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 of
the 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 experimental
techniques 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)
880.P20 Winter 2006
Richard Kass
1
Scintillators
Properties of common plastic scintillators
Emission spectrum of NE102A
Plastic scintillator
violet
blue
Typical cost 1$/in2
A typical plastic
Scintillator system
880.P20 Winter 2006
Richard Kass
2
Photomultiplier Tubes
light
e’s
violet blue green
Electric field accelerates electrons
Quantum efficiency of bialkali cathode vs wavelength
Electrons crash into dynodes  create more electrons
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 current
Properties of phototubes:
In situations where a lot of light
very high gain, low noise current amplifier
is produced (>103 photons) a
6
gains 10 possible
photodiode can be used in place of a
possible to count single photons
phototube, e.g. BaBar’s calorimeter
Off the shelf item, buy from a company
wide variety to choose from (size, gain, sensitivity)
tube costs range from $102-$103
Sensitive to magnetic fields (shield against earth’s): use “mu-metal”
880.P20 Winter 2006
Richard Kass
3
Scintillation Counter Example
Some typical parameters for a plastic scintillation counter are:
energy loss in plastic scintillator:
scintillation efficiency of plastic:
collection efficiency (# photons reaching PMT):
quantum efficiency of PMT
2MeV/cm
1 photon/100 eV
0.1
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 2x104g’s
of which 2x103g’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 =8x10-11C
Assume charge is collected in 50nsec (5x10-8s)
current=dq/dt=(8x10-11 coulombs)/(5x10-8s)=1.6x10-3A
Assume this current goes through a 50 W resistor
V=IR=(50 W )(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 n e n 
P ( n) 
n!
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
880.P20 Winter 2006
Richard Kass
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Time of flight with Scintillators
Time of Flight (TOF) is a particle identification technique.
measure particle speed and momentum determine mass
t=x/v=x/(bc) with b=pc/E=pc/[(mc2)2+(pc)2]1/2
x(( mc) 2  p 2 )1/ 2
t
pc
Actually, we measure
the time it takes for the
particle to travel a
known distance.
Consider two particles with different masses but same momentum:
2
2
2
2
2
2
2
2
2
x
((
m
c
)

p
)
x
((
m
c
)

p
)
x
(
m

m
1
2
1
2)
t12  t22 


( pc) 2
( pc) 2
p2
t12  t22  (t1  t2 )( t1  t2 )
x
x 2 ( m12  m22 )
t1  t2 
(t1  t 2 ) p 2
For high momentum (e.g. p>1 GeV/c for p’s):
t1+t2=2t and x/tc
x(m12  m22 )
x(m12  m22 )
t1  t2 
 1667
psec/meter
2
2
2cp
p
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Richard Kass
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Time of Flight with Scintillators
x(m12  m22 )
x(m12  m22 )
t  t1  t2 
 1667
psec/meter
2
2
2cp
p
As an example, assume m1=mp (140MeV) , m2=mk (494MeV), and x=10m
t=3.8 nsec for p=1 GeV
t=0.95 nsec for p=2GeV
Scintillator+phototubes are capable of measuring such small time differences
Time resolution of a “good” TOF system is 150ps (0.15 ns)
In colliding beam experiments, 0.5 <x< 1 m small x puts a limit of t.
For x=1 m, p=1 GeV K/p separation t psec < 3  separation
x =1 meter
1.4 GeV/c p’s and K’s
No pulse
height correction
with pulse
height correction
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Richard Kass
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Basic Physics Processes in a Sodium Iodide (NaI) Calorimeter
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, lmax=415 nm, decay time ~250nsec
NaI is often used to measure the energy low gamma rays
Photoelectric Effect
g absorbed by material,
electron ejected
Compton Scattering
ge-→ge- “elastic scattering”
g
g
Pair Production
g→e+e- creates anti-matter
g
e-
eNaI
hv < 0.05 MeV
eg
0.05 < hv < 10 MeV
e+
hv > 10 MeV g-ray must have E>2me
Attenuation of the gamma rays is energy dependent
radiation length of NaI ~2.5 cm
but only useful for E > few MeV
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Richard Kass
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NaI & Homeland Security
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Cs137
Example: Cs137 g-ray Spectrum in NaI
energy
b decay
b decay
Eg=662keV
b decay gives off electrons with a range of energies
Emax = 514 keV, 1170 keV
g decay gives off a monchromatic photon
forward
scattered
E = 662 keV
electron
NaI crystal ~ 5cm X 5cm
b decay
g
1800
backscatter
K-shell x-rays
Eg~35 keV
Eg=662keV
photopeak
e1800 backscatter
Eg=184keV
energy resolution:
E/E~2.5%@ 662KeV
Compton
scatterings
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Compton
Edge
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NaI is a Dirty Bomb Detector
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What’s in Your Air?
I set up a NaI counter in PRB3153 and took data for 24 hours.
Find lots of g-ray peaks
Use ROOT to fit the g-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
Bi214
K40
Pb214
Bi214
Bi214
Bi214
Energy (keV)
880.P20 Winter 2006
TL208
Energy (keV)
Richard Kass
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Cerenkov Light
The Cerenkov effect occurs when the velocity of a charged particle traveling through
a 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:
v
1
speed of particle > speed of light in medium
b 
c
n
For water n=1.33, will get Cerenkov light if v > 2.25x1010 cm/s
Huyghen’s wavefronts
radiation
(c/n)t

bct
1
Angle of Cerenkov Radiation: cos  
nb
880.P20 Winter 2006
No radiation
In a time t wavefront moves (c/n)t
but particle moves bct.
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Threshold Momentum for Cerenkov Radiation
Example: Threshold momentum for Cerenkov light:
1
n
1
1
gt 


bt 
n
1  b t2
n2 1 bt n2 1
b tg t 
1
n2 1

1
(n  1)(n  1)
Medium
helium
CO2
H2O
glass
d=n-1
3.3x10-5
4.3x10-4
0.33
0.46-0.75
gt
123
34
1.52
1.37-1.22
For gases it is convenient to let d=n-1. Then we have:
b tg t 
1
d (d  2)
The momentum (pt) at which we get Cerenkov radiation is:
pt  mb tg t 
m
d (d  2)
For a gas d+2 2 so the threshold momentum can be approximated by:
pt  mb tg t 
m
2d
For helium d=3.3x10-5 so we find the following thresholds:
electrons 63 MeV/c
pions
17 GeV/c
880.P20 Winter 2006
kaons
protons
61 GeV/c
115GeV/c
Richard Kass
13
Number of photons from Cerenkov Radiation
From classical electrodynamics (Frank&Tamm 1937, Nobel Prize 1958) we
find the following for the energy loss per wavelength (l) per dx for charge=1, bn>1:
dE
2pE
1

[
1

]
dxdl
l2
b 2 n (l ) 2
For example see Jackson section 13.5
With =fine structure constant, n(l) the index of refraction which in general depends
on the wavelength (l) of light.
We can re-write the above in terms of the number of photons (N) using:
dN=dE/E
dE
2pE
1
dN
2p
1

[
1

]


[
1

]
2
2
2
2
2
2
dxdl
l
b n(l )
dxdl
l
b n (l )
We can simplify the above by considering a region were n(l) is a constant=n:
dN
2p
1
dN
2p 2
1
2
2

[
1

]


sin 
1 2
 1  cos   sin  
2
2
2
2
2
dxd
l
l
b
n
(
l
)
dxd
l
l
b n(l )
We can calculate the number of photons/dx by integrating over the wavelengths that
can be detected by our phototube (l1, l2):
l2
dN
dl
1
1
 2psin 2   2  2psin 2 [  ]
dx
l
l1 l 2
l1
880.P20 Winter 2006
Note: if we are using a phototube
with a photocathode efficiency that
varies as a function of l then we have:
l2
dN
f (l )dl
2
 2p sin  
dx
l2
l1
Richard Kass
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Number of photons from Cerenkov Radiation
For a typical phototube the range of wavelengths (l1, l2) is (350nm, 500nm).
dN
1 1
1 1 105
2
2
 2p sin  [  ]  2p sin  [
 ]
 390 sin 2  photons/cm
dx
l1 l2
3.5 5 cm
We can simplify using:
sin 2   1  cos 2   1 
1
( bn  1)( bn  1)
(n  1)( n  1)


b 1
b 2n2
b 2n2
n2
For a highly relativistic particle going through a gas the above reduces to:
sin 2  
( bn  1)( bn  1)
(n  1)( n  1)

 2(n  1)
2 2
2
b

1
b

1, gas
b n
n
dN
 780(n  1) photons/cm
dx
GAS
For He we find: 2-3 photons/meter (not a lot!)
For CO2 we find: ~33 photons/meter
For H2O we find: ~34000 photons/meter
Photons are preferentially emitted
at small l’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.
880.P20 Winter 2006
Richard Kass
15
Types of Cerenkov Counters
There 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 (p, 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. <ng>=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.
880.P20 Winter 2006
Richard Kass
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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:
1
1
cos  


nb n( p / E )
m2  p2
np
Differential cerenkov counters typically on work over a fixed momentum range
(good for beam monitors, e.g. measure p 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.
Not all light will make it to phototube
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Richard Kass
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Ring Imaging Cerenkov Counters (RICH)
RICH counters use the cone of the Cerenkov light.
The ½ angle () of the cone is given by:
2
2 

1
1
1  m  p 
cos 
   cos


nb
np


2
r
L
The radius of the cone is: r=Ltan, with L the distance to the where the ring is imaged.
For a particle with p=1GeV/c, L=1 m, and LiF as the medium (n=1.392) we find:
p
K
P
(deg)
43.5
36.7
9.95
r(m)
0.95
0.75
0.18
Great p/K/p separation!
Thus 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”
880.P20 Winter 2006
Richard Kass
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CLEO’s Ring imaging Cerenkov Counter
Challenge is to separate p’s from K’s in the range 1.5 <p < 3GeV (Bpp Vs BKp)
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.
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Richard Kass
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CLEO’s Ring imaging Cerenkov Counter
Lithium Floride (LiF) radiator
Assembled radiators.
They are guarded by
Ray Mountain. Without
Ray “living”at the factory
that produced the LiF
radiators we would still
be waiting for the order
to be completed.
Assembled
photodetectors
A photodetector:
CaF2 window+cathode pads
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Richard Kass
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Performance of CLEO’s RICH
Number of detected
photons on 5 GeV
electrons
D*’s without/with
RICH information
Preliminary data
on p/K separation
A track in the
RICH
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The BaBar DIRC
Detector of Internally Reflected Cerenkov light
Here the challenge is to separate p’s and K’s in the range: 1.7<p< 4.2 GeV
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 <5Å
DIRC is a 3D device, measures x, y, and time of Cerenkov photons
Detect the photons with an array of phototubes
“Typical” photon has:
l=400 nm
200 bounces 5m path in quartz bar
10-60 ns propagation time
laser light propagating in a quartz bar
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Richard Kass
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The BaBar DIRC
Electromagnetic
Calorimeter
(EMC)
1.5 T Solenoid
Detector of
Internally
Recflected
Cherenkov
Light (DIRC)
Drift Chamber
(DCH)
Instrumented
Flux Return
(IFR)
Silicon Vertex
Tracker (SVT)
phototube array
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Richard Kass
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Performance of the BaBar DIRC
Timing information very useful to eliminate photons not associated with a track
Note: the pattern of phototubes with
signals is very complicated. The
detection surface is toroidal and therefore
the cerenkov rings are disjoint and distorted.
300 nsec window
500-1300 background hits
8 nsec window
1-2 background hits
Use a maximum likelihood analysis to separate p/K/p: L=L(c, t, ng)
DIRC works very well!
880.P20 Winter 2006
Richard Kass
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SuperK
SuperK is a water RICH. It uses phototubes to measure the Cerenkov ring.
Phototubes give time and pulse height information
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-K
detector (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
For water n=1.33
For b=1 particle cos=1/1.33, =41o
SuperK has: 50 ktons of H2O
Inner PMTS: 1748 (top and bottom) and 7650 (barrel)
outer PMTs: 302 (top), 308 (bottom) and 1275(barrel)
From SuperK site
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Richard Kass
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Askaryan Effect Radio Frequency Cerenkov Radiation
Askaryan Effect: EM showers in a dielectric medium generate coherent radio cerenkov emission
predicted 1962, observed 2000
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 - charges
Excess charge moving faster than speed of light will emit cerenkov radiation.
In ice the peak frequency of radiation ~ 2 GHz (l~15 cm).
The radiation is coherent (lrad  lateral shower size) and power ~ E2
Possible to observe very high n interactions in ice (or salt)
Radiation is linearly polarized
880.P20 Winter 2006
From: D. Saltzberg, Orion Workshop
Saltzberg, et al, Phys.Rev.Lett. 86 (2001) 2802-2805
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Radio Frequency Cerenkov Radiation from Ice
From: Andrea Silvestri, UCI,
International School
in Cosmic Ray Astrophysics,
July 2004, Erice-Sicily
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ANITA Experiment
Antarctic Impulsive Transient Antenna
ANITA is an experiment designed to detector ultra high energy neutrino interactions 1017<E<1020 eV
It relies on detecting Askaryan Cerenkov radiation from very high energy neutrino interactions in ice.
From: Andrea Silvestri, UCI, presented at International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily
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