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Scintillation Detectors

IntroductionComponents

ScintillatorLight GuidesPhotomultiplier Tubes

Formalism/ElectronicsTiming Resolution

Elton Smith JLab 2006 Detector/Computer Summer Lecture Series

Elton Smith / Scintillation Detectors

B field ~ 5/3 T

R = 3m

L = ½ π R = 4.71 m

p = 0.3 B R = 1.5 GeV/c

tπ = L/βπc = 15.77 ns

tΚ = L/βΚc = 16.53 ns

∆tπK = 0.76 ns

Experiment basics

βπ = p/√p2+mπ2 = 0.9957

βΚ = p/√p2+mΚ2 = 0.9496

Particle Identification by time-of-flight (TOF) requiresMeasurements with accuracies of ~ 0.1 ns


Measure the Flight Time between two Scintillators

400 cm

100 cm

300 cm

20 cmDisc

DiscTD

CStart

Stop

Particle Trajectory450 ns


Propagation velocities

c = 30 cm/ns

vscint = c/n = 20 cm/ns

veff = 16 cm/ns

vpmt = 0.6 cm/ns

vcable = 20 cm/ns

∆t ~ 0.1 ns∆x ~ 3 cm


TOF scintillators stacked for shipment


CLAS detector open for repairs


CLAS detector with FC pulled apart


Start counter assembly


Scintillator types

Organic

LiquidEconomicalmessy

SolidFast decay timelong attenuation lengthEmission spectra

Inorganic

AnthraceneUnused standard

NaI, CsIExcellent γ resolutionSlow decay time

BGOHigh density, compact


Photocathode spectral response


Scintillator thickness

Minimizing material vs. signal/background

CLAS TOF: 5 cm thickPenetrating particles (e.g. pions) loose 10 MeV

Start counter: 0.3 cm thickPenetrating particles loose 0.6 MeVPhotons, e+e− backgrounds ~ 1MeV contribute substantially to count rate

Thresholds may eliminate these in TOF


Light guides

GoalsMatch (rectangular) scintillator to (circular) pmtOptimize light collection for applications

TypesPlasticAirNone“Winston” shapes


Reflective/Refractive boundaries

acrylicScintillatorn = 1.58

PMT glassn = 1.5



Air withreflectiveboundary

Scintillatorn = 1.58

PMT glassn = 1.5

%5411 2

−≈⎟⎠⎞

⎜⎝⎛+−

=nnRair

(reflectance at normal incidence)



Scintillatorn = 1.58

PMT glassn = 1.5

air



acrylicScintillatorn = 1.58

PMT glassn = 1.5

Large-angle ray lost

Acceptance of incident rays at fixed angle depends on position at the exit face of the scintillator


Winston Cones - geometry


Winston Cone - acceptance


Photomultiplier tube, sensitive light meter Gain ~ 106 - 107

Electrodes

Photocathode Dynodes

Anode

γe−

56 AVP pmt


Window Transmittance


Voltage dividers

Equal voltage stepsMaximum gain

Progressive, higher voltage near anodeExcellent linearity, limited gain

Time optimized, higher voltage at cathodeGood gain, fast response

ZenersStabilize voltages independent of gain

Decoupling capacitors“reservoirs” of charge during pulsed operation


Voltage Dividersd1 d2 d3 dNdN-1dN-2

akg

4 2.5 1 1 1 1 1 1 1 1 1 1

16.5

RL

+HV−HV

Equal Steps – Max Gain

4 2.5 1 1 1 1 1 1 1.4 1.6 3 2.5

21

RL

Intermediate

6 2.5 1 1.25 1.5 1.5 1.75 2.5 3.5 4.5 8 10

44

RL

Progressive

Timing Linearity


VoltageDivider

Active componentsto minimize changesto timing and rate capability with HV

Capacitors for increasedlinearity in pulsed applications


High voltage

Positive (cathode at ground)low noise, capacitative coupling

NegativeAnode at ground (no HV on signal)

No (high) voltageCockcroft-Walton bases


Effect of magnetic field on pmt


Housing


Compact UNH divider design


Electrostatics near cathode at −HV

Stable performance with negative high voltage is achieved byEliminating potential gradients in the vicinity of the photocathode.The electrostatic shielding and the can of the crystal are bothMaintained at cathode potential by this arrangement.


Dark counts

Solid : Sea level

Dashed: 30 m underground

Thermal Noise

After-pulsing andGlass radioactivity

Cosmic rays


Signal for passing tracks


Single photoelectron signal


Pulse distortion in cable


Electronics

trigger

dynode

Measure timeMeasure pulse height

anode


Time-walk corrections


Formalism: Measure time and position

PL PR

TL TRX=0 XX=−L/2 X=+L/2

effLL vxTT /0 += effRR vxTT /0 −=

)()( 0021

21

RLRLave TTTTT +=+= Mean is independent of x!

[ ] )(2

)()(2

00RL

effRLRL

eff TTv

TTTTv

x −→−−−=


From single-photoelectron timing to counter resolutionThe uncertainty in determining the passage of a particlethrough a scintillator has a statistical component, dependingon the number of photoelectrons Npe that create the pulse.

)2/exp()2/()(

2212

0 λσσ

σσLNLns

pe

PTOF −⋅

⋅++= 1000≈peN

ns062.00 =σ Intrinsic timing of electronic circuits

ns1.21 =σ

cmnsP /0118.0=σ

)15(36.0134 counterscmLcm ⋅+=λ)22(430 counterscmcm=λ

Combined scintillator and pmt response

Average path length variations in scintillator

SinglePhotoelectronResponse }

Note: Parameters for CLAS


Average time resolution

CLAS in Hall B


Formalism: Measure energy loss

PL PR

TL TRX=0 XX=−L/2 X=+L/2

λ/0 xLL ePP −= λ/0 x

RR ePP =

00RLRL PPPPEnergy ⋅=⋅=

Geometric mean is independent of x!


Energy deposited in scintillator


UncertaintiesTiming

Assume that one pmt measures a time with uncertainty δt

Mass Resolution

2~

21 22 tttt RLave

δδδδ +=

2~)

21( 22 tvttvx effRLeff

δδδδ ⋅+⋅=

γEm = 2

2

2222 1)1( pEm ⎟

⎟⎠

⎞⎜⎜⎝

⎛ −=−=→

βββ

224

2

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=⎟

⎠⎞

⎜⎝⎛

pp

mm δ

βδβγδ


Integral magnetic shield


Example: Kaon mass resolution by TOF

cGeVPK /1= GeVEK 116.11495.0 2 =+=

896.0=⎟⎟⎠

⎞⎜⎜⎝

⎛=

K

KK E

Pβ 26.2=⎟⎟

⎠

⎞⎜⎜⎝

⎛=

K

KK m

Eγ

For a flight path of d = 500 cm, nsnscm

cmt 6.18/30896.0

500=⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅

=

nst 15.0=δ 01.0=⎟⎟⎠

⎞⎜⎜⎝

⎛ppδ

Assume

( ) 222

42

042.001.06.18

15.026.2 =+⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛

mmδ MeVmK 21~δ→

⎟⎟⎠

⎞⎜⎜⎝

⎛∞⎯⎯⎯ →⎯⎟

⎠⎞

⎜⎝⎛

∞→ βδβδ

γfixedfor

mm

2Note:


Velocity vs. momentum

π+

K+

p


Summary

Scintillator counters have a few simple components

Systems are built out of these countersFast response allows for accurate timing

The time resolution required for particle identification is the result of the time response of individual components scaled by √Npe


Magnetic fields

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