Scintillation Devices

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 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)
2
Scintillators
Properties of common plastic scintillators
Emission spectrum of NE102A Plastic scintillator
violet
blue
Typical cost 1/in2
A typical plastic Scintillator system
3
Photomultiplier Tubes
light
violet
green
blue
es
Electric field accelerates electrons Electrons
crash into dynodes create more electrons
Quantum efficiency of bialkali cathode vs
wavelength
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 very high gain, low
noise current amplifier gains 106
possible possible to count single photons 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 earths) use mu-metal
In situations where a lot of light is produced
(gt103 photons) a photodiode can be used in place
of a phototube, e.g. BaBars calorimeter
4
Scintillation Counter Example
Some 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
2x104gs of which 2x103gs reach PMT which
produce 500 photo-electrons Assume the PMT and
related electronics have the following
properties PMT gain106 so 500 photo-electrons
produces 5x108 electrons 8x10-11C Assume charge
is collected in 50nsec (5x10-8s) currentdq/dt(
8x10-11 coulombs)/(5x10-8s)1.6x10-3A Assume
this current goes through a 50 W
resistor VIR(50 W )(1.6x10-3A)80mV (big
enough to see with Oscope) So a minimum ionizing
particle produces an 80mV signal.
What is the efficiency of the counter? How often
do we get no signal (zero PEs)? The prob. of
getting n PEs when on average expect ltngt is a
Poisson process
The prob. of getting 0 photons is e-ltngt e-500
0. So this counter is 100 efficient. Note a
counter that is 90 efficient has ltngt2.3 PEs
5
Time of flight with Scintillators
Time of Flight (TOF) is a particle identification
technique. measure particle speed and momentum
determine mass
Actually, we measure the time it takes for
the particle to travel a known distance.
tx/vx/(bc) with bpc/Epc/(mc2)2(pc)21/2
Consider two particles with different masses but
same momentum
x
For high momentum (e.g. pgt1 GeV/c for
ps) t1t22t and x/tc
6
Time of Flight with Scintillators
As an example, assume m1mp (140MeV) , m2mk
(494MeV), and x10m Dt3.8 nsec for p1 GeV
Dt0.95 nsec for p2GeV
Scintillatorphototubes are capable of measuring
such small time differences
Time resolution of a good TOF system is s150ps
(0.15 ns)
In colliding beam experiments, 0.5 ltxlt 1 m Þ
small x puts a limit of Dt.
For x1 m, p1 GeV K/p separation Dt380 psec Þ lt
3 s separation
x 1 meter
1.4 GeV/c ps and Ks
No pulse height correction
with pulse height correction
7
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, lmax415 nm, decay time 250nsec
NaI is often used to measure the energy low gamma
rays
Compton Scattering ge-?ge- elastic scattering
Photoelectric Effect g absorbed by material,
electron ejected
Pair Production g?ee- creates anti-matter
g
g
g
e-
e-
e
e-
g
NaI
0.05 lt hv lt 10 MeV
hv gt 10 MeV g-ray must have Egt2me
hv lt 0.05 MeV
Attenuation of the gamma rays is energy dependent
radiation length of NaI 2.5 cm but only useful
for E gt few MeV
8
NaI Homeland Security
9
Example Cs137 g-ray Spectrum in NaI
Cs137
b decay
b decay gives off electrons with a range of
energies Emax 514 keV, 1170 keV g decay gives
off a monchromatic photon E 662 keV
energy
b decay
Eg662keV
forward scattered electron
NaI crystal 5cm X 5cm
b decay
g
1800 backscatter
K-shell x-rays Eg35 keV
Eg662keV photopeak
e-
1800 backscatter Eg184keV
energy resolution sE/E2.5_at_ 662KeV
Compton scatterings
Compton Edge
10
NaI is a Dirty Bomb Detector
11
Whats 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
K40
Bi214
Pb214
Bi214
Bi214
TL208
Bi214
Energy (keV)
Energy (keV)
12
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
speed of particle gt speed of light in medium
For water n1.33, will get Cerenkov light if v gt
2.25x1010 cm/s
Huyghens wavefronts
No radiation
radiation
(c/n)t
In a time t wavefront moves (c/n)t but particle
moves bct.
q
bct
Angle of Cerenkov Radiation
13
Threshold Momentum for Cerenkov Radiation
Example Threshold momentum for Cerenkov light
Medium dn-1 gt helium 3.3x10-5 123 CO2 4.3x10-
4 34 H2O 0.33 1.52 glass 0.46-0.75 1.37-1.22
For gases it is convenient to let dn-1. Then we
have
The momentum (pt) at which we get Cerenkov
radiation is
For a gas d2 2 so the threshold momentum can be
approximated by
For helium d3.3x10-5 so we find the following
thresholds
electrons 63 MeV/c kaons 61 GeV/c pions 17
GeV/c protons 115GeV/c
14
Number of photons from Cerenkov Radiation
From classical electrodynamics (FrankTamm 1937,
Nobel Prize 1958) we find the following for the
energy loss per wavelength (l) per dx for
charge1, bngt1
For example see Jackson section 13.5
With afine 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 dNdE/E
We can simplify the above by considering a region
were n(l) is a constantn
Þ
We can calculate the number of photons/dx by
integrating over the wavelengths that can be
detected by our phototube (l1, l2)
Note if we are using a phototube with a
photocathode efficiency that varies as a function
of l then we have
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Number of photons from Cerenkov Radiation
For a typical phototube the range of wavelengths
(l1, l2) is (350nm, 500nm).
We can simplify using
For a highly relativistic particle going through
a gas the above reduces to
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 ls
(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.
16
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. ltnggt3, the prob. of
zero photons P(0)e-35! Phototubes must be
shielded from magnetic fields above a few tenths
of a gauss.
17
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 cosq
is a function of mass
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
18
Ring Imaging Cerenkov Counters (RICH)
RICH counters use the cone of the Cerenkov
light. The ½ angle (q) of the cone is given by
r
2q
L
The radius of the cone is rLtanq, with L the
distance to the where the ring is imaged.
For a particle with p1GeV/c, L1 m, and LiF as
the medium (n1.392) we find q(deg) r(m) p 43.
5 0.95 K 36.7 0.75 P 9.95 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
19
CLEOs Ring imaging Cerenkov Counter
Challenge is to separate ps from Ks in the
range 1.5 ltp lt 3GeV (Bpp Vs BKp)
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.
20
CLEOs Ring imaging Cerenkov Counter
Assembled radiators. They are guarded by Ray
Mountain. Without Ray livingat the factory
that produced the LiF radiators we would
still be waiting for the order to be completed.
Lithium Floride (LiF) radiator
Assembled photodetectors
A photodetector CaF2 windowcathode pads
21
Performance of CLEOs RICH
Ds without/with RICH information
Number of detected photons on 5 GeV electrons
Preliminary data on p/K separation
A track in the RICH
22
The BaBar DIRC
Detector of Internally Reflected Cerenkov light
Here the challenge is to separate ps and Ks in
the range 1.7ltplt 4.2 GeV
DIRC uses quartz bars (490x1.7x3.5cm3) as
radiator (n1.473) and light guide The cerenkov
light is internally reflected to the end of a bar
bar must be very flat lt5Å DIRC is a 3D device,
measures x, y, and time of Cerenkov
photons Detect the photons with an array of
phototubes Typical photon has l400 nm
200 bounces 5m path in quartz bar 10-60 ns
propagation time
laser light propagating in a quartz bar
23
The BaBar DIRC
1.5 T Solenoid
Electromagnetic Calorimeter (EMC)
Detector of Internally Recflected Cherenkov Light
(DIRC)
e (3.1 GeV)
Drift Chamber (DCH)
e- (9 GeV)
Instrumented Flux Return (IFR)
Silicon Vertex Tracker (SVT)
phototube array
24
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 LL(qc, Dt, ng)
DIRC works very well!
25
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/tom
ba/sk/tscan/pictures.html
For water n1.33 For b1 particle cosq1/1.33,
q41o
SuperK has 50 ktons of H2OInner PMTS 1748 (top
and bottom) and 7650 (barrel) outer PMTs 302
(top), 308 (bottom) and 1275(barrel)
From SuperK site
26
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 airpb
In EM shower there will be more e-s than es
(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 (l15
cm). The radiation is coherent (lrad ³ lateral
shower size) and power E2 Possible to
observe very high n interactions in ice (or
salt) Radiation is linearly polarized
Saltzberg, et al, Phys.Rev.Lett. 86 (2001)
2802-2805
From D. Saltzberg, Orion Workshop
27
Radio Frequency Cerenkov Radiation from Ice
From Andrea Silvestri, UCI, International
School in Cosmic Ray Astrophysics, July 2004,
Erice-Sicily
28
ANITA Experiment
Antarctic Impulsive Transient Antenna
ANITA is an experiment designed to detector ultra
high energy neutrino interactions 1017ltElt1020
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