unit 2 instrumentation - international atomic energy agency · instrumentation experts teaching ......
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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
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• Simplest of the gas
filled detectors
• Range from hand-held
to room sized
• Gold standard for
exposure
measurements
• Sensitive to shock,
EMF, etc
Ion Chamber
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Ion Chamber
• Tritium-in-air monitor
with 4x100 cm3 ion
chambers – 2 sealed ion chambers
(measure background γ)
– 2 ‘open’ ion chambers
with 3-5 air changes/minute
(measure β from 3H & γ)
– Displays tritium
concentration
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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)
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Proportional Counters
• Gas amplification
improves signal-to-
noise 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)
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Proportional Counters
• Can be used for
spectrometry since
pulse height is
proportional to
energy
(obsolescent)
Tissue equivalent neutron detector – polyethylene moderator, 10B attenuator & sealed porprotional counter tube with 3He/BF3 count gas
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Proportional Counters
• Whole Body
Contamination
Monitors & Hand-
and-Foot Monitors
use gas
proportional
detectors (being
replaced by plastic
scintillators)
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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
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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
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GM Efficiency
Isotope Emission 4π Efficiency
C-14 156 keV β 5%
Tc-99 293 keV β & 89 keV γ 19%
Sr-90/Y-90 546 & 2280 keV β 22%
P-32 1711 keV β 32%
Pu-239 5156 keV α 15%
Tc-99m 143 keV γ 1%
I-125 35 keV γ 0.2%
4π efficiency = (number of counts) / (number of disintegrations)
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GM Meters
• GM meters are in wide
spread use as both
radiation survey
meters and
contamination meters
(frisker)
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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
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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
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Scintillation Detectors
• The scintillator,
photomultiplier
tube &
electronics are
housed within a
light-tight body
• Thin window
required for α and
β detectors
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Alpha Scintillation Detector
• Use ZnS(Ag) as
scintillator
ZnS usually directly on
photocathode of PMT
Covered by 0.8 or 1.2
mg/cm2 mylar film (easily
punctured)
4π efficiency ~33% for Pu-
239 alpha
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Beta Scintillation Detector
• Typically use bismuth
germanate or plastic
scintillators
Covered by 1.2 mg/cm2
mylar film
4π efficiency ~10% for C-14
beta
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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
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NaI Scintillation Detector
• High
efficiency but
energy
dependant
1 μGy/h Cs-
137 ϒ field
~9E4 cpm
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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
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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
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Other Gamma Scintillators
NaI(Tl) BGO CsI(Tl) CsI(Na) PVT
Density (g/cm3) 3.67 7.12 4.51 4.51 1.03
Melting Point (K) 924 1050 894 894 75
Hardness 2 5 2 2 0
Hygroscopic Yes No slightly Yes No
Wavelength (max, nm) 415 480 565 420 423
Decay time (μs) 0.23 0.30 1.00 0.63 0.0024
Afterglow (% after 6
s)
0.3-0.5 0.005 0.5-5.0 0.5-5.0 0.01
Resolution (% FWHM
@ Cs-137)
6 10 8 9 180
Light Yield
(photons/MeV)
38,000 8,200 52,000 39,000 10,000
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Plastic Scintillators
• PVT has low
efficiency & poor
resolution but it is:
Cheap
Easy to manufacture
Does not absorb
water
Spectrometry is
possible but difficult
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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
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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
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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
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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
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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’
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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 low-
level counting
• Capable of spectroscopy
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Liquid Scintillation Counting
• LSC is capable of performing
spectroscopy
Beta energies are not unique 3
H
14
C
32
P
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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
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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)
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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 (LN2)
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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
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Semiconductor Photon Detectors
• Comparison of different gamma spectrometry systems
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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
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Silicon Charged Particle Detectors
• Used to detect α, β, heavy
ions, etc
Material: silicon (boron
implanted, lithium drifted, etc)
Active area: 20 to 2000 mm2
Thickness: 0.1 to 2 mm
Bias Voltage: 15 to 24
Operating temperature: -196
C to +100 C
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Silicon Charged Particle Detectors
• Applications include:
Alpha/Beta Continuous Air
Monitors (CAM)
Alpha spectrometers
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Semiconductor Alpha Detectors
• Generally based on
ion-implanted
silicon detectors
• Requires extensive
sample preparation
• Count under
vacuum
• Does not require
(but may use)
cooling