![Material
Density
[g/cm3]
Emission
Max [nm]
Decay
Constant
(1)
Refractive
Index (2)
Conversion
Efficiency
(3)
Hygro-
scopic
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
CsF 4.64 390 3 - 5 ns 1.48 5 - 7 yes
(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](https://image.slidesharecdn.com/radiationdetectioninstrumentationfundamentals30thmarch-200422052457/85/Radiation-detection-instrumentation-fundamentals-74-320.jpg)
![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.
Commonly Used Scintillators](https://image.slidesharecdn.com/radiationdetectioninstrumentationfundamentals30thmarch-200422052457/85/Radiation-detection-instrumentation-fundamentals-75-320.jpg)
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 (common axis) 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 R Output Aor Anode (+) Cathode (-) End window Or wall
- 7. Indirect Ionization Process wall Incident gamma photon
- 8. Direct Ionization Process wall Incident charged particle e - e - e - e - e - e -e - e - beta (β-)
- 9. Competing Processes - recombination R Outpute - e - + +
- 10. Voltage versus Ions Collected Voltage Number of Ion Pairs collected Ionization region Saturation Voltage 100 % of initial ions are collected Recom- bination region
- 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 Detector Voltage Pulse Height Alpha Particles Beta Particles Gamma Photons
- 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 chamber
- 15. Ionization Chambers, continued • Chamber’s construction determines its 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
- 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
- 18. Current vs. Voltage for Fill Gases in a Cylindrical Ion Chamber Applied Voltage (volts) Relative Current (%) Helium at low pressure Air at low pressure Helium at high pressure Air at high pressure 0.1 1.0 10 100
- 19. Correcting Ion Chambers for T- Temperature, P- Pressure • 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 • ρ detector gas density • V detector volume • STP standard temp and pressure (273K, 760 torr (1 atm) X P P T T VρkI stp stp
- 21. Operating Regions of Gas-Filled DetectorsRecombinationRegion IonizationRegion ProportionalRegion LimitedProportional Region Geiger-MuellerRegion ContinuousDischargeRegion PulseHeight Voltage
- 22. Examples
- 23. 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
- 24. Pulse-Height Versus Voltage RecombinationRegion Ionization Region Proportional Region PulseHeight Voltage
- 25. 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
- 26. 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
- 28. Gas-Flow Proportional Counter Fill gas outlet Fill gas inlet Detector sample Sample planchet O-ring (window- optional) anode
- 29. 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)
- 30. 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
- 31. • No proportional relationship between energy of incident radiation and number of ionizations detected • A level pulse height occurs throughout the entire voltage range Geiger Mueller Detectors, continued
- 32. 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
- 33. 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?
- 34. Points to Remember for Gas-filled Detectors • Power supply requirements – Stable? – Batteries ok? • Temperature, pressure correction requirements • Calibration – Frequency – Nuclides
- 35. 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!
- 36. Issues with Gas Filled Detectors: Recovery Time • Time interval between dead time and full recovery • Recovery Time = Resolving time- dead time
- 37. 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.
- 38. 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 dead time – R0 is the observed count rate and – RC is the corrected count rate TR1 R R 0 o c
- 39. Relationship among dead time, recovery time, and resolving time Pulse Height Time, microseconds 100 200 300 400 5000 Recovery timeDead Time Resolving time
- 40. 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.
- 41. 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.
- 42. Example of Compensated GM RadEye component
- 43. 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
- 44. 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
- 45. 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 Rejection The NBR measurement technology has been developed by Thermo Electron for the supression of alarms caused by variations of the natural background.
- 47. Scintillators • Emit light when irradiated – promptly (<10-8s) • fluorescence – delayed (>10-8s) • phosphorescence • Can be – liquid – solid – gas – organic – inorganic
- 48. Basis of Scintillation - Energy Structure in an Atom Excited state Ground state, last filled (outer) orbital Energy
- 49. Basis of Scintillation - Energy Structure in a Molecule Excited state Ground state Interatomic distance Energy Ao A1 Bo B1 EA0 EA1 EB1 EB0
- 50. 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
- 51. 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.
- 52. Organic Scintillators • Examples • Differences Anthracene CH3 Excited state Ground state Interatomic distance Energy Ao A1 Bo B1 EA0 EA1 EB1 EB0 Toluene
- 53. 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
- 54. 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)
- 55. Solid Scintillators • Solids have – Lattice structure (molecular level) – Quantized energy levels – Valence bands – Conduction bands
- 57. Creation of Quantized “Bands” Conduction Band Valence Band + ++ - - - 0 Eo Eo + Eg EF
- 58. Introduction of Impurities Conduction Band Valence Band ~1 eV Donor impurity levels Acceptor impurity levels ~0.01 eV ~ 0.01eV
- 59. 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
- 60. Cutaway diagram of solid-fluor scintillation detector
- 61. Cutaway diagram of solid-fluor scintillation detector Gamma ray Scintillation event Reflector housing Fluor crystal NaI (Tl) Photocathode Photoelectrons Dynodes Photomultiplier tube
- 62. 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
- 63. Generalized Detection System using a Scintillator Oscilloscope Scaler Multi- Channel Analyzer DiscriminatorAmplifier Pre- Amp High Voltage Detector (Crystal & Photomultiplier)
- 64. 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
- 65. 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
- 66. 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
- 67. 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
- 68. 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
- 69. 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).
- 70. 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
- 71. 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).
- 72. 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
- 73. 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).
- 74. Material Density [g/cm3] Emission Max [nm] Decay Constant (1) Refractive Index (2) Conversion Efficiency (3) Hygro- scopic 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 CsF 4.64 390 3 - 5 ns 1.48 5 - 7 yes (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
- 75. 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. Commonly Used Scintillators
- 76. 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
- 77. 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.
- 79. 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.
- 80. Radiation Damage in Scintillators • Radiation damage results in change 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.
- 81. 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.
- 82. 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.
- 83. Emission Spectra of Scintillators
- 84. Emission Spectra of Scintillators
- 85. 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.
- 86. Temperature Influence on the Scintillation Response
- 87. 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)?
- 88. 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) Non-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 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
- 89. Material Important Properties Major Applications 6LiI(Eu) High neutron cross-section, high light output Thermal neutron detection and spectroscopy 6Li - glass High neutron cross-section, non-hygroscopic 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
- 90. 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.
- 92. 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.
- 93. 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
- 94. 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%
- 95. 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)
- 96. 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
- 97. • 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) 44-2 Gamma Scintillator
- 98. 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
- 100. Idealized Gamma-Ray Spectrum in NaI Energy Counts per Energy Interval Eo theoretical Actual
- 101. Components of Spectrum Energy Counts per Energy Interval Eo Photopeak Compton edge Backscatter Peak Annihilation Peak X-ray Peak
- 102. NaI(Tl) vs. HPGE
- 103. NaI(Tl) vs. HPGE
- 104. 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
- 106. Basic Nature of Semiconductors • Schematic view of lattice of Group IV-B (Group 4B) element Si • Dots represent electron pair bonds between the Si atoms Si Si Si Si Si Si
- 107. Basic Nature, cont’d • Schematic diagram of energy levels of crystalline Si. • Pure Si is a poor conductor of electricity Conduction Band Valence Band Energy 1.08 eV Forbidden Gap
- 108. 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 Si Si P
- 109. Basic Nature, cont’d • 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 Valence Band Energy 0.05 eV Donor level
- 110. 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 Si Si B
- 111. Basic Nature, cont’d • 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 Valence Band Energy 0.08 eV Acceptor level
- 112. Occupation of energy states for n and p-type semiconductors Conduction Band Valence Band After Turner 0.67 eV As donor impurity levels Ga acceptor impurity levels 0.013 eV 0.011eV
- 113. 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”
- 114. Energy-level diagram for n-p junction Conduction Band Valence Band After Turner n-type p-type Junction region
- 115. 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
- 116. 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
- 118. 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.
- 119. 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
- 120. Solid State Counters – A - very thin metal (gold) electrode. – P - thin layer of p- type 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.
- 121. 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.
- 122. 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.
- 123. 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.
- 124. Another type of Solid State / Scintillation system Thermoluminescent Dosimeters
- 125. 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.
- 126. 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
- 127. TL Process Valence Band (outermost electron shell) Conduction Band (unfilled shell) Phosphor atom Incident radiation Electron trap (metastable state) -
- 128. TL Process, continued Valence Band (outermost electron shell) Conduction Band Phosphor atom Thermoluminescent photon Heat Applied-
- 129. 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
- 130. Trap Depths - Equate to LongTerm Stability of Information Time or temperature
- 131. TLD Reader Construction Power Supply PMT DC Amp Filter Heated Cup TL material To High Voltage To ground Recorder or meter
- 132. 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
- 133. TLD Disadvantages • Lack of uniformity – batch calibration needed • Storage instablity • Fading • Light sensitivity • Spurious TL (cracking, contamination) • Reader instability • No permanent record
- 134. 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.
- 135. 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.
- 136. 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
- 137. 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
- 138. Thank You Nitish Virmani Lecturer Department of Radio-Imaging Technology Faculty of Allied Health Sciences SGT University Email- nitishvirmani18@gmail.com +918890526809