ES_scintillation_detectors_06.ppt

Scintillation Detectors
Elton Smith JLab 2006 Detector/Computer Summer Lecture Series
Introduction
Components
Scintillator
Light Guides
Photomultiplier Tubes
Formalism/Electronics
Timing Resolution

Elton Smith / Scintillation Detectors
B field ~ 5/3 T
R = 3m
L = ½ p R = 4.71 m
p = 0.3 B R = 1.5 GeV/c
tp = L/bpc = 15.77 ns
tK = L/bKc = 16.53 ns
DtpK = 0.76 ns
Experiment basics
bp = p/√p2+mp
2 = 0.9957
bK = p/√p2+mK
2 = 0.9496
Particle Identification by time-of-flight (TOF) requires
Measurements with accuracies of ~ 0.1 ns

Measure the Flight Time between two
Scintillators
Disc
Disc
TDC
Start
Stop
Particle Trajectory

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
Dt ~ 0.1 ns
Dx ~ 3 cm

TOF scintillators stacked for shipment

CLAS detector open for repairs

CLAS detector with FC pulled apart

Start counter assembly

Scintillator types
 Organic
 Liquid
 Economical
 messy
 Solid
 Fast decay time
 long attenuation length
 Emission spectra
 Inorganic
 Anthracene
 Unused standard
 NaI, CsI
 Excellent g resolution
 Slow decay time
 BGO
 High density, compact

Photocathode spectral response

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

Light guides
 Goals
Match (rectangular) scintillator to (circular) pmt
Optimize light collection for applications
 Types
Plastic
Air
None
“Winston” shapes

acrylic
Reflective/Refractive boundaries
Scintillator
n = 1.58
PMT glass
n = 1.5

Air with
reflective
boundary
Scintillator
n = 1.58
PMT glass
n = 1.5
%
5
4
1
1
2











n
n
Rair
(reflectance at normal incidence)

Scintillator
n = 1.58
PMT glass
n = 1.5
air

acrylic
Scintillator
n = 1.58
PMT glass
n = 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
Photocathode
Electrodes
Dynodes
Anode
56 AVP pmt
g
e−
Gain ~ 106 - 107

Voltage Dividers
d1 d2 d3
dN
dN-1
dN-2
a
k
g
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

Voltage
Divider
Active components
to minimize changes
to timing and rate
capability with HV
Capacitors for increased
linearity in
pulsed applications

High voltage
 Positive (cathode at ground)
low noise, capacitative coupling
 Negative
Anode at ground (no HV on signal)
 No (high) voltage
Cockcroft-Walton bases

Effect of magnetic field on pmt

Housing

Compact UNH divider design

Dark counts
Solid : Sea level
Dashed: 30 m underground
Thermal
Noise
After-pulsing and
Glass radioactivity
Cosmic rays

Signal for passing tracks

Single photoelectron signal

Pulse distortion in cable

Electronics
trigger
dynode
Measure time
Measure pulse height
anode

Formalism: Measure time and position
PL PR
TR
TL
X=0 X
X=−L/2 X=+L/2
eff
L
L v
x
T
T /
0


)
(
)
( 0
0
2
1
2
1
R
L
R
L
ave T
T
T
T
T 


 Mean is independent of x!
eff
R
R v
x
T
T /
0


  )
(
2
)
(
)
(
2
0
0
R
L
eff
R
L
R
L
eff
T
T
v
T
T
T
T
v
x 






From single-photoelectron timing to
counter resolution
The uncertainty in determining the passage of a particle
through a scintillator has a statistical component, depending
on the number of photoelectrons Npe that create the pulse.
)
2
/
exp(
)
2
/
(
)
(
2
2
1
2
0





L
N
L
ns
pe
P
TOF





 1000

pe
N
Note: Parameters for CLAS
ns
062
.
0
0 
 Intrinsic timing of electronic circuits
ns
1
.
2
1 

cm
ns
P /
0118
.
0


)
15
(
36
.
0
134 counters
cm
L
cm 



)
22
(
430 counters
cm
cm


Combined scintillator and pmt response
Average path length variations in scintillator
Single
Photoelectron
Response


Average time resolution
CLAS in Hall B

Formalism: Measure energy loss
PL PR
TR
TL
X=0 X
X=−L/2 X=+L/2

/
0 x
L
L e
P
P 
 
/
0 x
R
R e
P
P 
0
0
R
L
R
L P
P
P
P
Energy 



Geometric mean is independent of x!

Energy deposited in scintillator

Uncertainties
Timing
Mass Resolution
Assume that one pmt measures a time with uncertainty dt
2
~
2
1 2
2 t
t
t
t R
L
ave
d
d
d
d 

2
~
)
2
1
( 2
2 t
v
t
t
v
x eff
R
L
eff
d
d
d
d 



g
E
m  2
2
2
2
2
2 1
)
1
( p
E
m







 




b
b
b
2
2
4
2
























p
p
m
m d
b
db
g
d

Example: Kaon mass resolution by TOF
c
GeV
PK /
1
 GeV
EK 116
.
1
1
495
.
0 2



896
.
0










K
K
K
E
P
b 26
.
2










K
K
K
m
E
g
For a flight path of d = 500 cm, ns
ns
cm
cm
t 6
.
18
/
30
896
.
0
500











ns
t 15
.
0

d 01
.
0









p
p
d
Assume
  2
2
2
4
2
042
.
0
01
.
0
6
.
18
15
.
0
26
.
2 














m
m
d
MeV
mK 21
~
d

Note: 










 








 b
db
d
g
fixed
for
m
m
2

Velocity vs. momentum
p+
K+
p

Summary
 Scintillator counters have a few simple
components
Systems are built out of these counters
Fast 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

ES_scintillation_detectors_06.ppt

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