EN ISO 20042:2021

SLOVENSKI STANDARD
01-oktober-2021
Merjenje radioaktivnosti - Radionuklidi, ki sevajo žarke gama - Splošna preskusna
metoda z uporabo spektrometrije žarkov gama (ISO 20042:2019)
Measurement of radioactivity - Gamma-ray emitting radionuclides - Generic test method
using gamma-ray spectrometry (ISO 20042:2019)
Bestimmung der Radioaktivität - Gammastrahlung emittierende Radionuklide -
Allgemeines Messverfahren mittels Gammaspektrometrie (ISO 20042:2019)
Mesurage de la radioactivité - Radionucléides émetteurs gamma - Méthode d’essai
générique par spectrométrie gamma (ISO 20042:2019)
Ta slovenski standard je istoveten z: EN ISO 20042:2021
ICS:
13.280 Varstvo pred sevanjem Radiation protection
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 20042
EUROPEAN STANDARD
NORME EUROPÉENNE
August 2021
EUROPÄISCHE NORM
ICS 13.280
English Version
Measurement of radioactivity - Gamma-ray emitting
radionuclides - Generic test method using gamma-ray
spectrometry (ISO 20042:2019)
Mesurage de la radioactivité - Radionucléides Bestimmung der Radioaktivität - Gammastrahlung
émetteurs gamma - Méthode d'essai générique par emittierende Radionuklide - Allgemeines
spectrométrie gamma (ISO 20042:2019) Messverfahren mittels Gammaspektrometrie (ISO
20042:2019)
This European Standard was approved by CEN on 25 July 2021.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 20042:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 20042:2019 has been prepared by Technical Committee ISO/TC 85 "Nuclear energy,
nuclear technologies, and radiological protection” of the International Organization for Standardization
(ISO) and has been taken over as EN ISO 20042:2021 by Technical Committee CEN/TC 430 “Nuclear
energy, nuclear technologies, and radiological protection” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by February 2022, and conflicting national standards
shall be withdrawn at the latest by February 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document is read in conjunction with EN XXX.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 20042:2019 has been approved by CEN as EN ISO 20042:2021 without any modification.

INTERNATIONAL ISO
STANDARD 20042
First edition
2019-06
Measurement of radioactivity —
Gamma-ray emitting radionuclides —
Generic test method using gamma-ray
spectrometry
Mesurage de la radioactivité — Radionucléides émetteurs de
rayons gamma — Méthode d’essai générique par spectrométrie à
rayons gamma
Reference number
ISO 20042:2019(E)
©
ISO 2019
ISO 20042:2019(E)
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
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Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
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Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 20042:2019(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and units . 5
5 Principle . 6
5.1 General . 6
5.2 Summing method . 6
5.3 Fitting method . 7
6 Validating measurements by gamma-ray spectrometry . 7
6.1 General . 7
6.2 Step 1: customer requirements . 8
6.3 Step 2: technical requirements . 8
6.4 Step 3: detailed design .10
6.5 Step 4: installation .10
6.6 Step 5: validation studies .10
6.7 Step 6: robustness .11
6.8 Step 7: operation and maintenance .11
7 Nuclear decay data .11
7.1 Recommended nuclear decay data .11
7.2 Selection of gamma-ray photopeaks for inclusion in spectrum analysis libraries .12
7.3 Decay chains .12
8 Detector energy and efficiency calibration .13
8.1 Energy calibration .13
8.2 Efficiency calibration .13
8.3 Source(s) for energy calibration .14
8.4 Reference source(s) for efficiency calibration .15
8.4.1 General.15
8.4.2 Reference sources for laboratory systems .15
8.4.3 Reference sources used with numerical methods .15
9 Sample container .15
10 Procedure.16
10.1 Sample measuring procedure .16
10.1.1 Sampling.16
10.1.2 Sample preparation .16
10.1.3 Loading the sample container .18
10.1.4 Recording the sample spectrum .18
10.2 Analysis of the spectrum .18
10.2.1 Procedure for laboratory-based measuring systems .18
10.2.2 Background corrections .19
11 Expression of results .20
11.1 Calculation of activity and activity per kg (or m ) of sample .20
11.2 Determination of the characteristic limits .21
12 Test report .21
Annex A (informative) Quality assurance and quality control program .22
Annex B (informative) Corrections to the analysis process .24
Annex C (informative) Uncertainty budget .29
ISO 20042:2019(E)
Annex D (informative) Detector types .32
Annex E (informative) Example: Calculation of Cs activity content and characteristic
limits in an aqueous sample .35
Annex F (informative) Example: Simulating correction factors for sample positioning,
geometry, matrix, density and true summing .40
Bibliography .49
iv © ISO 2019 – All rights reserved

ISO 20042:2019(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies,
and radiological protection, SC 2, Radiological protection.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
ISO 20042:2019(E)
Introduction
Everyone is exposed to natural radiation. The natural sources of radiation are cosmic rays and
naturally occurring radioactive substances which exist in the earth and flora and fauna, including the
human body. Human activities involving the use of radiation and radioactive substances add to the
radiation exposure from this natural exposure. Some of those activities, such as the mining and use
of ores containing naturally-occurring radioactive materials (NORM) and the production of energy
by burning coal that contains such substances, simply enhance the exposure from natural radiation
sources. Nuclear power plants and other nuclear installations use radioactive materials and produce
radioactive effluent and waste during operation and decommissioning. The use of radioactive materials
in industry, agriculture, medicine and research is expanding around the globe.
All these human activities give rise to radiation exposures that are only a small fraction of the global
average level of natural exposure. The medical use of radiation is the largest and a growing man-made
source of radiation exposure in developed countries. It includes diagnostic radiology, radiotherapy,
nuclear medicine and interventional radiology.
Radiation exposure also occurs as a result of occupational activities. It is incurred by workers in
industry, medicine and research using radiation or radioactive substances, as well as by passengers and
crew during air travel. The average level of occupational exposures is generally similar to the global
average level of natural radiation exposure (see Reference [1]).
As uses of radiation increase, so do the potential health risk and the public's concerns. Thus, all these
exposures are regularly assessed in order to,
a) improve the understanding of global levels and temporal trends of public and worker exposure,
b) evaluate the components of exposure so as to provide a measure of their relative importance, and
c) identify emerging issues that may warrant more attention and study.
While doses to workers are mostly measured directly, doses to the public are usually assessed indirectly
using the results of radioactivity measurements of waste, effluent and/or environmental samples.
To ensure that the data obtained from radioactivity monitoring programs support their intended use, it
is essential that the stakeholders (for example nuclear site operators, regulatory and local authorities)
agree on appropriate methods and procedures for obtaining representative samples and for handling,
storing, preparing and measuring the test samples. An assessment of the overall measurement
uncertainty also needs to be carried out systematically. As reliable, comparable and ‘fit for purpose’
data are an essential requirement for any public health decision based on radioactivity measurements,
international standards of tested and validated radionuclide test methods are an important tool for
the production of such measurement results. The application of standards serves also to guarantee
comparability of the test results over time and between different testing laboratories. Laboratories
apply them to demonstrate their technical competences and to complete proficiency tests successfully
during interlaboratory comparisons, two prerequisites for obtaining national accreditation.
Today, over a hundred International Standards are available to testing laboratories for measuring
radionuclides in different matrices.
Generic standards help testing laboratories to manage the measurement process by setting out the
general requirements and methods to calibrate equipment and validate techniques. These standards
underpin specific standards which describe the test methods to be performed by staff, for example, for
different types of sample. The specific standards cover test methods for
40 3 14
— naturally-occurring radionuclides (including K, H, C and those originating from the thorium
226 228 234 238 210
and uranium decay series, in particular Ra, Ra, U, U and Pb) which can be found in
materials from natural sources or can be released from technological processes involving naturally
occurring radioactive materials (e.g. the mining and processing of mineral sands or phosphate
fertilizer production and use), and
vi © ISO 2019 – All rights reserved

ISO 20042:2019(E)
— human-made radionuclides, such as transuranium elements (americium, plutonium, neptunium,
3 14 90
and curium), H, C, Sr and gamma-ray emitting radionuclides found in waste, liquid and gaseous
effluent, in environmental matrices (water, air, soil and biota), in food and in animal feed as a result
of authorized releases into the environment, fallout from the explosion in the atmosphere of nuclear
devices and fallout from accidents, such as those that occurred in Chernobyl and Fukushima.
The fraction of the background dose rate to man from environmental radiation, mainly gamma
radiation, is very variable and depends on factors such as the radioactivity of the local rock and soil, the
nature of building materials and the construction of buildings in which people live and work.
A reliable determination of the activity concentration of gamma-ray emitting radionuclides in various
matrices is necessary to assess the potential human exposure, to verify compliance with radiation
protection and environmental protection regulations or to provide guidance on reducing health risks.
Gamma-ray emitting radionuclides are also used as tracers in biology, medicine, physics, chemistry, and
engineering. Accurate measurement of the activities of the radionuclides is also needed for homeland
security and in connection with the Non-Proliferation Treaty (NPT).
This document describes the generic requirements to quantify the activity of gamma-ray-emitting
radionuclides in samples after proper sampling, sample handling and test sample preparation in a
testing laboratory or in situ.
This document is to be used in the context of a quality assurance management system (ISO/IEC 17025).
It forms the basis for measurement tasks using gamma-ray spectrometry, such as those set out in
ISO 18589-3, ISO 18589-7, ISO 10703, ISO 13164-2 and ISO 13165-3.
This document is one of a set of generic International Standards on measurement of radioactivity such
as ISO 19361.
INTERNATIONAL STANDARD ISO 20042:2019(E)
Measurement of radioactivity — Gamma-ray emitting
radionuclides — Generic test method using gamma-ray
spectrometry
1 Scope
This document describes the methods for determining the activity in becquerel (Bq) of gamma-ray
emitting radionuclides in test samples by gamma-ray spectrometry. The measurements are carried out
in a testing laboratory following proper sample preparation. The test samples can be solid, liquid or
gaseous. Applications include:
— routine surveillance of radioactivity released from nuclear installations or from sites discharging
enhanced levels of naturally occurring radioactive materials;
— contributing to determining the evolution of radioactivity in the environment;
— investigating accident and incident situations, in order to plan remedial actions and monitor their
effectiveness;
— assessment of potentially contaminated waste materials from nuclear decommissioning activities;
— surveillance of radioactive contamination in media such as soils, foodstuffs, potable water,
groundwaters, seawater or sewage sludge;
— measurements for estimating the intake (inhalation, ingestion or injection) of activity of gamma-
ray emitting radionuclides in the body.
It is assumed that the user of this document has been given information on the composition of the test
sample or the site. In some cases, the radionuclides for analysis have also been specified if characteristic
limits are needed. It is also assumed that the test sample has been homogenised and is representative of
the material under test.
General guidance is included for preparing the samples for measurement. However, some types of sample
are to be prepared following the requirements of specific standards referred to in this document. The
generic recommendations can also be useful for the measurement of gamma-ray emitters in situ.
This document includes generic advice on equipment selection (see Annex A), detectors (more detailed
information is included in Annex D), and commissioning of instrumentation and method validation.
Annex F summarises the influence of different measurement parameters on results for a typical
gamma-ray spectrometry system. Quality control and routine maintenance are also covered, but
electrical testing of the detector and pulse processing electronics is excluded. It is assumed that any
data collection and analysis software used has been written and tested in accordance with relevant
software standards such as ISO/IEC/IEEE 12207.
Calibration using reference sources and/or numerical methods is covered, including verification of
the results. It also covers the procedure to estimate the activity content of the sample (Bq) from the
spectrum.
The principles set out in this document are applicable to measurements by gamma-ray spectrometry in
testing laboratories and in situ. However, the detailed requirements for in situ measurement are given
in ISO 18589-7 and are outside the scope of this document.
This document covers, but is not restricted to, gamma-ray emitters which emit photons in the energy
range of 5 keV to 3 000 keV. However, most of the measurements fall into the range 40 keV to 2 000 keV.
The activity (Bq) ranges from the low levels (sub-Bq) found in environmental samples to activities
found in accident conditions and high level radioactive wastes.
ISO 20042:2019(E)
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 542, Oilseeds — Sampling
ISO 707, Milk and milk products — Guidance on sampling
ISO 5500, Oilseed residues — Sampling
ISO 5538, Milk and milk products — Sampling — Inspection by attributes
ISO 5667-1, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes and
sampling techniques
ISO 5667-10, Water quality — Sampling — Part 10: Guidance on sampling of waste waters
ISO 10703, Water quality — Determination of the activity concentration of radionuclides — Method by
high resolution gamma-ray spectrometry
ISO 11929, Determination of the characteristic limits (decision threshold, detection limit and limits of the
confidence interval) for measurements of ionizing radiation — Fundamentals and application
ISO 17604, Microbiology of the food chain — Carcass sampling for microbiological analysis
ISO 18400-101, Soil quality — Sampling — Part 101: Framework for the preparation and application of a
sampling plan
ISO 18400-102, Soil quality — Sampling — Part 102: Selection and application of sampling techniques
ISO 18400-103, Soil quality — Sampling — Part 103: Safety
ISO 18400-104, Soil quality — Sampling — Part 104: Strategies
ISO 18400-107, Soil quality — Sampling — Part 107: Recording and reporting
ISO 18400-202, Soil quality — Sampling — Part 202: Preliminary investigations
ISO 18400-203, Soil quality — Sampling — Part 203: Investigation of potentially contaminated sites
ISO 18400-204, Soil quality — Sampling — Part 204: Guidance on sampling of soil gas
ISO 18400-205, Soil quality — Sampling — Part 205: Guidance on the procedure for investigation of
natural, near-natural and cultivated sites
ISO 18589-2, Measurement of radioactivity in the environment — Soil — Part 2: Guidance for the selection
of the sampling strategy, sampling and pre-treatment of samples
ISO 18589-7, Measurement of radioactivity in the environment — Soil — Part 7: In situ measurement of
gamma-emitting radionuclides
ISO 24333, Cereals and cereal products — Sampling
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM: 1995)
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2 © ISO 2019 – All rights reserved

ISO 20042:2019(E)
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
background continuum
events in the spectrum that form a smooth curve onto which the photopeaks are superimposed
Note 1 to entry: The continuum may arise from gamma-rays scattered inside the test sample or any surrounding
materials, from cosmic radiation or from radionuclides in the surrounding materials.
3.2
blank sample
sample of a similar material to the test sample but containing radioactive impurities negligible in
comparison with the test sample
3.3
calcination
thermal treatment of the powder in order to remove volatile impurities or to change the density or
specific surface area of the powder
[SOURCE: ISO 13779-6:2015, 3.4]
Note 1 to entry: Calcination is commonly used for samples such as soil.
3.4
comminution
operation of reducing particle size by crushing, grinding or pulverisation
3.5
dead time
time during spectrum acquisition (real time) during which pulses are not recorded or processed
Note 1 to entry: Dead time is given by real time minus live time.
Note 2 to entry: The time is given in seconds.
3.6
decision threshold
value of the estimator of the measurand, which when exceeded by the result of an actual measurement
using a given measurement procedure of a measurand quantifying a physical effect, one decides that
the physical effect is present
[SOURCE: ISO 11929:2010, 3.6]
3.7
detection efficiency
probability that a gamma-ray emitted at a particular energy (keV) in the decay of a radionuclide in a
test sample is detected in the photopeak corresponding to that energy
3.8
detection limit
smallest true value of the measurand which ensures a specified probability of being detectable by the
measurement procedure
[SOURCE: ISO 11929:2010, 3.7]
ISO 20042:2019(E)
3.9
fractionation
separation of a product into several fractions by an appropriate technique such as distillation or
crystallization
[SOURCE: ISO 1998-4:1998, 4.20.300]
3.10
full width half maximum
FWHM
width of a gamma-ray photopeak at half the maximum of the photopeak distribution
Note 1 to entry: The width is given in kiloelectronvolts.
3.11
in situ
use of a portable gamma-ray spectrometer for the direct measurement (e.g. in the environment and
buildings) for determination of activity such as per unit of surface area or per mass unit of gamma-
emitting radionuclides present in or deposited on the soil surface or content of large items such as
waste drums
3.12
live time
time during which pulses are processed during an acquisition (real) time
Note 1 to entry: The time is given in seconds.
3.13
net photopeak area
area (number of counts) observed in the photopeak
3.14
pathlength
distance a photon travels through matter
3.15
peak-to-Compton ratio
ratio of the number of counts in the biggest channel of the 1 332,5 keV Co peak to the average number
of counts in the channels representing the range from 1 040 through 1 096 keV
[SOURCE: 325-1996-IEEE Standard Test Procedures for Germanium Gamma-Ray Detectors]
3.16
percolation
separation technique to enrich selective ions of one element (e.g. by ion exchange or precipitation)
3.17
photopeak
peak observed above the background continuum in a gamma-ray spectrum due to events that deposit
the full energy of the photon in the detector material, usually approximately Gaussian in shape
3.18
radionuclide
radioactive nuclide
[SOURCE: IEV 881-02-36]
3.19
real time
time taken to acquire a spectrum
Note 1 to entry: The time is given in seconds.
4 © ISO 2019 – All rights reserved

ISO 20042:2019(E)
3.20
reference source
source containing one or more radionuclides in solid, liquid or gaseous form, sealed in a suitable
-1 -1
container, of known activity (Bq, Bq·g or Bq·ml ), prepared such that the activity is traceable to
national or international primary standards of radioactivity
3.21
region of interest
part of the spectrum that brackets a photopeak
3.22
spectrometry system
complete assembly of the sensor and associated pulse-processing electronics that converts the gamma-
rays detected by the sensor into a pulse-height spectrum
3.23
test sample
artefact (for example sample of soil in a plastic container) for measurement of the content of gamma-ray
emitting radionuclides
3.24
true coincidence summing
simultaneous detection of two or more gamma-rays in the spectrometry system, due to the emission of
a cascade of gamma-rays in the decay of a single nucleus in the test sample
4 Symbols and units
For the purpose of this document, the following symbols apply.
Table 1 — Symbols and units
A Activity (Bq) of each radionuclide in the calibration source at the time of calibration (t ).
c
-1
a, a Activity (Bq) of radionuclide in the sample, activity per unit mass (Bq·kg ), in the sample
m
a* Decision threshold (Bq)
#
a Detection limit (Bq)
True value of the activity (Bq)

a
ε Detection efficiency at energy, E
E
f Factor to correct for the radioactive decay during the counting time, t and t
d i
P Probability of the emission by a radionuclide of a gamma-ray with energy, E, per decay
E
−1 −1
λ Decay constant of a radionuclide (s ). The decay constant equals ln2·t
½
m Sample mass (kg)
n , Number of counts in the net area of the photopeak at energy, E, in the sample spectrum
N,E
n Number of counts in the net area of the photopeak at energy, E, in calibration spectrum
Ns,E
u Standard uncertainty associated with the measurement result (Bq)
U Expanded uncertainty calculated by U = k·u where k is the coverage factor (Bq)
t Sample spectrum counting time (live time) (s)
t Time between the reference time for the results and the start of the count time (s)
i
t Calibration spectrum counting time (live time) (s)
s
t Half-life of a radionuclide (s)
½
V Sample volume (m )
ISO 20042:2019(E)
5 Principle
5.1 General
The activity of gamma-ray emitting radionuclides in test samples is commonly determined using high
resolution gamma-ray spectrometry techniques based on the analysis of the energies and the areas of
the photopeaks. These techniques allow the identification and the quantification of the radionuclides
and are normally performed by the analysis software.
NOTE Lower-resolution detectors, such as sodium iodide or other scintillation materials, can be used for
the measurement of radioactivity in test samples in certain cases (see ISO 19581). For example, low-resolution
detectors are useful for rapid screening of samples of foodstuffs in the case of a nuclear incident but high-
resolution spectrometry is essential for samples that can contain complex mixtures of radionuclides, such as
environmental samples.
The nature and geometry of the detectors as well as the test samples call for appropriate energy and
efficiency calibrations. For semi-conductor detectors, freed charge is generated by the interaction of
ionising radiation with the detector material (through the photoelectric effect, the Compton effect
or pair production). A high-voltage supply applies a bias voltage to the detector crystal resulting in
an electric field. The freed charge is accelerated by the electric field towards the detector electrodes.
The collected charge is converted into an output voltage pulse by a preamplifier and the output pulse
is shaped and amplified by the main amplifier. The pulse amplitude is converted to a digital value by
an analog-to-digital converter (ADC) and the pulse-height histogram (spectrum) is stored using a
multichannel analyzer (MCA). The height of the pulse is proportional to the amount of freed charge and
hence to the energy of the ionising radiation striking the sensitive volume of the detector. Digital data
acquisition systems are also available that carry out the same function as the analogue electronics.
The spectrum stored by the MCA shows a set of peaks (photopeaks) superimposed on a background
continuum from scattered radiation; Reference [21] contains examples of gamma-ray spectra. The
photopeaks are approximately Gaussian in shape. The channel number of the photopeak centroid
depends on the energy of the photon detected. The net photopeak area is proportional to the number of
photons of that energy that have interacted with the detector during the counting period (corrected for
dead time). The net photopeak area is normally determined in the analysis software package by one of
two different techniques – summation or fitting.
5.2 Summing method
The number of counts in the photopeak is calculated by summing the total number of counts in a region
of interest around the photopeak and subtracting counts in the background continuum. The total
number of counts is given by:
H
NC= (1)
∑ i
iL=
where
N is the total number of counts from channel L (lowest) to channel H (highest) in the region of
interest;
C is the number of counts in channel number i.
i
Assuming the background continuum under the photopeak is linear, the background in the same region
of interest is given by:
nC()+C
LH
B = (2)
where
6 © ISO 2019 – All rights reserved

ISO 20042:2019(E)
B is the number of counts in the background from channel L to channel H;
n is the number of channels in the region of interest (n = H – L + 1).
The net photopeak area is given by N − B and the standard uncertainty in the photopeak area (assuming
a Poisson distribution for the contents of each channel) is given by:
H 2 2
 
nC +C
()
LH
 
uC=+ (3)
i
∑
 4 
iL=
 
Different software packages use different methods to determine the upper and lower bounds of the
region of interest and the shape of the background function. The region of interest shall be selected
carefully, particularly when the photopeak is near to discontinuities in the spectrum, near another
photopeak or located on a high background continuum (see Reference [2]).
The photopeak position is generally determined from the net counts in each channel:
H
′
iC⋅
∑ i
iL=
C = (4)
h
H
′
C
i
∑
iL=
where
C is the photopeak position (channel);
h
C' is the net count in channel i.
i
5.3 Fitting method
In this method, the net photopeak area is determined by non-linear least squares fitting of an analytical
function to the counts in the region of interest. The analytical function for an individual photopeak
is normally Gaussian, but some approaches include one or more exponential tails to approximate the
photopeak shape more closely. The net photopeak area and photopeak position are determined from
the values of the fitted parameters. Further details on the uncertainty in the photopeak area using this
approach are given in Reference [2].
The fitting method shall be used to determine the net areas of overlapping photopeaks in a spectrum.
The radionuclides in the test sample may be identified from the energies of the photopeaks present; the
activity (Bq) in the test sample may also be determined from the count rate observed in the photopeak,
corrected for factors such as detection efficiency, gamma-ray-emission probability and decay. Care
shall also be taken to apply corrections for effects not covered by many commercial spectrum analysis
software packages, such as true coincidence summing.
NOTE This description applies to semi-conductor detectors including CdZnTe but similar principles can also
be applied to other detectors [NaI(Tl), LaBr (Ce), CeBr , etc.].
3 3
6 Validating measurements by gamma-ray spectrometry
6.1 General
This subclause describes the steps to be followed from setting out the customer requirements and
selecting the equipment through to operation and maintenance, as also required by ISO/IEC 17025.
Documented evidence shall be available to demonstrate that the measurement procedures meet
customer requirements. The validation process is summarized in Figure 1.
ISO 20042:2019(E)
Figure 1 — Schematic diagram of validation process
6.2 Step 1: customer requirements
The specification for the spectrometry system shall be defined, including the energy range,
characteristic limits, maximum activity and uncertainty required by the customer. In consultation with
other members of staff as appropriate, the specification shall also take into account:
— compatibility with existing equipment;
— location (for example proximi
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