Unit 2 Instrumentation - International Atomic Energy Agency · Instrumentation Experts Teaching ... Melting Point (K) 924 1050 894 894 75 Hardness 2 5 2 2 0 ... •Require calibration

Unit 2

Instrumentation

Experts Teaching from Practical Experience

© Kinectrics Inc. 2012 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


• 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


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


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)



• 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)



• 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



• Whole Body

Contamination

Monitors & Hand-

and-Foot Monitors

use gas

proportional

detectors (being

replaced by plastic

scintillators)


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


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


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)


GM Meters

• GM meters are in wide

spread use as both

radiation survey

meters and

contamination meters

(frisker)


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


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


Scintillation Detectors

• The scintillator,

photomultiplier

tube &

electronics are

housed within a

light-tight body

• Thin window

required for α and

β detectors


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


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


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


NaI Scintillation Detector

• High

efficiency but

energy

dependant

1 μGy/h Cs-

137 ϒ field

~9E4 cpm



• 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

http://www.mjwts.com/images/fairweathercart.jpg



• NaI(Tl) detectors can be

used in low resolution

gamma spectrometers

Small, light

Operate at room

temperature

On-board microprocessor

or interface with PC




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


Plastic Scintillators

• PVT has low

efficiency & poor

resolution but it is:

Cheap

Easy to manufacture

Does not absorb

water

Spectrometry is

possible but difficult

http://www.radcommsystems.com/images/PDF/rc4000-radcomm.pdf


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



• 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



• 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


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


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’



• 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



• LSC is capable of performing

spectroscopy

Beta energies are not unique 3

H

14

C

32

P



• 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


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)


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)


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







• Comparison of different gamma spectrometry systems



• 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


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


Silicon Charged Particle Detectors

• Applications include:

Alpha/Beta Continuous Air

Monitors (CAM)

Alpha spectrometers


Semiconductor Alpha Detectors

• Generally based on

ion-implanted

silicon detectors

• Requires extensive

sample preparation

• Count under

vacuum

• Does not require

(but may use)

cooling


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

Documents

Unit 2 Instrumentation - International Atomic Energy Agency · Instrumentation Experts Teaching ... Melting Point (K) 924 1050 894 894 75 Hardness 2 5 2 2 0 ... •Require calibration