880.P20 Winter 2006 Richard Kass 1 Scintillation Devices As a charged particle traverses a medium it...

880.P20 Winter 2006 Richard Kass 1

Scintillation Devices

As a charged particle traverses a medium it excites the atoms (or molecules)in the the medium. In certain materials called scintillators a small fraction ofthe energy released when the atoms or molecules de-excite goes into light. ENERGY IN LIGHT OUT

The use of materials that scintillate is one of the most common experimentaltechniques in physics.

Used by Rutherford in his scattering experiments

Scintillation light can be used to: Signal the presence of a charged particle Measure the time it takes for a charged particle to travel a known distance

(“time of flight technique”) Measure energy since the amount of light is proportional to energy deposition

There are lots of different types of materials that scintillate:non-organic crystals (NaI, CsI, BGO)organic crystals (Anthracene)Organic plastics (see table on next page)Organic liquids (toluene, xylene)Our atmosphere (nitrogen)


Scintillators

A typical plasticScintillator system

violet blue

Emission spectrum of NE102APlastic scintillator

Properties of common plastic scintillators

Typical cost 1$/in2


Photomultiplier Tubes

We need a way to convert the scintillation photons into an electrical signal.Photons photoelectric effect electrons

Use a photomultiplier tube to convert scintillation light into electrical currentProperties of phototubes: very high gain, low noise current amplifier

gains 106 possiblepossible to count single photons

Off the shelf item, buy from a companywide variety to choose from (size, gain, sensitivity)tube costs range from $102-$103

Sensitive to magnetic fields (shield against earth’s): use “mu-metal”

In situations where a lot of lightis produced (>103 photons) aphotodiode can be used in place of a phototube, e.g. BaBar’s calorimeter

Quantum efficiency of bialkali cathode vs wavelength

violet blue green

Electric field accelerates electronsElectrons crash into dynodes create more electrons

light

e’s


Scintillation Counter ExampleSome typical parameters for a plastic scintillation counter are: energy loss in plastic scintillator: 2MeV/cm scintillation efficiency of plastic: 1 photon/100 eV collection efficiency (# photons reaching PMT): 0.1 quantum efficiency of PMT 0.25

What size electrical signal can we get from a plastic scintillator 1 cm thick? A charged particle passing perpendicular through this counter:

deposits 2MeV which produces 2x104’s of which 2x103’s reach PMT which produce 500 photo-electrons

Assume the PMT and related electronics have the following properties:PMT gain=106 so 500 photo-electrons produces 5x108 electrons =8x10-11CAssume charge is collected in 50nsec (5x10-8s)

current=dq/dt=(8x10-11 coulombs)/(5x10-8s)=1.6x10-3AAssume this current goes through a 50 resistor

V=IR=(50 )(1.6x10-3A)=80mV (big enough to see with O’scope)

So a minimum ionizing particle produces an 80mV signal.What is the efficiency of the counter? How often do we get no signal (zero PE’s)?The prob. of getting n PE’s when on average expect <n> is a Poisson process:

!)(

n

ennP

nn

The prob. of getting 0 photons is e-<n> =e-500 0. So this counter is 100% efficient.Note: a counter that is 90% efficient has <n>=2.3 PE’s


Time of flight with ScintillatorsTime of Flight (TOF) is a particle identification technique. measure particle speed and momentum determine mass

t=x/v=x/(c) with =pc/E=pc/[(mc2)2+(pc)2]1/2

pc

pmcxt

2/122 ))((

Consider two particles with different masses but same momentum:

2

22

21

2

2

222

2

2

221

222

21

)(

)(

))((

)(

))((

p

mmx

pc

pcmx

pc

pcmxtt

))(( 212122

21 tttttt

221

22

21

2

21 )(

)(

ptt

mmxtt

For high momentum (e.g. p>1 GeV/c for ’s):t1+t2=2t and x/tc

psec/meter)(

16672

)(2

22

21

2

22

21

21 p

mmx

cp

mmxtt

Actually, we measurethe time it takes for theparticle to travel a known distance.

x


Time of Flight with Scintillators

psec/meter)(

16672

)(2

22

21

2

22

21

21 p

mmx

cp

mmxttt

As an example, assume m1=m (140MeV) , m2=mk (494MeV), and x=10mt=3.8 nsec for p=1 GeVt=0.95 nsec for p=2GeV

Time resolution of a “good” TOF system is 150ps (0.15 ns)

Scintillator+phototubes are capable of measuring such small time differences

In colliding beam experiments, 0.5 <x< 1 m small x puts a limit of t.

x =1 meter

For x=1 m, p=1 GeV K/ separation t psec < 3 separation

1.4 GeV/c ’s and K’s

No pulse height correction

with pulse height correction


Photoelectric Effect absorbed by material, electron ejected

Compton Scatteringe-→e- “elastic scattering”

Pair Production→e+e- creates anti-matter

Basic Physics Processes in a Sodium Iodide (NaI) Calorimeter

e- e- e-

e+

hv < 0.05 MeV 0.05 < hv < 10 MeV hv > 10 MeV -ray must have E>2me

The amount of light given off by NaI is proportional to the amount energy absorbed. The light yield is ~ 1 photon per 25 eV deposited in NaI, max=415 nm, decay time ~250nsec

NaI

radiation length of NaI ~2.5 cmbut only useful for E > few MeV

NaI is often used to measure the energy low gamma rays

Attenuation of the gamma rays is energy dependent


NaI & Homeland Security


Example: Cs137 -ray Spectrum in NaI

E=662keVphotopeak

1800 backscatterE=184keV

K-shell x-raysE~35 keV

Compton scatterings

Compton Edge

ene

rgy

decay

decay E=662keV

decay gives off electrons with a range of energiesEmax = 514 keV, 1170 keV

decay gives off a monchromatic photonE = 662 keV

decay

Cs137

e-

1800

backscatter

forwardscatteredelectron

energy resolution:E/E~2.5%@ 662KeV

NaI crystal ~ 5cm X 5cm


NaI is a Dirty Bomb Detector


What’s in Your Air?

I set up a NaI counter in PRB3153 and took data for 24 hours.Find lots of -ray peaks Use ROOT to fit the -ray peaks to a Gaussian (signal) + linear background Pb214, Bi214 are Radon (Rn222) by-products (~1pc/L in PRB3153) K40 is common in many building materials (and bananas) TL208 (Thallium 208) is from Rn220

Energy (keV) Energy (keV)

Pb214

Bi214

Bi214 K40

Bi214

TL208Bi214


Cerenkov Light

The Cerenkov effect occurs when the velocity of a charged particle traveling througha dielectric medium exceeds the speed of light in the medium. Index of refraction (n) = (speed of light in vacuum)/(speed of light in medium)Will get Cerenkov light when:

Angle of Cerenkov Radiation:

nc

v 1

For water n=1.33, will get Cerenkov light if v > 2.25x1010 cm/s

n

1cos

No radiationradiation

ct(c/n)t

In a time t wavefront moves (c/n)tbut particle moves ct.

Huyghen’s wavefronts

speed of particle > speed of light in medium


Threshold Momentum for Cerenkov

Radiation Example: Threshold momentum for Cerenkov light:

nt

1

1

1

11

1222

nn

n

tt

t

)1)(1(

1

1

12

nnn

tt

For gases it is convenient to let =n-1. Then we have:

)2(

1

tt

The momentum (pt) at which we get Cerenkov radiation is:

)2(

m

mp ttt

For a gas +2 so the threshold momentum can be approximated by:

2

mmp ttt

For helium =3.3x10-5 so we find the following thresholds:electrons 63 MeV/c kaons 61 GeV/cpions 17 GeV/c protons 115GeV/c

Medium =n-1 t

helium 3.3x10-5 123CO2 4.3x10-4 34H2O 0.33 1.52glass 0.46-0.75 1.37-1.22


Number of photons from Cerenkov RadiationFrom classical electrodynamics (Frank&Tamm 1937, Nobel Prize 1958) wefind the following for the energy loss per wavelength () per dx for charge=1, n>1:

])(

11[

2222

n

E

dxd

dE

With =fine structure constant, n() the index of refraction which in general depends on the wavelength () of light.We can re-write the above in terms of the number of photons (N) using: dN=dE/E

For example see Jackson section 13.5

])(

11[

2]

)(

11[

2222222

ndxd

dN

n

E

dxd

dE

We can simplify the above by considering a region were n() is a constant=n:

2222

sincos1)(

11

n

2

2222sin

2]

)(

11[

2

dxd

dN

ndxd

dN

We can calculate the number of photons/dx by integrating over the wavelengths thatcan be detected by our phototube (1, 2):

]11

[sin2sin221

22

22

1

d

dx

dNNote: if we are using a phototubewith a photocathode efficiency thatvaries as a function of then we have:

2

1

22 )(

sin2

df

dx

dN


Number of photons from Cerenkov RadiationFor a typical phototube the range of wavelengths (1, 2) is (350nm, 500nm).

photons/cmsin39010

]5

1

5.3

1[sin2]

11[sin2 2

52

21

2

cmdx

dN

We can simplify using:

21222222 )1)(1()1)(1(1

1cos1sinn

nn

n

nn

n

For a highly relativistic particle going through a gas the above reduces to:

)1(2)1)(1()1)(1(

singas,12122

2

nn

nn

n

nn

photons/cm)1(780 ndx

dN

For He we find: 2-3 photons/meter (not a lot!)For CO2 we find: ~33 photons/meterFor H2O we find: ~34000 photons/meter

GAS

Photons are preferentially emittedat small ’s (blue)

For most Cerenkov counters the photon yield is limited (small) due to space limitations, the index of refraction of the medium, and the phototube quantum efficiency.


Types of Cerenkov CountersThere are three different types of Cerenkov counters used to identify particles.Listed in order of their sophistication they are:

Threshold counter (on/off device)Differential counter (makes use of the angle of the Cerenkov radiation)Ring imaging counter (makes use of the “cone” of light)

Each of the above counter is designed to work in a certain momentum range.Threshold counter: Identify the particle(s) which give off light. Can use to separate electrons from heavier particles (, K, p) since electrons will give off light at a much lower momentum (e.g. 68 MeV/c vs 17 GeV/c for He)Problems with device: above a certain momentum several particles will give light. usually threshold counters use gas which implies low light levels (n-1 small) low light levels leads to inefficiency, e.g. <n>=3, the prob. of zero photons: P(0)=e-3=5%!Phototubes must be shielded from magnetic fields above a few tenths of a gauss.


Types of Cerenkov Counters

Differential Cerenkov Counter:Makes use of the angle of Cerenkov radiation and only samples light at certain angles.For fixed momentum cos is a function of mass:

Not all light will make it to phototube

np

pm

Epnn

22

)/(

11cos

Differential cerenkov counters typically on work over a fixed momentum range (good for beam monitors, e.g. measure or K content of beam).Problems with differential Cerenkov counters: Optics are usually complicated. Have problems in magnetic fields since phototubes must be shielded from B-fields above a few tenths of a gauss.


Ring Imaging Cerenkov Counters (RICH)RICH counters use the cone of the Cerenkov light.The ½ angle () of the cone is given by:

np

pm

n

2211 cos

1cos

The radius of the cone is: r=Ltan, with L the distance to the where the ring is imaged.

L

r

For a particle with p=1GeV/c, L=1 m, and LiF as the medium (n=1.392) we find:

deg r(m) 43.5 0.95

K 36.7 0.75P 9.95 0.18Thus by measuring p and r we can identify what type of particle we have.

Problems with RICH: optics very complicated (projections are not usually circles) readout system very complicated (e.g. wire chamber readout, 105-106 channels) elaborate gas system photon yield usually small (10-20), only a few points on “circle”

Great /K/p separation!


CLEO’s Ring imaging Cerenkov Counter

The figures below show the CLEO III RICH structure. The radiator is LiF, 1 cm thick, followed by a 15.7 cm expansion volume and photon detector consisting of a wire chamber filled with a mixture of TEA and CH4 gas. TEA is photosensitive. The resulting photoelectrons are multiplied by the HV on the wires and the resulting signals are sensed by a rectangular array of pads coupled with highly sensitive electronics.

Challenge is to separate ’s from K’s in the range 1.5 <p < 3GeV (B Vs BK)


CLEO’s Ring imaging Cerenkov Counter

Lithium Floride (LiF) radiator

Assembled radiators. They are guarded by Ray Mountain. WithoutRay “living”at the factory that produced the LiF radiators we would stillbe waiting for the orderto be completed.

A photodetector:CaF2 window+cathode pads

Assembledphotodetectors


Performance of CLEO’s RICH

Number of detectedphotons on 5 GeV electrons

A track in theRICH

D*’s without/withRICH information

Preliminary dataon /K separation


The BaBar DIRC

Here the challenge is to separate ’s and K’s in the range: 1.7<p< 4.2 GeV Detector of Internally Reflected Cerenkov light

DIRC uses quartz bars (490x1.7x3.5cm3) as radiator (n=1.473) and light guide The cerenkov light is internally reflected to the end of a bar bar must be very flat <5ÅDIRC is a 3D device, measures x, y, and time of Cerenkov photonsDetect the photons with an array of phototubes “Typical” photon has: =400 nm 200 bounces 5m path in quartz bar 10-60 ns propagation time

laser light propagating in a quartz bar


The BaBar DIRC

1.5 T Solenoid Electromagnetic Calorimeter

(EMC)Detector of Internally

Recflected Cherenkov

Light (DIRC)

Instrumented Flux Return

(IFR) Silicon Vertex Tracker (SVT)

Drift Chamber (DCH)e- (9 GeV)

e+ (3.1 GeV)

phototube array


Performance of the BaBar DIRCTiming information very useful to eliminate photons not associated with a track

300 nsec window500-1300 background hits

8 nsec window1-2 background hits

Note: the pattern of phototubes withsignals is very complicated. Thedetection surface is toroidal and thereforethe cerenkov rings are disjoint and distorted.

Use a maximum likelihood analysis to separate /K/p: L=L(c, t, n)

DIRC works very well!


SuperK

481 MeV muon neutrino produces 394 MeV muon which later decays at rest into 52 MeV electron. The ring fit to the muon is outlined. Electron ring is seen in yellow-green in lower right corner. This is perspective projection with 110 degrees opening angle, looking from a corner of the Super-Kdetector (not from the event vertex). Color corresponds to time PMT was hit by Cerenkov photon from the ring. Color scale is time from 830 to 1816 ns with 15.9 ns step. In the charge weighted time histogram to the right two peaks are clearly seen, one from the muon, and second one from the delayed electron from the muon decay. Size of PMT corresponds to amount of light seen by the PMT. From: http://www.ps.uci.edu/~tomba/sk/tscan/pictures.html

SuperK is a water RICH. It uses phototubes to measure the Cerenkov ring.Phototubes give time and pulse height information

From SuperK site

SuperK has: 50 ktons of H2OInner PMTS: 1748 (top and bottom) and 7650 (barrel)outer PMTs: 302 (top), 308 (bottom) and 1275(barrel)

For water n=1.33For =1 particle cos=1/1.33, =41o


Askaryan Effect Radio Frequency Cerenkov RadiationAskaryan Effect: EM showers in a dielectric medium generate coherent radio cerenkov emission

From: D. Saltzberg, Orion Workshop

An EM shower propagating in air+pb

In EM shower there will be more e-’s than e+’s (~20%), a net current which can radiate. No radiation if exactly same amount of + and - chargesExcess charge moving faster than speed of light will emit cerenkov radiation. In ice the peak frequency of radiation ~ 2 GHz (~15 cm).The radiation is coherent (rad lateral shower size) and power ~ E2

Possible to observe very high interactions in ice (or salt) Radiation is linearly polarized

predicted 1962, observed 2000

Saltzberg, et al, Phys.Rev.Lett. 86 (2001) 2802-2805


Radio Frequency Cerenkov Radiation from IceFrom: Andrea Silvestri, UCI, International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily


ANITA ExperimentAntarctic Impulsive Transient Antenna

From: Andrea Silvestri, UCI, presented at International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily

ANITA is an experiment designed to detector ultra high energy neutrino interactions 1017<E<1020 eVIt relies on detecting Askaryan Cerenkov radiation from very high energy neutrino interactions in ice.

880.P20 Winter 2006 Richard Kass 1 Scintillation Devices As a charged particle traverses a medium it...

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