Radiation Detection Instrumentation Fundamentals

1
Radiation Detection Instrumentation Fundamentals
2
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.

3
General Principles of Radiation Detection
4
Outline

• Gas-Filled Detectors
• Scintillation Detectors
• Solid State Detectors
• Others

5
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

6
Gas- Filled Detectors - one example
wall
fill gas
Output
Anode ()
End window Or wall
Cathode (-)
or
R
7
Indirect Ionization Process
wall
e -
e -
e -
e -
e -
e -
e -
e -
Incident gamma photon
8
Direct Ionization Process
wall
e -
beta (ß-)
e -
e -
e -
e -
e -
e -
e -
Incident charged particle
9
Competing Processes - recombination
Output
e -
e -
R
10
Voltage versus Ions Collected
Recom- bination region
Ionization region
Number of Ion Pairs collected
Saturation Voltage
100 of initial ions are collected
Voltage
11
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

12
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

13
Pulse Height Variation
Alpha Particles
Pulse Height
Beta Particles
Gamma Photons
Detector Voltage
14
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

15
Ionization Chambers, continued

• Chambers 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 C?V

16
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)

17
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

18
Current vs. Voltage for Fill Gases in a
Cylindrical Ion Chamber
Air at high pressure
100
Helium at high pressure
Relative Current ()
10
Air at low pressure
1.0
Helium at low pressure
0.1
Applied Voltage (volts)
19
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.

20
Correcting Ion Chambers for T, P

• Detector current (I) and exposure rate (X)
  related by
• k, conversion factor
• ? detector gas density
• V detector volume
• STP standard temp and pressure (273K, 760 torr (1
  atm)

21
Operating Regions of Gas-Filled Detectors
Proportional Region
Pulse Height
Ionization Region
Continuous Discharge Region
Recombination Region
Limited Proportional Region
Geiger-Mueller Region
?
?
?
Voltage
22
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

23
Examples
24
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

25
Pulse-Height Versus Voltage
Ionization Region
Proportional Region
Recombination Region
Pulse Height
?
?
Voltage
26
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

27
Alpha Beta-Gamma Plateau
Ionization Current
Beta-Gamma Plateau
Alpha Plateau
Detector Voltage
28
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

29
Gas-Flow Proportional Counter
30
Gas-Flow Proportional Counter
Fill gas outlet
Fill gas inlet
anode
(window- optional)
Detector
O-ring
sample
Sample planchet
31
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)

32
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

33
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

34
Advantages/Disadvantages of Gas-Filled 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

35
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?

36
Points to Remember for Gas-filled Detectors

• Power supply requirements
• Stable?
• Batteries ok?
• Temperature, pressure correction requirements
• Calibration
• Frequency
• Nuclides

37
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!

38
Issues with Gas Filled Detectors Recovery Time

• Time interval between dead time and full recovery
• Recovery Time Resolving time- dead time

39
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.

40
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
• T is the resolving time
• R0 is the observed count rate and
• RC is the corrected count rate

41
Relationship among dead time, recovery time, and
resolving time
100
200
300
400
500
0
Pulse Height
Dead Time
Recovery time
Resolving time
Time, microseconds
42
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.

43
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
  detectors response at other radiation energies.

44
Energy Response of GM Uncompensated
R
20
1.2
1.0
0.8
10
100
1000
E, keV
45
COMPENSATED GEIGER DOSE RATE METERS

• Detectors 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
  instruments directional (polar) response and
  prevent beta and very low energy photon
  radiations from reaching the Geiger tube.

46
Energy Response of GM Uncompensated and
Compensated
47
Example Polar Response
48
Example of Compensated GM

• RadEye component

49
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

50
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

51
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
  GM-tube for dose rate measurement for gamma and
  x-ray.
• NBR Natural Background RejectionThe NBR
  measurement technology has been developed by
  Thermo Electron for the supression of alarms
  caused by variations of the natural background.

52
SCINTILLATION DETECTORS
53
Scintillators

• Emit light when irradiated
• promptly (lt10-8s)
• fluorescence
• delayed (gt10-8s)
• phosphorescence
• Can be
• liquid
• solid
• gas
• organic
• inorganic

54
Basis of Scintillation - Energy Structure in an
Atom
Energy
Excited state
Ground state, last filled (outer) orbital
55
Basis of Scintillation - Energy Structure in a
Molecule
Energy
Excited state
A1
EA1
Ground state
EB1
B1
EB0
Bo
EA0
Ao
Interatomic distance
56
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

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Liquid scintillation counting

• Standard laboratory method for measuring
  radiation from beta-emitting 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.

58
Organic Scintillators

• Examples
• Differences

Energy
CH3
Excited state
A1
EA1
Ground state
Toluene
EB1
B1
EB0
Bo
EA0
Ao
Anthracene
Interatomic distance
59
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

60
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)

61
Solid Scintillators

• Solids have
• Lattice structure (molecular level)
• Quantized energy levels
• Valence bands
• Conduction bands

62
Crystal Lattice
e-
As
Shared electron pair
63
Creation of Quantized Bands
64
Introduction of Impurities
Conduction Band
Donor impurity levels
0.01 eV
1 eV
0.01eV
Acceptor impurity levels
Valence Band
65
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

66
Cutaway diagram of solid-fluor scintillation
detector
67
Cutaway diagram of solid-fluor scintillation
detector
Photocathode
Scintillation event
Photomultiplier tube
Gamma ray
Dynodes
Photoelectrons
Fluor crystal NaI (Tl)
Reflector housing
68
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

69
Generalized Detection System using a Scintillator
Scaler
(Crystal Photomultiplier)
Detector
Amplifier
Pre- Amp
Discriminator
Multi- Channel Analyzer
High Voltage
Oscilloscope
70
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

71
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

72
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

73
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

74
Circuitry in LSC systems

• Shielded counting well
• Two (or more) PMTs 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

75
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

76
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

77
A simple coincidence circuit
Timing
Multi-channel Analyzer
Detector
Coincidence Unit
Source
Gate
Detector
Scaler
Timing
After Tsoulfanidis, 1995
78
Basic LSC System
Beckman LS 6500 Liquid Scintillation
Counting System.
79
Single summed pulse spectra
With pulse summation
Counts/ Min
Without pulse summation
Pulse Height
80
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)

81
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

82
Alternative Methods

• Internal standard method
• older technique
• sample is counted
• known quantity of radioisotope is added
• sample recounted
• Efficiency (cpm(stdsample)
  cpm(sample))/dpm(std)
• Most accurate method
• requires ability to add same amount of
  radionuclide each time
• more costly time consuming

83
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

84
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

85
Pulse Height Discriminationfor three common beta
emitters
3H
14C
32P
Count Rate
Pulse Height
86
Background Efficiency Checks on LSC

• Essential - LSCs 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

87
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

88
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 (gt1 MeV).

89
Linear Attenuation of NaI
90
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

91
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).

92
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

93
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).

94
Commonly Used Scintillators
(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.
95
Commonly Used Scintillators
(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.
96
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

97
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.

98
Neutron Detection
99
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.

100
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.

101
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 (gt 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.

102
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.

103
Emission Spectra of Scintillators
104
Emission Spectra of Scintillators
105
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.

106
Temperature Influence on the Scintillation
Response
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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)?

108
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110
PRACTICAL SCINTILLATION COUNTERS

• Highly sensitive surface contamination probes
  incorporate a range phosphors
• Examples include
• zinc sulphide (ZnS(Ag)) powder coatings (510
  mgcm2) 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.

111
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112
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.

113
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

114
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

115
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 5F(-15C) to 122F(50C) May
  be certified to operate from -40F(-40C) to
  150F(65C)
• 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)

116
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

117
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)

118
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

119
Semi-Conductor Detectors
120
Idealized Gamma-Ray Spectrum in NaI
theoretical
Counts per Energy Interval
Actual
Energy
Eo
121
Components of Spectrum
Compton edge
Photopeak
Backscatter Peak
Counts per Energy Interval
X-ray Peak
Annihilation Peak
Energy
Eo
122
NaI(Tl) vs. HPGE
123
NaI(Tl) vs. HPGE
124
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

125
Crystal Lattice
e-
Ge
As
Shared electron pair
126
Basic Nature of Semiconductors

• Schematic view of lattice of Group IVB element Si
• Dots represent electron pair bonds between the Si
  atoms

127
Basic Nature, contd

• Schematic diagram of energy levels of crystalline
  Si.
• Pure Si is a poor conductor of electricity

Conduction Band
1.08 eV
Forbidden Gap
Energy
Valence Band
128
Basic Nature, contd

• Schematic view of lattice of Group IV element Si,
  doped with P (Group VB) as an impurity note
  extra electron

129
Basic Nature, contd

• Schematic diagram of disturbed energy levels of
  crystalline Si.
• Si with Group V impurities like P is said to be
  an n-type silicon because of the negative charge
  carriers (the electrons)

Conduction Band
0.05 eV
Donor level
Energy
Valence Band
130
Basic Nature, contd

• 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
131
Basic Nature, contd

• Schematic diagram of disturbed energy levels of
  crystalline Si with B impurity.
• Si with Group III impurities is said to be a
  p-type silicon because of the positive charge
  carriers (the holes)

Conduction Band
Energy
Acceptor level
0.08 eV
Valence Band
132
Occupation of energy states for n and p-type
semiconductors
As donor impurity levels
Conduction Band
0.013 eV
0.67 eV
0.011eV
Ga acceptor impurity levels
Valence Band
After Turner
133
Operating Principles of Semiconductor detectors

• Si semiconductor is a layer of p-type Si in
  contact with n-type 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

134
Energy-level diagram for n-p junction
Conduction Band
n-type
Junction region
Valence Band
p-type
After Turner
135
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

136
Detector Specifics, contd

• 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

137
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138
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.

139
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 mSvh1

140
Solid State Counters

• A - very thin metal (gold) electrode.
• P - thin layer of p-type semiconductor.
• D - depletion region, 310 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.

141
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.

142
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.

143
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 170C is capable of
  a very high gamma resolution of 0.5. The
  temperature dependence and high cost add to their
  impracticality.

144
Another type of Solid State / Scintillation
system

• Thermoluminescent Dosimeters

145
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. 

146
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

147
TL Process
Conduction Band (unfilled shell)
Electron trap (metastable state)
-
Phosphor atom
Valence Band (outermost electron shell)
Incident radiation
148
TL Process, continued
Conduction Band
Thermoluminescent photon
-
Heat Applied
Phosphor atom
Valence Band (outermost electron shell)
149
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

150
Trap Depths - Equate to LongTerm Stability of
Information
Time or temperature
151
TLD Reader Construction
DC Amp
To High Voltage
To ground
PMT
Recorder or meter
Filter
TL material
Heated Cup
Power Supply
152
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

153
TLD Disadvantages

• Lack of uniformity batch calibration needed
• Storage instablity
• Fading
• Light sensitivity
• Spurious TL (cracking, contamination)
• Reader instability
• No permanent record

154
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.

155
Non TL Dosimeters, continued

• Uses thin layer of Al2O3C
• Has a TL sensitivity 50 times greater than
  TLD-100 (LiFMg,Ti)
• Almost tissue equivalent.
• Strong sensitivity to light
• Thermal quenching.
• Readout stimulated using laser
• Dosimeter luminesces in proportion to radiation
  dose. 

156
Summary

• Wide range of detection equipment available
• Understand strengths and weaknesses of each
• No single detector will do everything
• Well get to selection issues in the next two days

157
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