plateau

many other small corrections

Measured andcalculated dE/dx

2

2ln

14 2max

2

222

21

2222

TI

cm

A

ZzcmrN

dx

dE eeeA



Landau tails

Real detectors (limited granularity) do not measure<dE/dx>, but the energy ∆E deposited in a layer offinite thickness δx.∆E is the result of a certain number i of collisions withenergy transfers Ei, with cross-section dσ/dE

Problem: Relate F(∆E,δx) to dσ/dESolutions: Williams(1929), Landau(1944),

Allison and Cobb(1980) and others.

ionization by close collisions

-electrons

exci

tati

on

ion

izat

ion

by

dis

tan

t co

llisi

on

bel

ow

exc

itat

ion

th

resh

old

energy transfer/photon exchange

rel.

pro

bab

ility

Schematically !(after Gilmore)

Monte-Carlo approach:1.) divide absorber layer into thin slices. Probability

for any energy transfer in the slice is low.2.) Choose energy transfer randomly from above

distribution.3.) Sum over all slices.



Landau tails

For thin layers (and low density materials):→ Few collisions, some with high energy transfer.→ Energy loss distributions show large fluctuations

towards high losses: ”Landau tails”

data: Harr is et al. (1977)dotted curve: Landau(1944)dashed curve: Maccabee &

Papwor th(1969)(Landau’s method)

solid curve: Allison and Cobb

Argon + 7% CH4

Argon + 7% CH4

For thick layers and high density materials:→ Many collisions.→ Central Limit Theorem → Gaussian shape

distributions.



Ionization of gases

Primary and total ionization

Fast charged particles ionize the atoms of a gas.

Often the resulting primary electron will have enoughkinetic energy to ionize other atoms.

total number of createdelectron-ion pairs.∆E = total energy lossWi = effective <energy loss>/pairprimarytotal

iitotal

nn

W

xdxdE

W

En

43

Number ofprimaryelectron/ion pairsin frequentlyused (detector)gases.

(Lohse and Witzeling,Instrumentation In HighEnergy Physics, WorldScientific,1992)

41

37

36

3126

28

33

22

Wi (eV)

Primary ionization Total ionisation



Ionization of gases

2 remarks:

1. The actual number of primary electron-ion pairs isPoisson distributed

The detection efficiency is therefore limited to

For thin layers εdet can then be significantly lowerthan 1.

Example Ar: d = 1mm → nprimary = 2.5. → εdet = 0.92

2. The total number of electron-ion pairs is subject to

large, non-poissonian, fluctuations (Landau tails).

Detectors which are able to identify primary

clusters, may provide a better energy resolution.

→ cluster counting (see lecture Particle ID).

!)(

m

enmP

nm

neP 1)0(1det



Proportional Counter

≈ 100 electron-ion pairs are not easy to detect!Noise of amplifier ≈1000 e- (ENC) !We need to increase the number of e-ion pairs.

Gas amplificationConsider cylindrical field geometry (simplest case):

Electrons drift towards the anode wire (≈ stop and go!More details in next lecture!).Close to the anode wire the field is sufficiently high(some kV/cm), so that e- gain enough energy for furtherionization → exponential increase of number of e--ionpairs.

a

b

r

E

1/r

a

cathode

anode

gas

Ethreshold

a

rCVrV

r

CVrE

ln2

)(

12

0

0

0

0

C = capacitance / unit length




0CVkeM

xrxE ennenn 00 or α: First Townsend coefficient

(e--ion pairs/cm)

(O. Allkofer, Spark chambers, Theimig München, 1969)

1 λ: mean free path

kNFor low e- energies ε

C

r

a

drrn

nM exp

0Gain

(F. Sauli, CERN 77-09)




Choice of gas:

Dense noble gases. Energy dissipation mainly byionization! High specific ionization.

De-excitation of noble gases only possible viaemission of photons, e.g. 11.6 eV for Argon.

This is above ionization threshold of metals, e.g.Copper 7.7 eV.

Ar *11.6 eV

Cu

e-

cathode

→ new avalanches →permanent discharges !

(MAGBOLTZ)




Solution: Add poly-atomic gases as quenchers.Absorption of photons in a large energy range (manyvibrational and rotational energy levels).

E

r

Energy dissipationby collisions ordissociation intosmaller molecules.

Methane: absorption band 7.9 - 14.5 eV

(MAGBOLTZ)



Signalformation

Avalanche formation within a few wire radii and withint < 1 ns!

Signal induction both on anode and cathode due tomoving charges (both electrons and ions).


drdr

dV

lCV

Qdv

0

Electrons collected by anode wire, i.e. dr issmall (few µm). Electrons contribute only verylittle to detected signal (few %).

Ions have to driftback to cathode,i.e. dr is big.Signal durationlimited by total iondrift time !

Need electronic signal differentiation to limit dead time.





Operation modes: ionization mode: full charge collection, but no

charge multiplication.

Proportional mode: above threshold voltagemultiplication starts. Detected signal proportionalto original ionization → energy measurement(dE/dx). Secondary avalanches have to bequenched. Gain 104 - 105.

Limited Proportional → Saturated → Streamermode: Strong photo-emission. Secondaryavalanches, merging with original avalanche.Requires strong quenchers or pulsed HV. Highgain (1010), large

.

Operation modes of chambers

Geiger mode:Massive photoemission. Fulllength of anodewire affected. Stopdischarge bycutting down HV.Strong quenchersneeded as well.

signals → simpleelectronics.



Multi wire proportional chambers (MWPC)(G. Charpak et al. 1968, Nobel prize 1992)

Capacitive coupling of non-screened parallel wires?Negative signals on all wires? Compensated bypositive signal induction from ion avalanche.

Typical parameters: L=5mm,d=1mm,awire=20µm.

Address of fired wire(s) give only 1-dimensionalinformation. Secondary coordinate ….

Multi wire proportional chambers

field lines and equipotentials around anode wires

Normally digital readout:spatial resolution limited to

12

dx

(d=1mm, σx=300 µµm)



Secondary coordinate

Charge division. Resistive wires (Carbon,2kΩ/m).

Timing difference (DELPHI Outer detector, OPAL vertex detector)

1 wire plane + 2 segmented cathode planes

Multi wire proportional chambers

y

L

QBQA

track

%4.0 toup

L

y

QQ

Q

L

y

BA

B

Crossed wire planes. Ghost hits.Restricted to low multiplicities. Alsostereo planes (crossing under small

angle).

yL

track

CFD CFDT

)(OPAL4)(

100)(

cmy

psT

Analog readout ofcathode planes.→ σ ≈ 100 µm



Some ‘derivatives’

Multistep avalanche chambers

Derivatives of proportional chambers

amplification 1 G=103

amplification 2G=103

conversion

transfer

low field

high field

high field

low field

wire meshes

≥ 2 parallel plane avalanche chambers (PPAC),separated by transfer fields, operated at low gain.

Achieve high total gain (106-108) by reducing feedbackphoton problem.

Electron transparency of wire meshes depends on fieldratio Eout/Ein. (Eout/Ein = 0.1→ T ≈ 0.1)

Charge amplification by Penning process at rel. lowfield:

Application: Single photo electron detection.

A* + C → A + C+ + e-



Thin gap chambers (TGC)


G10 (support)

cathode pads ground plane

graphite

3.2

mm

2 mm

4kV50 m

Operation in saturated mode. Signal amplitudelimited by by the resistivity of the graphite layer(≈ 40kΩ/).

Fast (2 ns risetime), large signals (gain 106), robust

Application: OPAL pole tip hadron calorimeter.G. Mikenberg, NIM A 265 (1988) 223

ATLAS muon endcap trigger, Y.Arai et al. NIM A 367 (1995) 398

Gas:CO2/n-pentane

(≈ 50/50)




Resistive plate chambers (RPC)2

mm bakelite

(melamine phenolic laminate)

pickup strips

10 kV

spacer

Gas: C2F4H2, (C2F5H) + few % isobutane(ATLAS, A. Di Ciaccio, NIM A 384 (1996) 222)

Time dispersion ≈ 1..2 ns → suited as trigger chamberRate capability ≈ 1 kHz / cm2

15 kV

Double andmultigapgeometries →improve timingand efficiency

Problem: Operation close to streamer mode.

Gaseous and solid state tracking Scintillators, photo ...

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