Practical Health Physics Techniques - PowerPoint Presentation

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Practical Health Physics Techniques - PPT Presentation

An Introduction to Radiation Protection An Introduction to Radiation Protection 6e 2012 Martin Harbison Beach ColeCRC Press Introduction Properties of Radionuclides Analysis Techniques ID: 792747

radiation count introduction rate count radiation rate introduction protection crc press cole beach martin harbison 2012 time counting beta

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Slide1

Practical Health

Physics Techniques

An Introduction to Radiation Protection

Harbison

, Beach, Cole/CRC Press

Slide2

Introduction

Properties of Radionuclides

Analysis TechniquesEnergy determination

Measuring half livesGross alpha and beta counting

Slide3

Properties of Radionuclides

Radionuclides emit radiation of a specific type and energy, a bit like having their own fingerprint

For example:

Co-60 emits 0.31 MeV β-, 1.48 MeV β-

, 1.17 MeV γ, 1.33 MeV γ and has a half life of 5.27 yearsSr-90 emits 0.56 MeV β- and has a half life of 28.8 yearsAm-241 emits 5.443 MeV α, 5.486 MeV α, 59.5 keV γ and has a half life of 432.2 yearsAn Introduction to Radiation Protection 6e © 2012 Martin, Harbison, Beach, Cole/CRC Press

Slide4

Identification of Radionuclides

Identification of an unknown radionuclide can be achieved by determining:

the type and energy of radiation that it emits and/or

its radioactive half lifeComparing this information against reference data for all known radionuclides allows the identity of an unknown radionuclide to be determined

Slide5

Energy Determination

Gamma spectrometry - easy and convenient method (see Chapter 7)

Alpha spectrometry - much less common than gamma spectrometry and generally involves a significant amount of sample preparation

Beta absorption methods – can be carried out easily with limited equipment, if access to a gamma spectrometer is not possible to access a gamma spectrometer or the radionuclide is a pure beta emitter An Introduction to Radiation Protection 6e © 2012 Martin, Harbison, Beach, Cole/CRC Press

Slide6

Beta Absorption Method 1

Count the sample in a beta counting system e.g. GM detector in a lead castle

Measure the count rate with increasing thicknesses of aluminium between the sample and the detector

Slide7

Beta Absorption Method 1

Draw an absorption graph by plotting count rate (background corrected) against absorber thickness (expressed in g/cm

) on log-linear paper

Compare result with curves for various known β energies and identify the one which aligns with plotted results

Slide8

Beta Absorption Method 2

Count the sample in a beta counting system e.g. GM detector in a lead castle

Measure the count rate with increasing thicknesses of aluminium between the sample and the detector until the count rate has reduced to about one quarter of the initial value

Slide9

Beta Absorption Method 2

Plot count rate (background corrected) against absorber thickness (expressed in g/cm

)

Determine the thickness that would reduce the count rate to one half – the half-value thickness (HVT)In this case the HVT is 0.074g/cm2

Slide10

Beta Absorption Method 2

Determine the beta maximum energy by reading off a HVT-beta energy graph for the absorber used (in this case aluminium)

Maximum beta energy is

1.4 MeV

This corresponds to sodium-24 which has a maximum beta particle energy of

1.39 MeV

0.074g/cm

1.4 MeV

Slide11

Issues with Identifying Nuclides by Determining the Maximum Beta Energy

Difficult to do in practice as likely to be more than one nuclide and consequently several beta energies present

Difficult to identify nuclides solely from information about maximum beta energies

Slide12

Determination of Half Life

The activity of a nuclide at time t (A

) is given by:

where A

is the activity when t=0 and T

1/2

is the half life

Take a series of counts at suitable intervals (such that the count rate decreases by 10-15% between counts)

Plot the corrected count rate against time on a log-linear scale. The half life can then be determined from the graph by assessing the time it takes for the count rate to reduce by half

Alternatively, the half life can be determined statistically as shown on the next slides

Slide13

Determination of Half Life

Time (h)

Count rate (cpm)

6720

6050

5690

4563

3930

2989

Plotting the log

of the count rate against the time gives the following graph

Slide14

Determination of Half Life

Hence the gradient of the graph is equal to

–

The gradient of the graph is

-0.0449

using linear regression

Therefore

= 15.4 hours

A nuclide with a half life of 15.4 hours is

Na-24

Rearranging gives

and rearranging further gives

Now

Slide15

Issues with Identifying Nuclides by Determining the Half Life

Only suitable for nuclides with half lives between a few minutes and a few months

Difficult to do in practice as likely to be more than one nuclide present

Slide16

Identifying a Nuclides when another short lived Nuclide is present

This is the decay plot of Na-24, although there is also a shorter lived nuclide present

Rate of total decay decreases to eventually give a straight line when only the longer lived nuclide remains

Extrapolate the straight line back to t=0. Use the revised line (shown in green) to determine the half live of the longer lived nuclide. Time taken for count rate to reduce from 1000

cpm to 500

cpm

is about 15 hours (actual nuclide is Na-24 with a half life of 15.4 hours)

Extrapolation of long lived nuclide

Slide17

Produce a decay plot for the short lived nuclide by deducting the contribution of the longer lived nuclide count rate (using the extrapolated line) from the total count

rate

Use this decay plot (red line) to determine the half live of the shorter lived nuclide. Time taken for count rate to reduce from 2000 counts/min to 250 counts/min (3 half lives) is about 2 hours, giving a half life of about

40 minutes (actual

nuclide is

Cl-38

with a half life of

37.3 minutes)

Identifying a Nuclides when another short lived Nuclide is present

Slide18

Gross Alpha and Beta Counting

Generally done with a high sensitivity counting system

Can use an alpha detector (drawer assembly) or beta detector (drawer assembly or in a beta castle)

where

= background corrected count rate in cps and

= percentage counting system efficiency

Power supply

Detector

Amplifier

Discriminator

Scaler

Slide19

Gross Alpha and Beta Counting

Efficiency

is defined as the fraction of particles counted compared with the total number emitted

It is determined by measuring the count rate from a source of known emission rate in the counting position to be used for the samples to be counted (i.e. same shelf if a beta castle is used)

Example

If the known source activity is 220

its emission rate will be 220 x 60 = 13200

dpm

. If it gives a corrected count rate of 1980

cpm

the detector efficiency will be

Efficiency (%) =

x 100

Slide20

Gross Alpha and Beta Counting

Example

Calculate the activity of a source which gives an uncorrected count rate of 4925

cpm

in a detector which has an efficiency of 15% and gives a background count rate of 65

cpm

Slide21

Counting a Smear Sample

where

= background corrected count rate in

cps

= percentage counting system

efficiency

A = area smear in cm

and

= percentage of the contamination picked up by the smear paper

Note: E

is difficult to determine and is dependent on physical and chemical nature of contamination, nature of the surface being smeared etc. It is usually assumed to be 10%, but could be as high as 100%

Slide22

Counting an Air Sample

where

= background corrected count rate in

cps

= percentage counting system

efficiency

V = volume of air sampled in m

Note: Take care when interpreting results as radon daughters may be present. Allow the sample to decay for 24 hours and then recount if the presence of radon daughters is suspected. Consider using radon compensating counters in areas where radon is known to be present

Slide23

Issues with Gross Alpha and Beta Counting

Need to correct for resolving time of counter (see later)

Efficiency is dependent on:

Geometry of the counting system

Backscatter

Self absorption in the source

Absorption in the counter window

Absorption in the air gap between the source and detector

Many of these factors are dependent on the energy of the radiation & hence the efficiency is energy dependent

Slide24

Issues with Gross Alpha and Beta Counting

Ideally the counter should be calibrated with a source of the same nuclide and the same geometry as the samples to be counted

Not practicable to calibrate the counter for range of energies

For health physics purposes it is usual to calibrate with one typical source and accept possible errors when assessing nuclides of other energies

However, this error can be particularly significant for some nuclides e.g. low energy beta emitters, so corrections may need to be made

Slide25

Corrections for Resolving Time

Resolving time

is the short period of time (order of 100 µs) after a

particle or

photon has just been detected that other particles/photons cannot be detected

It is the time required for the charged particles generated by the interaction of the ionising radiation in the detector to be collected

Slide26

Corrections for Resolving Time

Sometimes referred to as the

dead time

of the detector because during this time it cannot respond to any new event

fixed dead time

, which is a function of the circuitry, can be introduced into the counting system

Slide27

Corrections for Dead Time

The dead time can be corrected for dead time by the following equation

where

true count rate in cps

c = observed count rate in cps

dead time in s

Slide28

Corrections for Dead Time

Example

Calculate the true count rate of a sample if the observed count rate is 500 cps and the dead time is 200 µs

Slide29

Counting Statistics

Radioactive decay is a

random

process

The number of counts in a given time will fluctuate around an average value

The

standard deviation

is a measure of the scatter of the counts about their average value

If the average of a number of counts is

, the standard deviation is

√

If N = 900 counts,

Slide30

Counting Statistics

If a count is taken over time t and N counts are recorded:

Example

A 10 s count gives a result of 400 counts. What is the count rate and standard deviation?

Slide31

Counting Statistics

68% of the observations are within one standard deviation of the true count rate i.e. for the previous example there is a 68% chance that the count rate is between 38 cps and 42 cps

The standard deviation is a measure of the accuracy of the measurement

Greater accuracy can be achieved by increasing the total count recorded

Slide32

Counting Statistics

Example

100 counts in 5s: count rate = 20 ± 2 cps

1000 counts in 50s: count rate = 20 ± 0.63 cps

10000 count in 500s: count rate = 20 ± 0.2 cps

The standard deviation can also be expressed as a percentage.

N=100,

= 10 (10%)

N= 1000,

= 31.6 (3.2%)

N= 10,000,

= 100 (1%)

To be accurate to 1% the counting period must be long enough to give at least 10,000 counts

Slide33

Counting Statistics

The background corrected count rate S, is given by:

where

N is the total count rate (with background) in time t

B is the background count in time t

and

Slide34

Counting Statistics

Example

5 minute count gave

a result of

5325 counts. The background count in 10 minutes was 267 counts. What is the corrected count rate and the standard deviation?

Slide35

Counting Statistics

For low activity sources long count times will be required to achieve an acceptable accuracy

Need to choose the most efficient split of time between the sample count and the background count

Highest accuracy is achieved when

where

is the sample count time

is the background count time

k is the ratio of the sample count rate to the background count rate

Need to do a short count first to determine the count rates and hence determine the value of k

Slide36

Counting Statistics

Example

Sample count rate is about 320

cpm

and the background count rate is 20

cpm

. A total of 10 hours count time is available. How much time should be spend counting the background?

Therefore sample count time should be 4 times the background count time

the background should be counted for 2 hours (and the sample for 8 hours)

Slide37

Leak Testing Sealed Sources

Sealed sources should be leak tested at regular intervals (in UK generally every two years)

Risk assessment should determine most appropriate method – either

direct smearing of the

source

or indirectly (

if source is inaccessible or wiping it could damage the integrity of the source) e.g. by smearing the surface of the source

container. You

might also need to use shielding or source handling tools

etc

Pass/fail criteria should be specified

Slide38

Leak Testing Sealed Sources

Smear paper should be assessed in a suitable counter to determine the presence of any radioactive material, at levels below the pass/fail criteria, that has leaked from the source

Records should be maintained

Further information in ISO 9978: Sealed Radioactive Sources – Leakage Test Methods

Slide39

Summary (1)

Identification of Nuclides –

can be by

α, β or γ spectrometry,

β absorption or half life measurementsDetermination of Sample Activity – need to know counting efficiency, taking into account energy of radiation and also the radiation backgroundSurface Contamination Level

Airborne Contamination Level

Slide40

Summary (2)

Resolving Time –

time that the detector is unable to detect radiation after it has registered a pulse. Reduces effective counting time. More significant for higher count rates

Counting Statistics – standard deviation (σ

) is a measure of the accuracy of a count. . More counts gives better accuracy e.g. 10,000 counts gives 1% accuracy. Accuracy is also affected by the background count rate An Introduction to Radiation Protection 6e © 2012 Martin, Harbison, Beach, Cole/CRC Press

Practical Health Physics Techniques - PowerPoint Presentation

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