Transcript Radiation Detectors

Radiation Sensors
Zachariadou K. | TEI of Piraeus
Radiation Sensors
Part-IV
Scintillation Sensors
Part-IV
Scintillation Sensors
The course is largely based on :
 G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New York, Wiley, 2000
 Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray Spectrometry”; Willey
, 21008
 Richard Fernow. “Introduction to experimental particle physics”, Cambridge
University Press, 1986
 Christian Joram “CERN Summer Student Lectures on Particle Detectors” 2003
 C. Zorn, “Instrumentation in High Energy Physics” World Scientific, 1992
 Claude LeRoy, P. G. Rancoita “ Principles Of Radiation Interaction In Matter And
Detection”
Scintillation Sensors
Main components:
Based on a material
(Scintillator) that
produces a pulse of
light shortly after the
passage of a particle
Scintillator
The light produced is
propagated through light
guides and directed on
the face pf a
photomultiplier tube
Light guide
Photolectrons emitted
from the cathode of
the tube are amplified
to give a fast
electronic pulse
Photo multiplier tube
Scintillation Sensors
They emit light when irradiated
 promptly (<10-8s) Fluorescence
 delayed (>10-8s) Phosphorescence
A good scintillator: its material
converts as large a fraction as
possible of the incident radiation
energy to prompt fluorescence
Scintillation Sensors principles
Detect ionizing radiation by the scintillation light produced
Scintillation material requirements:
 Convert the kinetic energy of charged particles into detectable light with
high scintillation efficiency
 Linearity: the light yield should be proportional to deposited energy
 The medium should be transparent to the wavelength of its own emission
for good light collection
 The material should be of good optical quality and able to be
manufactured in large sizes
 The index of refraction should be near that of glass (~1.5) to permit
efficient coupling of the scintillator light to a photomultiplier tube or
other light sensor
Scintillator types
Organic
(Plastic or Liquid solutions)
(They yield less light but they are faster)
Liquid
• Economical
• messy
Solid
• Fast decay time (ns to μs)
• long attenuation length
Inorganic
(They yield more light but they are
slower)
 NaI, CsI
• Excellent gamma
resolution
• Best light output
• Best linearity
• Slow decay time (μs)
• Expensive
 BGO
•High density
•compact
Inorganic:
high density crystals , high Z
Organic (hydrogen content) :
 gamma spectroscopy
beta and fast neutron detection
 Medical imaging
Inorganic Crystal Scintillators
Crystals of alkali metals (iodides)
NaI(Tl)
CsI(Tl)
CaI(Na)
LiI(Eu)
CaF2(Eu)
Sodium iodide activated with thallium [NaI(Tl)], coupled to Photomultiplier tubes
(PMTs) and operated in pulse mode, is used for most nuclear medicine applications
 Bismuth germanate (BGO) is coupled to PMTs and used in pulse mode as detectors
in most PET scanners
Inorganic Crystal Scintillators
Scintillation Mechanism
NaI, CsI. BaF2..
Inorganic Crystal Scintillators
Scintillation Temperature Dependence
The light output shows
strong temperature
dependence
Inorganic Scintillators
Liquid Noble gases
Scintillation mechanism:
Inorganic Scintillators
Lead tungstate crystals used in
the CMS electromagnetic
calorimeter :
Organic Scintillators
 Monocrystals: aromatic compounds
(consist of planar molecules made of
benzene rings) examples:Toluene,
Anthracene
The potential energy of a molecule (which comes
from electronic, vibrational and translational
energies) changes with interatomic distance.
 Light production in organic
scintillators is the result of molecular
transitions.
The radiation raises the molecule to
an excited stat (A1)
 The molecule returns to the ground
state by losing energy through
vibrations (heat) to move to point B1 and
then by emitting a photon to point B0.
 The transition is rapid (10-8 s), and
results in a measurable photon of
energy different than the incident one.
Energy
Liquid of plastic: mixtures of compounds
that consist of a solvent (highest
concentration) and solute (s). Examples of
solvents benzene, toluene, and p-Xylene
EA1
Excited state
A1
Ground state
EB1
EB0
EA0
B1
Bo
Ao
Inter-atomic distance
Organic Scintillators
 Emitted light is in the UV range
 Shift emission to longer wavelengths
 Longer absorption length and efficient
read out device
Wavelength shifting:
Scintillator readout
In a plastic scintillator for 3keV ionization energy deposited one photon
is produced. This results to 500 photons per cm of scintillator material
This low intensity scintillation light has to be optically
coupled to a photomultiplier for amplification and
transformation to an electrical signal
 Light guides
 Wavelength shifter bars
 Photomultiplier tubes
Light guides
Goals:
 To match (rectangular) scintillator to (circular) Photomultiplier tube
 To optimize light collection for applications
Fish tail light guide
Wavelength shifter bars
 Collect the light at a location distant from the scintillator
 A wavelength shifter absorbs the light and re-emits it at a
larger wavelength assuring its transfer over a relative long
distance
 The choice of the material of a WLS : to match the emission spectrum to
the absorption spectrum of the photocathode of the PMT
Photomultiplier tubes (PMT)
PMTs perform two functions:
Conversion of ultraviolet and visible light photons into
an electrical signal
Signal amplification, on the order of millions to billions
Photomultiplier tubes (PMT)
Main Components
A PMT consists of
 an evacuated glass tube containing a photocathode
 typically 10 to 12 electrodes called dynodes (which eject
additional electrons after being struck by an electron. Multiple
dynodes result in 106 or more signal enhancement
 Anode-collector (accumulates all electrons produced from final
dynode)
 Resistor
The collected current passes through a resistor to generate
voltage pulse
Photomultiplier tubes
 Electrons emitted by the photocathode are attracted to the first dynode and are
accelerated to kinetic energies equal to the potential difference between the
photocathode and the first dynode
 When these electrons strike the first dynode, about 5 electrons are ejected from the
dynode for each electron hitting it
 These electrons are attracted to the second dynode, and so on, finally reaching the
anode
Reflector
Glass
Emitted
electron
Dynodes
(secondary e- emission)
+200V
Anode
+600V
+50V
Coaxial out
Scintillation photon
+400V
Photocathode
+800V
Gain ~ 106 - 107
Photomultiplier tubes
Principle of detection
E  hf  W
In the photocathode the photons are converted
into photoelectrons via the photoelectric effect:
E=kinetic energy of the photo-electron
f= frequency of the incident photon
W=working function
The photocathode must be made of a low W
material to maximize the emission of photolectrons
Quantum efficiency of the PMT’s photocathode is the
probability for an electron creation per striking photon
QE ( ) 
number of emitted photoelectrons
number of incident photons
Typical value: 20-25%
Photomultiplier tubes
 The total amplification of the PMT is the product of the individual
amplifications at each dynode
 If a PMT has ten dynodes and the amplification at each stage is 5, the
total amplification will be approximately 10,000,000
 The amplification can be adjusted by changing the voltage applied to
the PMT
Photomultiplier tubes
 The intensity (I) of light transported in the scintillator and the light guide is
attenuated by atomic absorption and scattering in the reflecting surface.
The reduction is a function of the distance l and the light wavelength λ:
I (l ,  )  I (0,  )e
l
I ph (  )
 The number of photoelectrons
produced by the photocathode:
Ι(0,λ)=initial light intensity
Iph(λ)=Photon attenuation length @ λ
n pe  n ph  I (L, )QE() f ()dλ
Nph=number of photons produced in the
scintillator of length L
F(λ)=fraction of light trapped
Photomultiplier tubes
A numerical example:
• Scintillator thickness=1cm
• Light output=25%
•1 MeV deposited energy corresponds to the production of 104 γ
•Α minimum ionizing particle produces 2 MeV/cm
2X104 γ are emitted in 1cm of the scintillator
I (l ,  )  I (0,  )e
l
I ph (  )
Assume
• quantum efficiency 20%
• Optical collection efficiency 50%
The number of photoelectrons
follow Poisson statistics
If the attenuation length =250cm the
intensity loss is 0.4% in 1 cm
The number of photoelectrons
coming off the photocathode:
Npe=360
n nen
P ( n) 
n!
Photomultiplier tubes
A numerical example:
Anode: 0 V
Cathode: -1400V 2200V
Multiplication factor M between the first
stage of the n-dynodes and the anode:
For M=107 the charge
collected at the anode is:
M  1   2   3     n
Q  eM  1.6 1019C 107  1.6 pC
Typically the charge is collected within 5μs 
the current in the anode is:
dq
i
 0.32mA
dt
Commonly Used Scintillators
Density
[g/cm3]
Emission
Max [nm]
Decay
Constant
(1)
Refractive
Index (2)
Conversion
Efficiency
(3)
Hygroscopic
NaI(Tl)
3.67
415
0.23 ms
1.85
100
yes
CsI(Tl)
4.51
550
0.6/3.4 ms
1.79
45
no
CsI(Na)
4.51
420
0.63 ms
1.84
85
slightly
CsI
undoped
4.51
315
16 ns
1.95
4-6
no
CaF2
(Eu)
3.18
435
0.84 ms
1.47
50
no
6LiI
(Eu)
4.08
470
1.4 ms
1.96
35
yes
6Li
glass
2.6
390 - 430
60 ns
1.56
4-6
no
4.64
390
3 - 5 ns
1.48
5-7
yes
Material
CsF
(1) Effective average decay time For g-rays.
(2) At the wavelength of the emission maximum.
(3) Relative scintillation signal at room temperature for g-rays when coupled to a
photomultiplier tube with a Bi-Alkalai photocathode.
Commonly Used Scintillators
Material
Density
[g/cm3]
Emission
Maximum
[nm]
Decay
Constant
(1)
Refractive
Index (2)
Conversion
Efficiency
(3)
Hygros
copic
BaF2
4.88
315
220
0.63 ms
0.8 ns
1.50
1.54
16
5
no
YAP (Ce)
5.55
350
27 ns
1.94
35 - 40
no
GSO (Ce)
6.71
440
30 - 60 ns
1.85
20 - 25
no
BGO
7.13
480
0.3 ms
2.15
15 - 20
no
CdWO4
7.90
470 / 540
20 / 5 ms
2.3
25 - 30
no
Plastics
1.03
375 - 600
1 - 3 ms
1.58
25 - 30
no
(1) Effective agerage decay time For g-rays.
(2) At the wavelength of the emission maximum.
(3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier
tube with a Bi-Alkalai photocathode.
Compare Scintillators
Choice of a certain scintillation crystal in a radiation detector depends strongly on the application.
Material
Important Properties
Major Applications
NaI(Tl)
Very high light output,
energy resolution
CsI(Tl)
Noon-hygroscopic, rugged, long
wavelength emission
Particle and high energy physics, general
radiation detection, photodiode readout,
phoswiches
CsI(Na)
High light output, rugged
Geophysical, general radiation detection
CsI
undoped
Fast, non-hygroscopic, radiation
hard, low light output
Physics (calorimetry)
CaF2(Eu)
Low Z, high light outut
b detection, a, b phoswiches
CdWO4
Very high density, low afterglow,
radiation hard
DC measurement of X-rays (high intensity),
readout with photodiodes, Computerized
Tomography (CT)
Plastics
Fast, low density and Z, high light
output
Particle detection, beta detection
good
General
scintillation
counting,
health
physics, environmental monitoring, high
temperature use
ScintillatorsNeutron detection
Neutrons do not produce ionization
directly in scintillation crystals
Neutrons can be detected through
their interaction with the nuclei of a
suitable element.
In 6LiI(Eu) crystals
neutrons interact with 6Li
nuclei to produce an alpha
particle and 3H which both
produce scintillation light
that can be detected.
Timing applications
Application: measurement of time intervals at the nanosecond level
System that uses of the excellent timing capabilities of scintillators: time of flight (TOF)
A time of flight (TOF) detector can discriminate between a lighter and a heavier
elementary particle of same momentum using their time of flight between two
scintillators.
The first of the scintillators activates a clock upon being hit while the other stops
the clock upon being hit.
Time of flight
difference:
P=momentum
E=energy
x x
t 
v bc
pc
b

E
x(( mc) 2  p 2 )1 / 2
t
pc
pc
mc  pc 
2 2
2 2
Consider two particles with same
momentum p and different masses
Timing applications-cont
t12
 t22

x 2 (( m1c) 2  p 2 )
( pc)
2

x 2 ((m2c) 2  p 2 )
( pc)
2

x 2 (m12  m22 )
p2
t12  t22  (t1  t2 )(t1  t2 )
x 2 (m12  m22 )
t1  t2 
(t1  t2 ) 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
Timing applications
Measure the Flight Time between two Scintillators (TOF)
Stop
Start
Disc
Disc
TDC
Particle Trajectory
Literature
 Crismatec, “Catalogue of Scintillation Detectors”, Saint-Gobain (1992);
 C. D’Ambrosio et al., “Low dose-rate irradiation set-up for scintillating
crystals”, NIM A, V. 388,1-2, (1997);
 C. D’Ambrosio et al., “A HPMT based set-up to characterize scintillating
crystals”,NIM A, V. 434, 2-3, (1999);
 M. Moszynski, “Inorganic scintillation detectors in γ-ray spectrometry”, NIM A,
V.505, 1-2, (2003);
 J. B. Birks, “Scintillation counters”, Pergamon Press, (1954) London;
 I. B. Berlmann, “Handbook of fluorescence spectra of aromatic molecules” ;
2nd ed.,Academic Press, (1971) New York
 H. Leutz,”Scintillating Fibres”, NIM A, V. 364, (1995) 422;
 RD7, DRDC Status Reports, CERN, Geneva;
 ATLAS Technical Design Report, CERN, (1999);
C. D’Ambrosio and H. Leutz, “Hybrid photon detectors” NIM A, V.501, 2-3,
(2003);