Unit 2. Instrumentation. Experts Teaching from Practical Experience

Transcription

1 Unit 2 Instrumentation Experts Teaching from Practical Experience

2 Gas-Filled Detectors Gas-filled detectors measure the charge released when radiation interacts with the gas Three types: Ion Chambers, Proportional Counters and Geiger- Muller Detectors Some commonly used gases are: Air (dry) Xenon (e.g.: pressurized ion chambers) P-10 (e.g.: gas flow proportional counters) Butane Kinectrics Inc

3 Ion Chamber Simplest of the gas filled detectors Range from hand-held to room sized Gold standard for exposure measurements Sensitive to shock, EMF, etc Kinectrics Inc

4 Ion Chamber Tritium-in-air monitor with 4x100 cm 3 ion chambers 2 sealed ion chambers (measure background γ) 2 open ion chambers with 3-5 air changes/minute (measure β from 3 H & γ) Displays tritium concentration Kinectrics Inc

5 Proportional Counters More sensitive than ion chambers because of gas amplification Will respond to α, β, γ, x-ray, neutrons, etc and can discriminate between different radiations Ludlum LB-122 Proportional Counter Pressurized xenon detector (βγ) Rechargeable butane detector (α) Kinectrics Inc

6 Proportional Counters Gas amplification improves signal-tonoise ratio Avalanche must be complete before PC can respond to next event (dead time typically 0.5 μs) Canberra Planchette Counter gas flow proportional counter (uses P-10 count gas) Kinectrics Inc

7 Proportional Counters Can be used for spectrometry since pulse height is proportional to energy (obsolescent) Tissue equivalent neutron detector polyethylene moderator, 10 B attenuator & sealed porprotional counter tube with 3 He/BF 3 count gas Kinectrics Inc

8 Proportional Counters Whole Body Contamination Monitors & Handand-Foot Monitors use gas proportional detectors (being replaced by plastic scintillators) Kinectrics Inc

9 Geiger-Muller Detectors First developed in 1908 Probably the most common type of radiation measurement instrument: Simple, cheap & rugged Respond to almost all types of ionizing radiation Do everything but don t do anything well Fill gases are typically helium or argon Kinectrics Inc

10 GM Tubes Common types of GM tube: End window Side window Pancake PGM & EWGM have a thin mica window, SWGM are usually metal & often have a beta shield Kinectrics Inc

11 GM Efficiency Isotope Emission 4π Efficiency C kev β 5% Tc kev β & 89 kev γ 19% Sr-90/Y & 2280 kev β 22% P kev β 32% Pu kev α 15% Tc-99m 143 kev γ 1% I kev γ 0.2% 4π efficiency = (number of counts) / (number of disintegrations) Kinectrics Inc

12 GM Meters GM meters are in wide spread use as both radiation survey meters and contamination meters (frisker) Kinectrics Inc

13 GM Dose Rate Meters Small GM Detectors are used in Electronic Personal Dosimeters The calibration is based on a reference radiation (e.g. Cs-137 γ) and the results can be misleading if the radiation field is significantly different than the reference radiation Kinectrics Inc

14 Scintillation Detectors Scintillation detectors contain a luminescent material: Inorganic solids e.g.: NaI(Tl), CsI(Tl), ZnS(Ag), bismuth germanate Organic solids e.g.: anthracene, stilbene, polyvinyl toluene (PVT) Organic liquids Aromatic solvents & phosphors Noble gases Kinectrics Inc

15 Scintillation Detectors The scintillator, photomultiplier tube & electronics are housed within a light-tight body Thin window required for α and β detectors Kinectrics Inc

16 Alpha Scintillation Detector Use ZnS(Ag) as scintillator ZnS usually directly on photocathode of PMT Covered by 0.8 or 1.2 mg/cm 2 mylar film (easily punctured) 4π efficiency ~33% for Pu- 239 alpha Kinectrics Inc

17 Beta Scintillation Detector Typically use bismuth germanate or plastic scintillators Covered by 1.2 mg/cm 2 mylar film 4π efficiency ~10% for C-14 beta Kinectrics Inc

18 Gamma Scintillation Detector Typically contain a NaI(Tl) scintillator 2 x2 mm for low energy ϒ 2 x2 or 3 x3 for mid to high energy ϒ Large volume crystals are available for aerial surveys, etc Kinectrics Inc

19 NaI Scintillation Detector High efficiency but energy dependant 1 μgy/h Cs- 137 ϒ field ~9E4 cpm Kinectrics Inc

20 NaI Scintillation Detector NaI scintillation detectors are combined with GPS to perform geocoded area surveys High sensitivity enable large areas to be surveyed quickly, but Only detect gamma contamination Kinectrics Inc

21 NaI Scintillation Detector NaI(Tl) detectors can be used in low resolution gamma spectrometers Small, light Operate at room temperature On-board microprocessor or interface with PC Kinectrics Inc

22 NaI Scintillation Detector Kinectrics Inc

23 Other Gamma Scintillators NaI(Tl) BGO CsI(Tl) CsI(Na) PVT Density (g/cm3) Melting Point (K) Hardness Hygroscopic Yes No slightly Yes No Wavelength (max, nm) Decay time (μs) Afterglow (% after 6 s) Resolution (% Cs-137) Light Yield (photons/mev) ,000 8,200 52,000 39,000 10,000 Kinectrics Inc

24 Plastic Scintillators PVT has low efficiency & poor resolution but it is: Cheap Easy to manufacture Does not absorb water Spectrometry is possible but difficult Kinectrics Inc

25 Liquid Scintillators Liquid scintillation counting is a standard laboratory technique for measuring beta-emitting nuclides Liquid scintillation cocktails contain: Aromatic solvent (e.g.: pseudocumene), generally 60-99% of volume Phosphors (e.g.: 2,5-diphenyloxazole, 1,4- bis[2-methylsteryl]benzene), generally < 1% of volume Emulsifying agents, etc Kinectrics Inc

26 Liquid Scintillators Beta particle interacts with solvents (particularly the π-electrons in an aromatic) Energy is transferred to the primary phosphor (e.g.: PPO) Secondary phosphor (e.g.: Bis-MSB) is included as a wavelength shifter Other agents included to improve performance of the cocktail Kinectrics Inc

27 Liquid Scintillators Primary scintillators can be excited by energy transferred from solvents but emit light < 400 nm PMT (particularly older ones) are less efficient at these wavelengths Kinectrics Inc

28 Liquid Scintillation Counting Sample is mixed with cocktails in a transparent or translucent container Placed in light-tight Liquid Scintillation Counter (LSC) PMT collects light emitted from vial Kinectrics Inc

29 Liquid Scintillation Counter Generally automated systems for many samples but single sample units are available (connect to laptop PC) Require calibration standards for energy & quench Kinectrics Inc

30 Liquid Scintillation Counting Primarily used for β but capable of measuring α, ϒ, etc Count times ~1 minute for common applications but up to hours for lowlevel counting Capable of spectroscopy Kinectrics Inc

31 Liquid Scintillation Counting LSC is capable of performing spectroscopy Beta energies are not unique 3 H 14 C 32 P Kinectrics Inc

32 Liquid Scintillation Counting Potential problems include: Quenching (physical, colour, chemical, etc) Reduces efficiency and shifts energy downward, more important for low energy β (e.g.: tritium) Static electricity Photoluminescene Chemoluminescene Bioluminescene Kinectrics Inc

33 Cherenkov Counting Cherenkov radiation produced by high energy γ (greater than 263 kev but generally used for higher energies such as Co-60) can also be counted in a LSC Does not require use of a cocktail Cherenkov photons are in the low energy counting region (0-50 kev) Kinectrics Inc

34 Semi-Conductor Detectors First introduced in the early 1960s Entered general use in the 1980s/1990s Advantages Small size, high density, fast response, ability to perform high-resolution spectroscopy Disadvantages Cost, degradation due to radiation damage, may require cooling (LN 2 ) Kinectrics Inc

35 Semiconductor Photon Detectors Si(Li) x-ray & Ge(Li) γ detectors introduced in 1960s HPGe introduced in 1970s & replaced Ge(Li) detectors by mid-1980s Kinectrics Inc

36 Semiconductor Photon Detectors Kinectrics Inc

37 Semiconductor Photon Detectors Kinectrics Inc

38 Semiconductor Photon Detectors Comparison of different gamma spectrometry systems Kinectrics Inc

39 Semiconductor Photon Detectors Primarily a laboratory instrument but can be used in field High resolution, allows both identification and quantification of nuclides High cost, fragile, requires skilled operator & cooling Kinectrics Inc

40 Silicon Charged Particle Detectors Used to detect α, β, heavy ions, etc Material: silicon (boron implanted, lithium drifted, etc) Active area: 20 to 2000 mm 2 Thickness: 0.1 to 2 mm Bias Voltage: 15 to 24 Operating temperature: -196 C to +100 C Kinectrics Inc

41 Silicon Charged Particle Detectors Applications include: Alpha/Beta Continuous Air Monitors (CAM) Alpha spectrometers Kinectrics Inc

42 Semiconductor Alpha Detectors Generally based on ion-implanted silicon detectors Requires extensive sample preparation Count under vacuum Does not require (but may use) cooling Kinectrics Inc

43 Semiconductor Alpha Detectors Components of an alpha spectrometer are similar to those of a gamma spectrometer (but may not include cooling) Good sample preparation is labour intensive and essential to spectrum quality Kinectrics Inc