radiation detection instrumentation fundamentals rev0

Radiation Detection Instrumentation and

Measurement Fundamentals

IntroductionRadiation cannot be perceived by human senses. There are two basic types of instruments used for its detection:

•Dose measuring instruments

•Particle counting instruments

With proper calibration, particle-counting instruments can be used to measure radiation. Internal dose can be estimated from bioassay measurements performed with particle counting instruments.

Dose measuring instrumentsRadiation cannot be perceived by human senses. There are two basic types of instruments used for its detection:

•Dose measuring instruments

•Particle counting instruments

With proper calibration, particle-counting instruments can be used to measure radiation. Internal dose can be estimated from bioassay measurements performed with particle counting instruments.

Dose measuring instruments• To measure radiation dose, the

instrument's response must be proportional to the energy absorbed from radiation.

• A "radiation flux" measuring instrument does not necessarily measure dose.

• To effectively measure dose, efficiency for different types of radiation and energy must be taken into account.

Dose measuring instruments• Pocket dosimeters, film badges, and

personal thermoluminescent dosimeters are all used to measure personal dose

• They are based on effects of accumulated irradiation in the material inside the instrument.

• Changes in the detector's material can be stored for later reading of the results.

Dose measuring instruments;

Personnel Monitoring• We determine the rate of radiation exposure

with units of counts per minute or mR per hour.

• For personnel exposure and radiation protection we are concerned with integrated dose.

• What is the total amount to which a person has been exposed? The measuring devices used are different from those already discussed. There are essentially three different types of integrated dose monitors including

Dose measuring instruments;

Personnel MonitoringThere are essentially three different types of integrated dose monitors including;

· Direct Reading Dosimeters

· Film badges

· Thermoluminescent Dosimeters (TLDs)

Personnel Monitoring; Film badges

• The film badge is a beta/gamma sensitive device that measures total whole body dose.

• The badge itself is a small plastic holder that contains a photographic film packet.

• Inside the packet are two pieces of photographic film, tightly wrapped in a paper envelope to prevent light from exposing the film.

• One piece of film is sensitive to low radiation exposure levels and the other is sensitive to high exposure levels.


• When radiation interacts with the film emulsion, it produces ions that chemically activate silver molecules in the emulsion.

• When the film is put into a developing solution, the chemically activated silver atoms are changed into elemental silver, which turns black.

• The degree of this blackness or its density is read on a machine called a densitometer and the reading is an indication of the beta and gamma dose.


• The film badge is a beta/gamma sensitive device that measures total whole body dose.

• The badge itself is a small plastic holder that contains a photographic film packet.

• Inside the packet are two pieces of photographic film, tightly wrapped in a paper envelope to prevent light from exposing the film.

• One piece of film is sensitive to low radiation exposure levels and the other is sensitive to high exposure levels.

Thermoluminescent Dosimeters (TLDs)

• Many stations now use thermoluminescent dosimeters (TLDs) instead of film badges.

• This is because the TLD is not subject to the interpretation of the densitometer.

• It is a more modern device and lends itself to automated reading and record keeping.


• Externally, the TLD looks the same as a film badge but it may be slightly larger or

• smaller. Inside, it is quite different. Instead of film, the TLD contains a piece of

• thermoluminescent material. Thermoluminescent material is material that will give off

• light when heated in proportion to the amount of radiation it has been exposed to.


• This is especially true for a crystalline material. If radiation imparts enough energy to one of these electrons, the electrons prefers to be in the ground state and will drop back to the ground state and emit the extra energy in the form of heat, X-rays, or light.


• In TLD material, there is an in between state called a metastable state, which acts as an electron trap.

• As shown in Figure above, when radiation interacts with the ground state electron, it jumps up and is trapped in the metastable state.


• It remains there until it gets enough energy to move up to the unstable state. This energy is supplied when the TLD chip is heated to a high enough temperature.

• Then the electron will drop back down to the ground state, and, because the TLD chip is a luminescent material, it will release its extra energy in the form of light.


• The total quantity of light emitted by electrons returning to the ground state is proportional to the number of electrons trapped in the metastable state.

• The number of electrons trapped in the metastable state is proportional to the amount of beta and gamma radiation that interacted with the material.

• This means that the amount of light emitted when the TLD is heated is proportional to the number of beta and gamma radiation interacting with the material.


• As shown in Figure before, the TLD reader consists of a heater and a photomultiplier tube like the ones used in scintillation detectors.

• When the TLD chip is heated, light from the chip is directed into the photomultiplier tube.

• In the photomultiplier tube, electrons are produced in the photocathode, multiplied across the dynodes, and finally collected on the anode.

• This then produces a pulse in the circuit that is proportional to the total amount of beta and gamma radiation absorbed by the TLD material.


• There are several reasons for using TLDs instead of film badges:

• One reason is size TLD chips are so small that they can be taped to the fingers to measure exposure to the extremities without interfering with work.

• A second reason is sensitivity. The TLD is generally more sensitive than a film badge, more accurate in the low mR range, and able to provide a better overall indication of the total beta/ga mma dose received.


• A third reason is that the TLD chip can be reused after it is read.

• A fourth reason is that the TLD is not as sensitive to moisture as is the film badge, so data would not be lost if the TLD became wet.

• The only disadvantage to the TLD, is that it is relatively expensive to use.

DIRECT READING DOSIMETERS

• Direct reading dosimeters allow you to determine how much gamma radiation you have been exposed to.

• Direct reading pocket ion chambers use a small capacitor, charged prior to use that is connected to glass fiber electroscope.

• This detector is mounted in a pen type housing that can be clipped to a pocket or lab coat.

• If the detector is exposed to ionizing radiation, a loss in charge potential of the chamber results.

DIRECT READING DOSIMETERS

• This loss of charge is indicated by a corresponding deflection of the glass fiber. The deflection can be viewed by means of a microscope built into the dosimeter.

• The amount of deflection and corresponding dose is indicated on a scale.

• The movement on the fiber, then, is a measure of the amount of gamma radiation absorbed by the dosimeter.

Dosimeter types and the radiations they measure

ASSESSMENT OF EXTERNAL EXPOSURES

• Personal dosimeters are designed to measure the dose in soft tissue at a defined depth below a specified point on the body.

• The quantity personal dose equivalent Hp(d) is normally determined at two depths, d = 0.07 and 10 mm, as measures of exposure to weakly and strongly penetrating radiations respectively.

• The former is representative of dose to skin and the latter represents dose to the blood forming organs. If exposure to the eye is of particular concern, a depth of 3 mm represents the eye lens.


• The personal dose equivalent at 10 mm depth, Hp(10), is used to provide an estimate of effective dose for comparison with the appropriate dose limits.

• As Hp(0.07) is used to estimate the equivalent dose to skin, it should be used for extremity monitoring, where the skin dose is the limiting quantity.

DIRECT READING DOSIMETERS• In direct reading pocket

dosimeters (Fig.), a scale is placed so that the hairline on the scale is the movable fiber.

• As the fiber moves, the scale indicates the total amount of gamma radiation absorbed by the dosimeter.

• A microscope inside the dosimeter enables you to read the scale and see the total gamma dose received

DIRECT READING DOSIMETERS: QUARTZ FIBRE ELECTROMETERS (QFE)

C — spring-loaded charging pin;

R — repellor;

F — quartz fibre;

S — reticle;

L — lens system.

QFE

• When plugged into a charger, electrical charge flows up the charging pin to the quartz fibre and repellor. A light illuminates the inside of the QFE so that the position of the quartz fibre is seen as the repellor and fibre repel each other.

• The amount of charge is adjusted so that the fibre’s deflection is set against zero on the scaled reticle.

• When ionizing radiation ionizes the air in the chamber, the charge on the fibre and repellor is reduced allowing the fibre to move towards the repellor.

• If the QFE is held up to light and viewed, the fibre appears to indicate the dose received on the reticle.

QFE

• QFEs with maximum ranges of 2 mSv to 10 Sv are available. Different types detect thermal or fast neutrons, betas or low or high energy photons. A tissue equivalent QFE is also available but it does not respond to all radiations. Accuracy of measurement is poor.

• QFEs are sensitive to shock, vibration, temperature, environmental contamination and other factors which can affect the rate at which charge dissipates to produce erroneous indications of the dose received. However, they are relatively inexpensive and provide immediate approximations of dose for emergency workers.

USE OF DIRECT READING DOSIMETERS

• Direct reading dosimeters have numerous applications which are complementary to those of passive dosimeters.

• Most importantly, in emergencies and other situations in which acute exposures are possible, they can confirm that the doses received do not exceed dose limits.

• Alarm dosimeters should be pre-set sufficiently below the limits to allow time for workers to retreat.

• Workers regularly at high risk should use direct reading dosimeters that cannot be switched off.


• Direct reading dosimeters typically measure doses as low as 1 mSv, which is at least ten times more sensitive than many passive devices.

• At low dose rates the accuracy of the measurement may be poor but adequate and frequent readings will permit work to be analysed to determine which parts of a procedure contribute most to the overall dose.

• Audible indications of dose rate also maintain a worker’s awareness of exposure so that procedures can be refined to optimize the dose received.

• Dosimeters issued to monitor personal doses should not be confused with others which may be used for environmental measurements.


Active (direct reading) dosimeters have many useful applications. Some dosimeters achieve high sensitivity but at reduced accuracy.

ALBEDO NEUTRON DOSIMETERS

• Albedo dosimeters are designed to record neutron doses by using the body as a moderator to reduce intermediate and fast neutrons to thermal energies.

• Doses as low as 100 mSv may be measured using LiF TLDs made with natural or lithium enriched in 6Li.

• For neutrons with energies above 10 keV the sensitivity is significantly reduced and the measurement must be multiplied by a correction factor which is dependent on the neutron spectrum.

ALBEDO NEUTRON DOSIMETERS

• The albedo method is only satisfactory if the spectrum remains nearly constant. It is unsatisfactory for applications in which a major fraction of the dose equivalent is due to neutrons above a few hundred keV.

• It is generally inappropriate, for example, for industrial use of neutron sources (252Cf, AmBe,

• etc.) or deuterium–tritium generators which have a large fraction of neutron energy above 1 MeV.

• In these situations, a constantly changing work geometry is likely to produce highly variable spectra.

Albedo TLDs measure neutrons that the body has thermalized

• Albedo TLDs measure fast and intermediate energy neutron doses.

• Albedo TLDs must be in close contact with the body.• Albedo TLDs may be unsuitable for general industrial application.

NUCLEAR EMULSION NEUTRON DOSIMETERS

• A nuclear emulsion (nuclear track analysis, NTA) dosimeter comprises a film and holder designed to detect fast and thermal neutrons of energies above 0.7 MeV.

• The film (typically an emulsion of a thickness of between 24 and 33 mm on a base) is prepared in dry nitrogen and sealed inside a special wrapper.

• When it is in the holder, incident fast neutrons interact (elastic scattering) with hydrogenous material surrounding the film and produce recoil protons.

• Thermal neutrons interact with nitrogen in the emulsion and thereby produce 0.6 MeV protons.

• The protons form ionization tracks in the emulsion. A dose of 50 mSv from AmBe neutrons will produce about one track per square millimetre.

• When the film is processed these can be counted using a high power (× 1000) microscope.

NUCLEAR EMULSION NEUTRON DOSIMETERS

• The difficult task of identifying tracks is aided by a lead filter in the front of the polypropylene holder which reduces fogging (film darkening) caused by X and gamma radiations.

• A boron loaded plastic filter at the back of the holder absorbs albedo thermal neutrons.

• Track fading is minimized by the moisture proof wrapper but the latent (undeveloped) images still fade with time, limiting possible wearing periods to one or two months in ideal conditions.

• The wrapper must not be damaged nor the film subjected to excessive heat.

An NTA dosimeter and processed film.

• An NTA dosimeter comprises a film and holder.• Processed films reveal tracks in the emulsion caused by

neutrons.

CRITICALITY NEUTRON DOSIMETERS

• Criticality dosimeters are used in nuclear installations where the movement of reactor fuel raises the possibility of a criticality accident.

• This includes fuel handling and reprocessing areas and facilities where 233U, 235U or 239Pu are used in quantities greater than a few grams.

• The activation foils used in such dosimeters become radioactive when irradiated by the large release of neutrons associated with a criticality accident.

• They are unsuitable for routine neutron dosimetry because of their poor sensitivity and rapid loss of information (decay) following exposure.

CRITICALITY NEUTRON DOSIMETERSA criticality dosimeter (locket or button) is a small box usually containing several foils to provide information on the neutron dose and energy spectrum:•gold — thermal neutron measurement (G1, G2)•indium — thermal and fast neutron measurement (I)•copper — intermediate neutron measurement (Cu)•sulphur — fast neutron measurement (S).

Cadmium (Cd) is used to shield foils to differentiate between thermal neutron and intermediate neutron exposure. Following exposure, the induced radioactivity is measured by counting the beta or gamma radiation emitted by the foil. The activity is proportional to the neutron dose.

CRITICALITY NEUTRON DOSIMETERS• In a different form of criticality dosimeter, fissile

materials are placed between track etch detectors (see Section 14).

• Fission fragments damage the plastic when the fissile materials are exposed to neutrons of certain energies.

• The materials used in these fission dosimeters are radioactive even before they have been exposed to neutrons.

An exploded view of the components of a criticality locket

Neutrons cause nuclear interactions, activation and fission in foils.Various activation foils are used in criticality lockets.

General Principles of Radiation Detection

Outline

• Gas-Filled Detectors

• Scintillation Detectors

• Solid State Detectors

• Others

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

Gas- Filled Detectors - one example

wall

fill gas

R

Output

Aor

Anode (+)

Cathode (-)

End windowOr wall

Indirect Ionization Process

wall

Incident gamma photon

e -

e -e -

e -

e -

e -

e -

e -

Direct Ionization Process

wall

Incident charged particle

e -

e -

e -

e -

e -

e -e -

e -

beta (β-)

Competing Processes - recombination

R

Outpute -

e -

+

+

Voltage versus Ions Collected

Voltage

Number of Ion Pairs collected

Ionization region

Saturation Voltage

100 % of initialions are collected

Recom-binationregion

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

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

Pulse Height Variation

Detector Voltage

PulseHeight

AlphaParticles

BetaParticles

Gamma Photons

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

Ionization Chambers, continued

• Chamber’s 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 = CΔV

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)

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

Current vs. Voltage for Fill Gases in a Cylindrical Ion Chamber

Applied Voltage (volts)

RelativeCurrent(%)

Helium at low pressure

Air at low pressure

Helium at high pressure

Air at high pressure

0.1

1.0

10

100

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.

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)

XPP

T

TVρkI

stp

stp

Operating Regions of Gas-Filled Detectors

Reco

mb

inati

on

Reg

ion

Ion

izati

on

Reg

ion

Pro

port

ion

al R

eg

ion

Lim

ited

Pro

port

ion

al

Reg

ion

Geig

er-

Mu

elle

r R

eg

ion

Con

tin

uou

s D

isch

arg

e R

eg

ion

Pu

lse H

eig

ht

Voltage

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

Examples

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

Pulse-Height Versus Voltage

Reco

mb

inati

on

Reg

ion

Ionization Region

Proportional Region

Pu

lse H

eig

ht

Voltage

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

Alpha & Beta-Gamma Plateau

Detector Voltage

IonizationCurrent

Alpha Plateau

Beta-Gamma Plateau

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

Gas-Flow Proportional Counter

Gas-Flow Proportional Counter

Fill gasoutlet Fill gas

inlet

Detector

sample

Sample planchet

O-ring

(window- optional)

anode

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)

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

• 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

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

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?

Points to Remember for Gas-filled Detectors

• Power supply requirements– Stable?– Batteries ok?

• Temperature, pressure correction requirements

• Calibration– Frequency– Nuclides

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!

Issues with Gas Filled Detectors: Recovery Time

• Time interval between dead time and full recovery

• Recovery Time = Resolving time- dead time

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.

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

TR1R

R0

oc

Relationship among dead time, recovery time, and resolving time

Pulse Height

Time, microseconds

100 200 300 400 5000

Recovery timeDeadTime

Resolving time

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.

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.

Energy Response of GM – Uncompensated

10 100 1000 E, keV

R

20

1.21.00.8

COMPENSATED GEIGER DOSE RATE METERS

• Detector’s 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 instrument’s directional (polar) response and prevent beta and very low energy photon radiations from reaching the Geiger tube.

Energy Response of GM – Uncompensated and Compensated

10 100 1000 E, keV

R

20

1.21.00.8

Example Polar Response

Example of Compensated GM

RadEye component

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

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

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.

SCINTILLATION DETECTORS

Scintillators

• Emit light when irradiated– promptly (<10-8s)

• fluorescence– delayed (>10-8s)

• phosphorescence

• Can be– liquid– solid– gas– organic– inorganic

Basis of Scintillation - Energy Structure in an Atom

Excited state

Ground state, last filled (outer) orbital

En

erg

y

Basis of Scintillation - Energy Structure in a Molecule

Excited state

Ground state

Interatomic distance

En

erg

y

Ao

A1

Bo

B1

EA0

EA1

EB1

EB0

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

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.

Organic Scintillators

• Examples

• Differences

Anthracene

CH3Excited state

Ground state

Interatomic distance

Ene

rgy

Ao

A1

Bo

B1

EA0

EA1

EB1

EB0

Toluene

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

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)

Solid Scintillators

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

Crystal Lattice

Ge

As+

e-

Shared electron pair

Creation of Quantized “Bands”

Conduction Band

Valence Band+ ++

- - -

0

Eo

Eo + Eg

EF

Introduction of Impurities

Conduction Band

Valence Band

~1 eV

Donorimpurity levels

Acceptorimpurity levels

~0.01 eV

~ 0.01eV

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

Cutaway diagram of solid-fluor scintillation detector

Cutaway diagram of solid-fluor scintillation detector

Gamma ray

Scintillationevent

Reflector housing

Fluor crystal NaI (Tl)

Photocathode

Photoelectrons

Dynodes

Photomultiplier tube

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

Generalized Detection System using a Scintillator

Oscilloscope

Scaler

Multi-ChannelAnalyzer

DiscriminatorAmplifierPre-Amp

HighVoltage

Detector

(Crystal &Photomultiplier)

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

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

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

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

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

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

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

A simple coincidence circuit

Coincidence Unit

Scaler

Multi-channelAnalyzer

Detector

Timing

Timing

Amplification

Detector

Source

Amplification

Gate

After Tsoulfanidis, 1995

Basic LSC System

Beckman LS 6500 Liquid Scintillation CountingSystem.

Single & summed pulse spectra

Counts/Min

Pulse Height

With pulse summation

Without pulse summation

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)

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

• Internal standard method– older technique– sample is counted– known quantity of radioisotope is added– sample recounted– Efficiency = (cpm(std+sample) – cpm(sample))/dpm(std)

• Most accurate method– requires ability to add same amount of

radionuclide each time– more costly & time consuming

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

Alternative Methods

• 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

Pulse Height Discrimination

Pulse Height Discriminationfor three common beta emitters

CountRate

Pulse Height

3H14C 32P

Background & Efficiency Checks on LSC

• Essential - LSC’s 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

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

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).

Linear Attenuation of

NaI

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

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).

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

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).

MaterialDensity [g/cm3]

Emission Max [nm]

Decay Constant

(1)

Refractive Index (2)

Conversion Efficiency

(3)

Hygro-scopic

NaI(Tl) 3.67 415 0.23 s 1.85 100 yes

CsI(Tl) 4.51 550 0.6/3.4 s 1.79 45 no

CsI(Na) 4.51 420 0.63 s 1.84 85 slightly

CsIundoped

4.51 315 16 ns 1.95 4 - 6 no

CaF2 (Eu) 3.18 435 0.84 s 1.47 50 no

6LiI (Eu) 4.08 470 1.4 s 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

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MaterialDensity [g/cm3]

Emission Maximum

[nm]

Decay Constant

(1)

Refractive Index (2)

Conversion Efficiency

(3)

Hygroscopic

BaF2 4.88315 220

0.63 s0.8 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 s 2.15 15 - 20 no

CdWO4 7.90 470 / 540 20 / 5 s 2.3 25 - 30 no

Plastics 1.03 375 - 600 1 - 3 s 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

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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

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.

Neutron Detection

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.

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.

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.

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.

Emission Spectra of Scintillators

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.

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Temperature Influence on the Scintillation Response

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)?

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)Noon-hygroscopic, rugged,

long wavelength emission

Particle and high energy physics, general radiation detection, photodiode readout, phoswiches

CsI(Na) High light output, ruggedGeophysical, general radiation

detection

CsI undoped

Fast, non-hygroscopic, radiation hard, low light output

Physics (calorimetry)

CaF2(Eu) Low Z, high light outut detection, , phoswiches

CdWO4Very high density, low

afterglow, radiation hard

DC measurement of X-rays (high intensity), readout with photodiodes, Computerized Tomography (CT)

PlasticsFast, low density and Z,

high light outputParticle detection, beta detection

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Material Important Properties Major Applications

6LiI(Eu)High neutron cross-section,

high light outputThermal neutron detection and

spectroscopy6Li -

glassHigh neutron cross-section,

non-hygroscopicThermal neutron detection

BaF2 Ultra-fast sub-ns UV emissionPositron life time studies, physics

research, fast timing

YAP(Ce)

High light output, low Z, fastMHz X-ray spectroscopy, synchrotron

physics

GSO(Ce)

High density and Z, fast, radiation hard

Physics research

BGO High density and ZParticle physics, geophysical research,

PET, anti-Compton spectrometers

CdWO4Very high density, low

afterglow, radiation hard

DC measurement of X-rays (high intensity), readout with photodiodes, Computerized Tomography (CT)

PlasticsFast, low density and Z, high

light outputParticle detection, beta detection



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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.

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.

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

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%

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)

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

• 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

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

Semi-Conductor Detectors

Idealized Gamma-Ray Spectrum in NaI

Energy

Countsper

EnergyInterval

Eo

theoretical

Actual

Components of Spectrum

Energy

Countsper

EnergyInterval

Eo

PhotopeakCompton edge

BackscatterPeak

AnnihilationPeak

X-rayPeak

NaI(Tl) vs. HPGE

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

Crystal Lattice

Ge

As+

e-

Shared electron pair

Basic Nature of Semiconductors• Schematic view of lattice of Group IVB element Si

• Dots represent electron pair bonds between the Si atoms

Si

Si

Si

Si

Si

Si

Basic Nature, cont’d

• Schematic diagram of energy levels of crystalline Si.

• Pure Si is a poor conductor of electricity

Conduction Band

Valence Band

En

erg

y

1.08 eV Forbidden Gap

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

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

En

erg

y 0.05 eV Donor level

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

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

En

erg

y

0.08 eV Acceptor level

Occupation of energy states for n and p-type semiconductors

Conduction Band

Valence Band

After Turner

0.67 eV

As donorimpurity levels

Ga acceptorimpurity levels

0.013 eV

0.011eV

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”

Energy-level diagram for n-p junction

Conduction Band

Valence Band

After Turner

n-type

p-type

Junctionregion

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

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

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.

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

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.

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.

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.

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.

Another type of Solid State / Scintillation system

Thermoluminescent Dosimeters

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.

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

TL Process

Valence Band (outermost electron shell)

Conduction Band (unfilled shell)

Phosphor atom

Incident radiation

Electron trap (metastable state)

-

TL Process, continued

Valence Band (outermost electron shell)

Conduction Band

Phosphor atom

Thermoluminescent photon Heat Applied-

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

Trap Depths - Equate to LongTerm Stability of Information

Time or temperature

TLD Reader Construction

Power Supply

PMT

DC Amp

Filter

Heated Cup

TL material

To High Voltage To ground

Recorder or meter

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

TLD Disadvantages

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

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.

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.

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

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

radiation detection instrumentation fundamentals rev0

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