Scintillation Detectors
Elton Smith JLab 2005 Detector/Computer Summer Lecture Series
IntroductionComponents
ScintillatorLight GuidesPhotomultiplier Tubes
Formalism/ElectronicsTiming Resolution
Elton Smith / Scintillation Detectors
B field ~ 5/3 T
R = 3m
L = ½ R = 4.71 m
p = 0.3 B R = 1.5 GeV/c
t = L/c = 15.77 ns
t = L/c = 16.53 ns
tK = 0.76 ns
Experiment basics
= p/√p2+m2 = 0.9957
= p/√p2+m2 = 0.9496
Particle Identification by time-of-flight (TOF) requiresMeasurements with accuracies of ~ 0.1 ns
Elton Smith / Scintillation Detectors
Measure the Flight Time between two Scintillators
400 cm
100 cm
300 cm
20 cmDisc
DiscT
DC
Start
Stop
Particle Trajectory450 ns
Elton Smith / Scintillation Detectors
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
Elton Smith / Scintillation Detectors
Scintillator types
Organic
Liquid Economical messy
Solid Fast decay time long attenuation length Emission spectra
Inorganic
Anthracene Unused standard
NaI, CsI Excellent resolution Slow decay time
BGO High density, compact
Elton Smith / Scintillation Detectors
Scintillator thickness
Minimizing material vs. signal/background
CLAS TOF: 5 cm thick Penetrating particles (e.g. pions) loose 10 MeV
Start counter: 0.3 cm thick Penetrating particles loose 0.6 MeV Photons, e+e− backgrounds ~ 1MeV
contribute substantially to count rate Thresholds may eliminate these in TOF
Elton Smith / Scintillation Detectors
Light guides
Goals Match (rectangular) scintillator to (circular) pmt Optimize light collection for applications
Types Plastic Air None “Winston” shapes
Elton Smith / Scintillation Detectors
acrylic
Reflective/Refractive boundaries
Scintillatorn = 1.58
PMT glassn = 1.5
Elton Smith / Scintillation Detectors
Air withreflectiveboundary
Reflective/Refractive boundaries
Scintillatorn = 1.58
PMT glassn = 1.5
%541
12
n
nRair
(reflectance at normal incidence)
Elton Smith / Scintillation Detectors
Reflective/Refractive boundaries
Scintillatorn = 1.58
PMT glassn = 1.5
air
Elton Smith / Scintillation Detectors
acrylic
Reflective/Refractive boundaries
Scintillatorn = 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
Elton Smith / Scintillation Detectors
Photomultiplier tube, sensitive light meter
Photocathode
Electrodes
Dynodes
Anode
56 AVP pmt
e−
Gain ~ 106 - 107
Elton Smith / Scintillation Detectors
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
Elton Smith / Scintillation Detectors
VoltageDivider
Active componentsto minimize timingand rate capabilitywith gain
Capacitors for increasedlinearity in pulsed applications
Elton Smith / Scintillation Detectors
High voltage
Positive (cathode at ground) low noise, capacitative coupling
Negative Anode at ground (no HV on signal)
No (high) voltage Cockcroft-Walton bases
Elton Smith / Scintillation Detectors
Dark counts
Solid : Sea level
Dashed: 30 m underground
Thermal Noise
After pulsing andGlass radioactivity
Cosmic rays
Elton Smith / Scintillation Detectors
Electronics
trigger
dynode
Measure timeMeasure pulse height
anode
Elton Smith / Scintillation Detectors
Formalism: Measure time and position
PL PR
TRTL
X=0 XX=−L/2 X=+L/2
effLL vxTT /0
)()( 002
1
2
1RLRLave TTTTT Mean is independent of x!
effRR vxTT /0
)(2
)()(2
00RL
effRLRL
effTT
vTTTT
vx
Elton Smith / Scintillation Detectors
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
LN
Lns
pe
PTOF
1000peN
Note: Parameters for CLAS
ns062.00 Intrinsic timing of electronic circuits
ns1.21
cmnsP /0118.0
)15(36.0134 counterscmLcm )22(430 counterscmcm
Combined scintillator and pmt response
Average path length variations in scintillator
SinglePhotoelectronResponse
Elton Smith / Scintillation Detectors
Formalism: Measure energy loss
PL PR
TRTL
X=0 XX=−L/2 X=+L/2
/0 xLL ePP /0 x
RR ePP
00RLRL PPPPEnergy
Geometric mean is independent of x!
Elton Smith / Scintillation Detectors
UncertaintiesTiming
Mass Resolution
Assume that one pmt measures a time with uncertainty t
2~
2
1 22 tttt RLave
2~)
2
1( 22 t
vttvx effRLeff
E
m 22
2222 1
)1( pEm
224
2
p
p
m
m
Elton Smith / Scintillation Detectors
Example: Kaon mass resolution by TOF cGeVPK /1 GeVEK 116.11495.0 2
896.0
K
KK E
P 26.2
K
KK m
E
For a flight path of d = 500 cm, nsnscm
cmt 6.18
/30896.0
500
nst 15.0 01.0
p
pAssume
222
42
042.001.06.18
15.026.2
m
m MeVmK 21~
Note:
fixedfor
m
m2
Elton Smith / Scintillation Detectors
Summary
Scintillator counters have a few simple components Systems are built out of these counters Fast response allows for accurate timing
The time resolution for required for particle identification is the result of the time response of individual components scaled by √Npe