Download Radiation Detection Instrumentation Fundamentals

Radiation Detection
Instrumentation Fundamentals
Radiation Detection
Instrumentation Fundamentals
• Includes
– Basic operation principles of different types of
radiation detectors;
– Physical processes underlying the principles
of operation of these devices, and
– Comparing and selecting instrumentation best
suited for different applications.
General Principles of
Radiation Detection
Outline
•
•
•
•
Gas-Filled Detectors
Scintillation Detectors
Solid State Detectors
Others
Gas-Filled Detectors Components
• Variable voltage source
• Gas-filled counting chamber
• Two coaxial electrodes well insulated from each
other
• Electron-pairs
–
–
–
–
produced by radiation in fill gas
move under influence of electric field
produce measurable current on electrodes, or
transformed into pulse
Gas- Filled Detectors - one
example
wall
End
window
Or wall
fill gas
Anode (+)
Cathode (-)
Output
R
or
A
Indirect Ionization Process
wall
Incident gamma photon
Direct Ionization Process
wall
beta (β-)
Incident
charged
particle
e
e
-
e
-
e
-
-
e
e
-
-
e
e
-
-
Competing Processes recombination
+
e
Output
-
+
e
-
R
Voltage versus Ions Collected
Number
of Ion
Pairs
collected
Recombination
region
Ionization region
Saturation Voltage
100 % of initial
ions are collected
Voltage
Saturation Current
• The point at which 100% of ions begin to be
collected
• All ion chambers operate at a voltage that
produces a saturation current
• The region over which the saturation current is
produced is called the ionization region
• It levels the voltage range because all charges
are already collected and rate of formation is
constant
Observed Output: Pulse Height
• Ions collected
• Number of ionizations relate to specific
ionization value of radiation
• Gas filled detectors operate in either
– current mode
• Output is an average value resulting from detection
of many values
– pulse mode
• One pulse per particle
Pulse Height Variation
Alpha
Particles
Pulse
Height
Beta
Particles
Gamma Photons
Detector Voltage
Ionization Region Recap
• Pulse size depends on # ions produced in
detector.
• No multiplication of ions due to secondary
ionization (gas amplification is unity)
• Voltage produced (V) = Q/C
• Where
– Q is total charge collected
– C is capacitance of the ion chanber
Ionization Chambers, continued
• Chamber’s construction determines is operating
characteristics
• Physical size, geometry, and materials define its
ability to maintain a charge
• Operates at a specific voltage
• When operating, the charge collected due to
ionizing events is
Q = CΔV
Ionization Chambers, continued
• The number of ions (N) collected can
be obtained once the charge is
determined:
N=Q/k
• Where k is a conversion factor
– (1.6 x 10-19C/e)
Other Aspects of Gas-Filled
Detectors
• Accuracy of measurement
– Detector Walls composed of air equivalent material or
– tissue equivalent
• Wall thickness
– must allow radiation to enter/ cause interactions
– alpha radiation requires thin wall (allowed to pass)
– gammas require thicker walls (interactions needed)
• Sensitivity
– Air or Fill gas Pressure
– see next graph
Current vs. Voltage for Fill Gases in
a Cylindrical Ion Chamber
100
Relative
10
Current
(%)
Air at high pressure
Helium at high pressure
Air at low pressure
1.0
Helium at low pressure
0.1
Applied Voltage (volts)
Correcting Ion Chambers for T, P
• Ion chambers operate in pressurized
mode which varies with ambient conditions
• Detector current (I) and exposure rate X
are functions of gas temperature and
pressure as well as physical size of
detector.
Correcting Ion Chambers for T, P
• Detector current (I) and exposure rate (X)
related by:
•
•
•
•
 Tstp  P  
X

I  kρV
P 
T

 stp 
k, conversion factor
ρ detector gas density
V detector volume
STP standard temp and pressure (273K, 760
torr (1 atm)



Voltage
Continuous Discharge Region
Geiger-Mueller Region
Limited Proportional
Region
Proportional Region
Ionization Region
Recombination Region
Pulse Height
Operating Regions of Gas-Filled
Detectors
Values of k, Conversion Factor
• Calculated as
• (2.58 x 10- 4C/kg-R)(1 h / 3600 s)( 1 A s / C)
• Yields 7.17 x 10- 8 A-h/R-kg
Examples
Proportional Counters
• Operates at higher voltage than ionization
chamber
• Initial electrons produced by ionization
– are accelerated with enough speed to cause
additional ionizations
– cause additional free electrons
– produces more electrons than initial event
• Process is termed: gas amplification
Recombination Region
Pulse Height
Pulse-Height Versus Voltage
Ionization
Region
Proportional Region


Voltage
Distinguishing Alpha & Beta
• Proportional counters
– can distinguish between different radiation types
– specifically alpha and beta-gamma
• Differential detection capability
– due to size of pulses produced by initial ionizing
events
– requires voltage setting in range of 900 to 1,300 volts
• alpha pulses above discriminator
• beta/gamma pulses too small
Alpha & Beta-Gamma Plateau
Ionization
Current
Beta-Gamma Plateau
Alpha Plateau
Detector Voltage
Gas Flow Proportional Counters
• Common type of proportional counter
• Fixed radiation detection instrument used in
counting rooms
• Windowed or windowless
• Both employ 2 geometry
– essentially all radiation emitted from the surface of the
source enters active volume of detector
• Windowless
– used for alpha detection
Gas-Flow Proportional Counter
Gas-Flow Proportional Counter
Fill gas
outlet
Fill gas
inlet
Detector
anode
sample
Sample planchet
(windowoptional)
O-ring
Gas Flow Proportional,
continued
• Fill gas
– selected to enhance gas multiplication
– no appreciable electron attachment
– most common is P-10 (90% Argon and 10%
methane)
Geiger Mueller Detectors
• Operate at voltages above proportional
detectors
• Each primary ionization
– produces a complete avalanche of ions throughout
the detector volume
– called a Townsend Avalanche
– continues until maximum number of ion pairs are
produced
– avalanche may be propagated by photoelectrons
– quenching is used to prevent process
Geiger Mueller Detectors,
continued
• No proportional relationship between
energy of incident radiation and number of
ionizations detected
• A level pulse height occurs throughout the
entire voltage range
Advantages/Disadvantages of GasFilled Detectors
• Ion Chamber: simple, accurate, wide range,
sensitivity is function of chamber size, no dead
time
• Proportional Counter: discriminate hi/lo LET,
higher sensitivity than ion chamber
• GM Tube: cheap, little/no amplification, thin
window for low energy; limited life
Points to Remember for Gas-filled
Detectors
• Know operating principles of your detector
– Contamination only?
– High range?
– Alpha / beta detection?
– Dose rate?
– Alpha/beta shield?
Points to Remember for Gas-filled
Detectors
• Power supply requirements
– Stable?
– Batteries ok?
• Temperature, pressure correction
requirements
• Calibration
– Frequency
– Nuclides
Issues with Gas Filled Detectors: Dead
Time
• Minimum time at which detector recovers
enough to start another avalanche (pulse)
• The dead time may be set by:
– limiting processes in the detector, or
– associated electronics
• “Dead time losses”
– can become severe in high counting rates
– corrections must be made to measurements
• Term is used loosely - beware!
Issues with Gas Filled Detectors:
Recovery Time
• Time interval between dead time and full
recovery
• Recovery Time = Resolving time- dead
time
Issues with Gas Filled Detectors:
Resolving Time
• Minimum time interval that must elapse
after detection of an ionizing particle
before a second particle can be detected.
Correcting for Dead Time
• For some systems (GMs) dead time may be
large.
• A correction to the observed count rate can
be calculated as:
• Where
Ro
Rc 
1  R0 T
– T is the resolving time
– R0 is the observed count rate and
– RC is the corrected count rate
Relationship among dead time,
recovery time, and resolving time
0
100
200
300
Pulse
Height
Dead
Time
Recovery time
Resolving time
Time, microseconds
400
500
Geiger Tube as Exposure Meter
• “Exposure” is the parameter measuring
the ionization of air.
• Geiger tube measures ionization pulses
per second - a “count rate”.
• The number of ionizations in the Geiger
tube is a constant for a particular energy
but is energy dependent.
COMPENSATED GEIGER DOSE
RATE METERS
• GMs have a high sensitivity but are very
dependent upon the energy of photon radiations.
• The next graph illustrates the relative response
(R) of a typical GM vs photon energy (E).
• At about 60 keV the response reaches a
maximum which may be thirty times higher than
the detector’s response at other radiation
energies.
Energy Response of GM – Uncompensated
R
20
1.2
1.0
0.8
10
100
1000
E, keV
COMPENSATED GEIGER DOSE RATE
METERS
• Detector’s poor energy response may be
corrected by adding a compensation sheath
– Thin layers of metal are constructed around the GM to
attenuate the lower photon energies, where the
fluence per unit dose rate is high, to a higher degree
than the higher energies.
– The modified or compensated response, shown as a
dashed line on the next graph, may be independent of
energy within ± 20% over the range 50 keV to 1.25
MeV.
– Compensation sheaths also influence an instrument’s
directional (polar) response and prevent beta and
very low energy photon radiations from reaching the
Geiger tube.
Energy Response of GM – Uncompensated
and Compensated
R
20
1.2
1.0
0.8
10
100
1000
E, keV
Example Polar Response
Example of Compensated GM
RadEye component
RadEye
• Pocket meter
– low power components
– automatic self checks
– essential functions accessed while wearing protective
gloves.
– Alarm-LED can be seen while the instrument is worn
in a belt-holster.
– Instrument also equipped with a built in vibrator and
an earphone-output for silent alarming or use in very
noisy environment.
• Number of optional components
RadEye
• Options
– RadEye PRD - High Sensitivity Personal
Radiation Detector
• The RadEye PRD is 5000 - 100000 times more
sensitive than typical electronic dosimeter.
• The RadEye PRD uses Natural Background
Rejection (NBR) technology. It is the only
instrument of its type and size to achieve this.
• Probably a plastic scintillator – more about this
later
RadEye
• Options
– RadEye G - Wide Range Gamma Survey Meter for
Personal Radiation Protection
• linearity over 6 decades of radiation intensity: from
background level to 5 R/h
• overrange indication up to 1000 R/h.
• RadEye G incorporates a large energy compensated GMtube for dose rate measurement for gamma and x-ray.
– NBR = Natural Background Rejection
The NBR measurement technology has been
developed by Thermo Electron for the supression of
alarms caused by variations of the natural
background.
SCINTILLATION
DETECTORS
Scintillators
• Emit light when irradiated
– promptly (<10-8s)
• fluorescence
– delayed (>10-8s)
• phosphorescence
• Can be
–
–
–
–
–
liquid
solid
gas
organic
inorganic
Energy
Basis of Scintillation - Energy
Structure in an Atom
Excited state
Ground state, last filled
(outer) orbital
Energy
Basis of Scintillation - Energy
Structure in a Molecule
EA1
Excited state
A1
EB1
EB0
EA0
Ground state
B1
Bo
Ao
Interatomic distance
Scintillator Properties
• A large number of different scintillation crystals
exist for a variety of applications.
• Some important characteristics of scintillators
are:
–
–
–
–
–
Density and atomic number (Z)
Light output (wavelength + intensity)
Decay time (duration of the scintillation light pulse)
Mechanical and optical properties
Cost
http://www.scionixusa.com/pages/navbar/scin_crystals.html
Liquid scintillation counting
• Standard laboratory method for measuring radiation from betaemitting nuclides.
• Samples are dissolved or suspended in a "cocktail" containing an
aromatic solvent (historically benzene or toluene, and small amounts
of other additives known as fluors.
– Beta particles transfer energy to the solvent molecules, which in turn
transfer their energy to the fluors;
– Excited fluor molecules dissipate the energy by emitting light.
– Each beta emission (ideally) results in a pulse of light.
– Scintillation cocktails may contain additives to shift the wavelength of
the emitted light to make it more easily detected.
• Samples are placed in small transparent or translucent (often glass)
vials that are loaded into an instrument known as a liquid scintillation
counter.
• Examples
• Differences
CH3
Toluene
Energy
Organic Scintillators
EA1
Excited state
A1
Ground state
EB1
EB0
EA0
Anthracene
B1
Bo
Ao
Interatomic distance
Inorganic (Crystal) Scintillators
• Most are crystals of alkali metals (iodides)
–
–
–
–
–
NaI(Tl)
CsI(Tl)
CaI(Na)
LiI(Eu)
CaF2(Eu)
• Impurity in trace amounts
– “activator” causes luminescence
– e.g., (Eu) is 10-3 of crystal
Organic vs. Inorganic
Scintillators
• Inorganic scintillators have greater:
– light output
– longer delayed light emission
– higher atomic numbers
– than organic scintillators
• Inorganic scintillators also
– linear energy response (light output is 
energy absorbed)
Solid Scintillators
• Solids have
– Lattice structure (molecular level)
– Quantized energy levels
– Valence bands
– Conduction bands
Crystal Lattice
e-
Ge
As+
Shared
electron pair
Creation of Quantized “Bands”
Conduction Band
-
-
Eo + Eg
EF
+
+
+
Valence Band
Eo
0
Introduction of Impurities
Conduction Band
Donor
impurity
levels
Acceptor
impurity
levels
~0.01 eV
~1 eV
~ 0.01eV
Valence Band
Detecting Scintillator Output:PhotoCathode & Photomultiplier
Tubes
• Radiation interaction in scintillator produces
light (may be in visible range)
• Quantification of output requires light
amplification and detection device(s)
• This is accomplished with the:
– Photocathode
– Photomultiplier tube
• Both components are
– placed together as one unit
– optically coupled to the scintillator
Cutaway diagram of solid-fluor
scintillation detector
Cutaway diagram of solid-fluor
scintillation
detector
Photocathode
Gamma ray
Scintillation
event
Fluor crystal NaI (Tl)
Reflector housing
Photomultiplier tube
Dynodes
Photoelectrons
Major components of PM Tube
• Photocathode material
• Dynodes
– electrodes which eject additional electrons after being
struck by an electron
– Multiple dynodes result in 106 or more signal
enhancement
• Collector
– accumulates all electrons produced from final dynode
• Resistor
– collected current passed through resistor to generate
voltage pulse
Generalized Detection System
using a Scintillator
(Crystal &
Photomultiplier)
Scaler
Detector
PreAmp
High
Voltage
Amplifier
Oscilloscope
Discriminator
MultiChannel
Analyzer
Liquid Scintillation Systems
• Used to detect low energy (ie., low range)
radiations
– beta
– alpha
• Sample is immersed in scintillant
• Provides 4  geometry
• Quenching can limit output
– chemical
– color quenching
– optical quenching
Chemical Quenching
• Dissipation of energy prior to transfer from
organic solvent to scintillator
• Reduces total light output
• Common chemical quenching agents
– Dissolved oxygen is most common
– Acids
– Excessive concentration of one component (e.g.,
primary fluor)
– Too little scintillation media
– halogenated hydrocarbons
Color Quenching
• Absorption of light photons after they are
emitted from the scintillator
• Reduces total light output
• Common color quenching agents:
– light absorbing contaminants
– blood
– urine
– tissues samples
Optical Quenching
• Absorption of light photons after they are
emitted from the scintillator liquid and
before they reach the PMT
• Reduces total light output
• Common optical quenching agents:
– fingerprints
– condensation
– dirt on the LS vials
Circuitry in LSC systems
• Shielded counting well
• Two (or more) PMT’s optically coupled to
sample well
• Coincidence circuitry to compare PMT
pulses
• Pulse Summation Circuit
– adds signals from PMTs
– gates single pulse to amplifier
– summation circuit doubles height of signal
Coincidence Circuitry
• Used to reduce noise
• Limit thermionic emissions
– spontaneous emissions from within the PMT
• Directly opposing PMT tubes
–
–
–
–
–
connected to coincidence circuit
gated outputs from both tubes
only simultaneous signal from both will be accepted
only one signal is not accepted
simultaneous signals are summed
• Applied to Liquid Scintillation Systems
Coincidence & Anticoincidence
Circuitry
• Sometimes desirable to discard pulses due to
some radiations & accept only those from a
single type of particle.
• Examples:
– detection of pair-production events (accept only
simultaneous detection of 180° apart photons)
– detection of internal conversion electrons
• radioisotopes with IC electrons emit gammas & X-rays.
• A single detector counts IC and compton electrons.
• Use X-rays that are emitted simultaneously with IC & block
Compton events
A simple coincidence circuit
Amplification
Timing
Detector
Source
Multi-channel
Analyzer
Coincidence
Unit
Detector
Gate
Scaler
Amplification
Timing
After Tsoulfanidis, 1995
Basic LSC System
Beckman
LS 6500 Liquid
Scintillation Counting
System.
Single & summed pulse spectra
With pulse summation
Counts/
Min
Without
pulse summation
Pulse Height
Correcting for Quench
• Quench correction
– any quenching that occurs in sample results in shift of
pulse height spectrum toward lower values
• Techniques
– purge sample with N2, CO2, or Ar (removes O2
chemical quench
– bleach or decolorize sample (reduces color quench)
– handle LSC vials by top/bottom & wiping vials clean
prior to counting (reduces optical quenching)
Alternative Methods
• Channel ratio method
– two energy windows established
– known amount of radioactivity is added to varying
concentrations of quenching agent
– ratio of net counts in upper channel over lower
channel vs quench correction is plotted
• Disadvantage
– low count rates require longer counting times
– multiple calibration curves may be required for
• range
• quenching agents
Alternative Methods
• Internal standard method
–
–
–
–
–
older technique
sample is counted
known quantity of radioisotope is added
sample recounted
Efficiency = (cpm(std+sample) – cpm(sample))/dpm(std)
• Most accurate method
– requires ability to add same amount of
radionuclide each time
– more costly & time consuming
Alternative Methods
• External standard method
– relies on gamma source (226Ra or 133Ba) adjacent to sample
– two sets of calibration curves are derived
– sample standard count is plotted versus amount of quench
agent
– Net External Counts - [External & Sample Std cpm] [Sample Standard cpm]
• Disadvantages
– least accurate of available methods
– samples must be counted twice
– sample uniformly dispersed in counting vials
Pulse Height Discrimination
• Light produced per disintegration of a
radioactive atom:
– is related to particle type (alpha, beta, gamma),
– and energy (keV - MeV).
• Pulse height increases with energy
• Example (follows) beta emitters of varying
energies:
– 3H, max 18.6 keV
– 14C, max 156 keV
– 32P, max 1.71 MeV
Pulse Height Discrimination
for three common beta emitters
3H
14C
32P
Count
Rate
Pulse Height
Background & Efficiency
Checks on LSC
• Essential - LSC’s are essentially proportional
counters; change in potential impacts gain
• Efficiency depends on several variables:
– temperature
– quenching ( determine counting efficiency for every
sample)
• Background & efficiency checks needed with
every run
– contamination
– efficiency changes
Field Applications for Liquid and Solid
Scintillation Counters
• Solid Scintillators
– in-situ measurement of low to high energy gammas
– laboratory systems
• spectroscopy
• SCA or MCA mode
• Liquid Scintillators
– wipe tests
– contaminants in solids (concrete)
– contaminants in aqueous/organic liquids
Selecting Scintillators - Density and
Atomic number
• Efficient detection of gamma-rays requires
material with a high density and high Z
• Inorganic scintillation crystals meet the
requirements of stopping power and
optical transparency,
– Densities range from roughly 3 to 9 g/cm3
– Very suitable to absorb gamma rays.
– Materials with high Z-values are used for
spectroscopy at high energies (>1 MeV).
Linear
Attenuation of
NaI
Relative Importance of Three Major
Interaction Mechanisms
• The lines show the values of Z and hv for which the two neighboring
effects are just equal
Light output of Scintillators
• Scintillation material with a high light
output is preferred for all spectroscopic
applications.
• Emission wavelength should be matched
to the sensitivity of the light detection
device that is used (PMT of photodiode).
Decay time
• Scintillation light pulses (flashes) are usually
characterized by a fast increase of the intensity
in time (pulse rise time) followed by an
exponential decrease.
• Decay time of a scintillator is defined by the time
after which the intensity of the light pulse has
returned to 1/e of its maximum value.
• Most scintillators are characterized by more than
one decay time and usually, the effective
average decay time is given
• The decay time is of importance for fast counting
and/or timing applications
Mechanical and Optical Properties
• NaI(Tl) is one of the most important scintillants.
– Hygroscopic
– Can only be used in hermetically sealed metal containers
• Some scintillation crystals may easily crack or cleave under
mechanical pressure
• CsI is “plastic” and will deform.
• Important aspects of commonly used scintillation materials are listed
on the next 2 slides.
• The list is not exhaustive, and each scintillation crystal has its own
specific application.
– For high resolution spectroscopy, NaI(Tl), or CsI(Na) (high light
output) are normally used.
– For high energy physics applications, the use of bismuth
germanate Bi4Ge3O12 (BGO) crystals (high density and Z)
improves the lateral confinement of the shower.
– For the detection of beta-particles, CaF2(Eu) can be used
instead of plastic scintillators (higher density).
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 -rays.
(2) At the wavelength of the emission maximum.
(3) Relative scintillation signal at room temperature for -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 -rays.
(2) At the wavelength of the emission maximum.
(3) Relative scintillation signal at room temperature for -rays when coupled to a photomultiplier tube
with a Bi-Alkalai photocathode.
Afterglow
• Defined as the fraction of scintillation light still present for
a certain time after the X-ray excitation stops.
– Originates from the presence of millisecond to even hour long
decay time components.
– Can be as high as a few % after 3 ms in most halide scintillation
crystals .
– CsI(Tl) long duration afterglow can be a problem for many
applications.
– Afterglow in halides is believed to be intrinsic and correlated to
certain lattice defects.
• BGO and Cadmium Tungstate (CdWO4) crystals are
examples of low afterglow scintillation materials
Scintillators - Neutron Detection
• Neutrons do not produce ionization directly
in scintillation crystals
• Can be detected through their interaction
with the nuclei of a suitable element.
– 6LiI(Eu) crystal -neutrons interact with 6Li
nuclei to produce an alpha particle and 3H
which both produce scintillation light that can
be detected.
– Enriched 6Li containing glasses doped with
Ce as activator can also be used.
Neutron Detection
Neutron Detection
• Conventional neutron meters surround a
thermal neutron detector with a large and
heavy (20 lb) polyethylene neutron
moderator.
• Other meters utilizes multiple windows
formed of a fast neutron scintillator (ZnS in
an epoxy matrix), with both a thermal
neutron detector and a photomultiplier
tube.
Radiation Damage in Scintillators
• Radiation damage results inchange in
scintillation characteristics caused by prolonged
exposure to intense radiation.
• Manifests as decrease of optical transmission of
a crystal
– decreased pulse height
– deterioration of the energy resolution
• Radiation damage other than activation may be
partially reversible; i.e. the absorption bands
disappear slowly in time.
Radiation Damage in Scintillators
• Doped alkali halide scintillators such as NaI(Tl)
and CsI(Tl) are rather susceptible to radiation
damage.
• All known scintillation materials show more or
less damage when exposed to large radiation
doses.
• Effects usually observed in thick (> 5 cm)
crystals.
• A material is usually called radiation hard if no
measurable effects occur at a dose of 10,000
Gray. Examples of radiation hard materials are
CdWO4 and GSO.
Emission Spectra of Scintillation
Crystals
• Each scintillation material has characteristic
emission spectrum.
• Spectrum shape is sometimes dependent on the
type of excitation (photons / particles).
• Emission spectrum is important when choosing
the optimum readout device (PMT /photodiode)
and the required window material.
• Emission spectrum of some common scintillation
materials shown in next two slides.
Emission Spectra of Scintillators
Emission Spectra of Scintillators
Temperature Influence on the
Scintillation Response
• Light output (photons per MeV gamma) of most
scintillators is a function of temperature.
– Radiative transitions, responsible for the production of
scintillation light compete with non-radiative
transitions (no light production).
– In most light output is quenched (decreased) at higher
temperatures.
– An exception is the fast component of BaF2 where
intensity is essentially temperature independent.
Temperature Influence on the
Scintillation Response
http://www.scionixusa.com/pages/navbar/scin_crystals.html
Choosing a Scintillator
• Following table lists characteristics such as high density,
fast decay etc.
• Choice of a certain scintillation crystal in a radiation
detector depends strongly on the application.
• Questions such as :
– What is the energy of the radiation to measure ?
– What is the expected count rate ?
– What are the experimental conditions (temperature, shock)?
Material
Important Properties
Major Applications
NaI(Tl)
Very high light output, good
energy resolution
General scintillation counting, health
physics, environmental
monitoring, high temperature use
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
Fast, non-hygroscopic,
radiation hard, low light
output
Physics (calorimetry)
CsI
undoped
CaF2(Eu) Low Z, high light outut
 detection, ,  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
Material Important Properties
6LiI(Eu)
6Li
High neutron cross-section,
high light output
High neutron cross-section,
glass
non-hygroscopic
Major Applications
Thermal neutron detection and
spectroscopy
Thermal neutron detection
BaF2
Ultra-fast sub-ns UV emission
Positron life time studies, physics
research, fast timing
YAP(Ce
)
High light output, low Z, fast
MHz X-ray spectroscopy, synchrotron
physics
GSO(Ce High density and Z, fast,
)
radiation hard
Physics research
BGO
High density and Z
Particle physics, geophysical
research, PET, anti-Compton
spectrometers
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
PRACTICAL SCINTILLATION
COUNTERS
• Highly sensitive surface contamination probes
incorporate a range phosphors
• Examples include:
– zinc sulphide (ZnS(Ag)) powder coatings (5–10
mg·cm–2) on glass or plastic substrates or coated
directly onto the photomultiplier window for detecting
alpha and other heavy particles;
– cesium iodide (CsI(Tl)) that is thinly machined (0.25
mm) and that may be bent into various shapes;
– and plastic phosphors in thin sheets or powders fixed
to a glass base for beta detection.
PRACTICAL SCINTILLATION
COUNTERS
• Probes (A and B previous slide) and their associated
ratemeters (C) tend not to be robust.
• Photomultipliers are sensitive to shock damage and are
affected by localized magnetic fields.
• Minor damage to the thin foil through which radiation
enters the detector allows ambient light to enter and
swamp the photomultiplier.
• Cables connecting ratemeters and probes are also a
common problem.
• Very low energy beta emitters (for example 3H) can be
dissolved in liquid phosphors in order to be detected.
43-93 Alpha/Beta Scintillator
• The Model 43-93 is a 100 cm² dual
phosphor alpha/beta scintillator that is
designed to be used for simultaneously
counting alpha and beta contamination
43-93 Alpha/Beta Scintillator
•
•
•
•
INDICATED USE: Alpha beta survey
SCINTILLATOR: ZnS(Ag) adhered to 0.010" thick plastic scintillation
material
WINDOW: 1.2 mg/cm² recommended for outdoor use
WINDOW AREA:
– Active - 100 cm²
– Open - 89 cm²
•
•
•
•
•
EFFICIENCY (4pi geometry): Typically 15% - Tc-99; 20% - Pu-239; 20% S-90/Y-90
NON-UNIFORMITY: Less than 10%
BACKGROUND: Alpha - 3 cpm or less
Beta - Typically 300 cpm or less (10 µR/hr field )
CROSS TALK:
– Alpha to beta - less than 10%
– Beta to alpha - less than 1%
43-93 Alpha/Beta Scintillator
• COMPATIBLE INSTRUMENTS: Models 2224, 2360
• TUBE: 1.125"(2.9cm) diameter magnetically shielded
photomultiplier
• OPERATING VOLTAGE: Typically 500 - 1200 volts
• DYNODE STRING RESISTANCE: 100 megohm
• CONNECTOR: Series “C” (others available )
• CONSTRUCTION: Aluminum housing with beige
polyurethane enamel paint
• TEMPERATURE RANGE: 5°F(-15°C) to 122°F(50°C)
May be certified to operate from -40°F(-40°C) to
150°F(65°C)
• SIZE: 3.2"(8.1 cm)H X 3.5"(8.9 cm)W X 12.2"(31 cm)L
• WEIGHT: 1 lb (0.5kg)
44-2 Gamma Scintillator
• The Model 44-2 is a 1" X 1" NaI(Tl)
Gamma Scintillator that can be used with
several different instruments including
survey meters, scalers, ratemeters, and
alarm ratemeters
44-2 Gamma Scintillator
•
•
•
•
•
•
•
•
•
•
•
•
INDICATED USE: High energy gamma detection
SCINTILLATOR: 1" (2.5 cm) diameter X 1" (2.5 cm) thick sodium iodide
(NaI)Tl scintillator
SENSITIVITY: Typically 175 cpm/microR/hr (Cs-137)
COMPATIBLE INSTRUMENTS: General purpose survey meters,
ratemeters, and scalers
TUBE: 1.5:(3.8cm) diameter magnetically shielded photomultiplier
OPERATING VOLTAGE: Typically 500 - 1200 volts
DYNODE STRING RESISTANCE: 100 megohm
CONNECTOR: Series "C" (others available )
CONSTRUCTION: Aluminum housing with beige polyurethane enamel
paint
TEMPERATURE RANGE: -4° F(-20° C) to 122° F(50° C)
May be certified for operation from -40° F(-40° C) to 150° F(65° C)
SIZE: 2" (5.1 cm) diameter X 7.3" (18.5 cm)L
WEIGHT: 1 lb (0.5kg)
Scintillation Detectors
• Best:
– Measure low gamma dose rates
• Also:
– Measure beta dose rates (with corrections)
• However:
– Somewhat fragile and expensive
• CANNOT:
– Not intended for detecting contamination, only
radiation fields
Semi-Conductor Detectors
Idealized Gamma-Ray
Spectrum in NaI
theoretical
Counts
per
Energy
Interval
Actual
Energy
Eo
Components of Spectrum
Counts
per
Energy
Interval
Backscatter
Peak
X-ray
Peak
Compton edge
Annihilation
Peak
Energy
Eo
Photopeak
NaI(Tl) vs. HPGE
NaI(Tl) vs. HPGE
Semiconductor Detectors
• Solids have
–
–
–
–
lattice structure (molecular level)
quantized energy levels
valence bands
conduction bands
• Semiconductors have lattice structure
– similar to inorganic scintillators
– composed of Group IVB elements
– ability to easily share electrons with adjoining atoms
Crystal Lattice
e-
Ge
As+
Shared
electron pair
Basic Nature of
Semiconductors
• Schematic view of lattice of Group IVB element Si
• Dots represent electron pair bonds between the Si
atoms
Si
Si
Si
Si
Si
Si
Basic Nature, cont’d
• Schematic diagram of energy levels of crystalline
Si.
Energy
Conduction Band
1.08 eV
Forbidden Gap
Valence Band
• Pure Si is a poor conductor of electricity
Basic Nature, cont’d
• Schematic view of lattice of Group IV element
Si, doped with P (Group VB) as an impurity –
note extra electron
Si
Si
Si
P
Si
Si
Basic Nature, cont’d
• Schematic diagram of disturbed energy levels of
crystalline Si.
Conduction Band
Energy
0.05 eV Donor level
Valence Band
• Si with Group V impurities like P is said to be an ntype silicon because of the negative charge carriers
(the electrons)
Basic Nature, cont’d
• Schematic view of lattice of Group IV element
Si, doped with B (Group IIIB) as an impurity –
note hole in electron orbital
Si
Si
Si
B
Si
Si
Basic Nature, cont’d
• Schematic diagram of disturbed energy levels of
crystalline Si with B impurity.
Energy
Conduction Band
0.08 eV Acceptor level
Valence Band
• Si with Group III impurities is said to be a p-type silicon
because of the positive charge carriers (the holes)
Occupation of energy states for
n and p-type semiconductors
As donor
impurity
levels
Ga acceptor
impurity
levels
Conduction Band
0.013 eV
0.011eV
0.67 eV
Valence Band
After Turner
Operating Principles of
Semiconductor detectors
• Si semiconductor is a layer of p-type Si in contact with ntype Si.
• What happens when this junction is created?
– Electrons from n-type migrate across junction to fill holes
in p-type
– Creates an area around the p-n junction with no excess of
holes or electrons
– Called a “depletion region”
• Apply (+) voltage to n-type and (-) to p-type:
– Depletion region made thicker
– Called a “reverse bias”
Energy-level diagram for n-p
junction
Conduction
Band
n-type
Junction
region
p-type
Valence
Band
After Turner
Detector specifics
• Depletion region acts as sensitive volume of
the detector
• Passage of ionizing radiation through the
region
–
–
–
–
–
Creates holes in valence band
Electrons in conduction band
Electrons migrate to positive charge on n side
Holes migrate to negative voltage on p side
Creates electrical output
• Requires about 3.6 eV to create an electron
hole pair in Si
Detector Specifics, cont’d
• Reverse bias n-p junction is good detector
– Depletion region
• Has high resistivity
• Can be varied by changing bias voltage
– Ions produced can be quickly collected
– Number of ion pairs collected is proportional to
energy deposited in detector
• Junction can be used as a spectrometer
• Types of detectors:
–
–
–
–
HPGe
GeLi (lithium drifted detectors)
Surface barrier detectors
Electronic dosimeters
SOLID STATE DETECTORS RECAP
• Solid state detectors utilize semiconductor
materials.
• Intrinsic semiconductors are of very high purity
and extrinsic semiconductors are formed by
adding trace quantities (impurities) such as
phosphorus (P) and lithium (Li) to materials such
as germanium (Ge) and silicon (Si).
• There are two groups of detectors:
– junction detectors and bulk conductivity detectors.
SOLID STATE DETECTORS
• Junction detectors are of either
– diffused junction or
– surface barrier type:
– an impurity is either diffused into, or spontaneously oxidized
onto, a prepared surface of intrinsic material to change a layer of
‘p-type’ semiconductor from or to ‘n-type’.
• When a voltage (reverse bias) is applied to the surface barrier
detector it behaves like a solid ionization chamber.
• Bulk conductivity detectors are formed from intrinsic semiconductors
of very high bulk resistivity (for example CdS and CdSe).
• They also operate like ionization counters but with a higher density
than gases and a ten-fold greater ionization per unit absorbed dose.
• Further amplification by the detector creates outputs of about one
microampere at 10 mSv·h–1
Solid State Counters
– A - very thin metal
(gold) electrode.
– P - thin layer of ptype semiconductor.
– D - depletion region,
3–10 mm thick
formed by the
voltage, is free of
charge in the absence
of ionizing radiations.
– N - n-type
semiconductor.
– B - thin metal
electrode which
provides a positive
potential at the n-type
semiconductor.
PRACTICAL SOLID STATE
DETECTORS
• The main applications for semiconductor detectors are in
the laboratory for the spectrometry of both heavy
charged (alpha) particle and gamma radiations.
• However, energy compensated PIN diodes and special
photodiodes are used as pocket electronic (active)
dosimeters.
– PIN diode: Acronym for positive-intrinsic-negative diode.
– A photodiode with a large, neutrally doped intrinsic region
sandwiched between p-doped and n-doped semiconducting
regions.
– A PIN diode exhibits an increase in its electrical conductivity as a
function of the intensity, wavelength, and modulation rate of the
incident radiation. Synonym PIN photodiode.
PIN Diodes
• Ordinary Silicon PIN photodiodes can serve as detectors for X-ray
and gamma ray photons. The detection efficiency is a function of the
thickness of the silicon wafer. For a wafer thickness of 300 microns
(ignoring attenuation in the diode window and/or package) the
detection efficiency is close to 100% at 10 KeV, falling to
approximately 1% at 150 KeV(3).
• For energies above approximately 60 KeV, photons interact almost
entirely through Compton scattering. Moreover, the active region of
the diode is in electronic equilibrium with the surrounding medium-the diode package, substrate, window and outer coating, etc., so
that Compton recoil electrons which are produced near--and close
enough to penetrate--the active volume of the diode, are also
detected.
• For this reason the overall detection efficiency at 150 KeV and
above is maintained fairly constant (approximately 1%) over a wide
range of photon energies.
• Thus, a silicon PIN diode can be thought of as a solid-state
equivalent to an ionization-chamber radiation detector.
PRACTICAL SOLID STATE
DETECTORS
• Specially combined thin and thick detectors provide the
means to identify charged particles.
– used to monitor for plutonium in air, discriminating against alpha
particles arising from natural radioactivity, and for monitoring for
radon daughter products in air.
– Small physical size and insensitivity to gamma radiation have
found novel applications: inside nuclear fuel flasks monitoring for
alpha contamination and checking sealed radium sources for
leakage.
• Bulk conductivity detectors can measure high dose rates
but with minute-long response times. A Ge(Li) detector
operated at –170°C is capable of a very high gamma
resolution of 0.5%. The temperature dependence and
high cost add to their impracticality.
Another type of Solid State /
Scintillation system
Thermoluminescent Dosimeters
Thermoluminescence
• (TL) is the ability to convert energy from
radiation to a radiation of a different wavelength,
normally in the visible light range.
• Two categories
– Fluorescence - emission of light during or immediately
after irradiation
– Not a particularly useful reaction for TLD use
– Phosphorescence - emission of light after the
irradiation period. Delay can be seconds to months.
• TLDs use phosphorescence to detect radiation.
Thermoluminescence
• Radiation moves electrons into “traps”
• Heating moves them out
• Energy released is proportional to
radiation
• Response is ~ linear
• High energy trap data is stored in TLD for
a long time
TL Process
Conduction Band (unfilled shell)
-
Phosphor atom
Incident
radiation
Electron trap
(metastable state)
Valence Band (outermost electron shell)
TL Process, continued
Conduction Band
Thermoluminescent
photon
Phosphor atom
-
Heat Applied
Valence Band (outermost electron shell)
Output – Glow Curves
•
•
•
•
•
•
•
•
•
A glow curve is obtained from heating
Light output from TLis not easily interpreted
Multiple peaks result from electrons in "shallow" traps
Peak results as traps are emptied.
Light output drops off as these traps are depleted.
Heating continues
Electrons in deeper traps are released.
Highest peak is typically used to calculate dose
Area under represents the radiation energy deposited in
the TLD
Trap Depths - Equate to LongTerm
Stability of Information
Time or temperature
TLD Reader Construction
To High
Voltage
DC Amp
To ground
PMT
Recorder or meter
Filter
TL material
Heated Cup
Power Supply
Advantages
• Advantages (as compared to film dosimeter
badges) includes:
– Able to measure a greater range of doses
– Doses may be easily obtained
– They can be read on site instead of being sent away
for developing
– Quicker turnaround time for readout
– Reusable
– Small size
– Low cost
TLD Disadvantages
•
•
•
•
•
•
•
Lack of uniformity – batch calibration needed
Storage instablity
Fading
Light sensitivity
Spurious TL (cracking, contamination)
Reader instability
No permanent record
NON-TL Dosimeters
• LUXEL DOSIMETER
• "Optically Stimulated Luminescence"
(OSL) technology
• Minimum detectable dose
– 1 mRem for gamma and x-ray radiation,
– 10 mRem for beta radiation.
Non TL Dosimeters, continued
• Uses thin layer of Al2O3:C
• Has a TL sensitivity 50 times greater than
TLD-100 (LiF:Mg,Ti)
• Almost tissue equivalent.
• Strong sensitivity to light
• Thermal quenching.
• Readout stimulated using laser
• Dosimeter luminesces in proportion to
radiation dose.
Summary
• Wide range of detection equipment
available
• Understand strengths and weaknesses of
each
• No single detector will do everything
• We’ll get to selection issues in the next
two days
Suggested Reading
• Glenn F. Knoll, Radiation Detection and
Measurement, John Wiley & Sons.
• Hernam Cember, Introduction to Health
Physics, McGraw Hill.
• Nicholas Tsoulfanidis, Measurement and
Detection of Radiation, Taylor & Francis.
• C.H. Wang, D.L.Willis, W.D. Loveland,
Radiotracer Methodology in the Biological,
Environmental and Physical Sciences,
Prentice-Hall