IEC 61453:2007

Nuclear instrumentation - Scintillation gamma ray detector systems for the assay of radionuclides - Calibration and routine tests

Specifies methods of calibration and routine tests of scintillation detector systems for the measurement of gamma-ray energies and emission rates of radionuclides and the assay of radioactivity. Is applicable to scintillation detector systems based on inorganic scintillators for photon measurements. This second edition reflects an expansion of detector types considered.

Instrumentation nucléaire - Equipements avec détecteurs à scintillation de rayonnement gamma, pour le dosage de radionucléides - Etalonnage et essais individuels

Spécifie les méthodes d'étalonnage et d'essais individuels des équipements de détection à scintillation destinés à la mesure des énergies des rayonnements gamma et des taux d'émission gamma de radionucléides, ainsi qu'au dosage de la radioactivité. Est applicable aux équipements de détection à scintillation basés sur des scintillateurs inorganiques pour la mesure des rayonnements photons. Cette deuxième édition inclue une extension des types de détecteurs considérés.

General Information

• Status: Published
• Publication Date: 09-Aug-2007
• ICS: 27.120.01 - Nuclear energy in general
• Technical Committee: TC 45 - Nuclear instrumentation
• Drafting Committee: WG 9 - TC 45/WG 9
• Current Stage: PPUB - Publication issued
• Start Date: 10-Aug-2007
• Completion Date: 15-Sep-2007

Relations

• Revises: IEC 61453:1997 - Nuclear instrumentation - Thallium activated sodium iodide detector systems for assay of radionuclides - Calibration and usage
• Effective Date: 05-Sep-2023

Overview

IEC 61453:2007 is an international standard developed by the International Electrotechnical Commission (IEC) specifically for nuclear instrumentation involving scintillation gamma ray detector systems. It addresses the calibration and routine testing methods required for accurate assay of radionuclides through gamma-ray energy measurements and emission rates. The standard is applicable to scintillation detector systems that use inorganic scintillators for photon detection and measurement.

This second edition of the IEC 61453 standard broadens its scope beyond sodium iodide detectors to include other inorganic scintillators, reflecting advancements in detector technology. The document supports nuclear instrumentation professionals in ensuring reliable, standardized measurements for various applications involving radioactivity assessments.

Key Topics

• Calibration Methods
  The standard outlines calibration procedures for energy calibration and efficiency calibration of gamma ray detector systems. It supports several counting techniques including:

  • Total spectrum counting systems: counting all pulses above a low-energy threshold.
  • Single-channel analyzer (SCA) counting systems: counting within specific energy windows.
  • Multichannel analyzer (MCA) counting systems: utilizing multiple energy windows to enhance measurement precision.

• Measurement Accuracy and Definitions
  The document provides detailed definitions for terms such as activity (measured in becquerels), detector efficiency, and resolving time correction. This ensures consistent terminology and understanding across professionals using scintillation gamma ray detector systems.

• Routine Testing
  Clear guidelines for routine tests are specified to confirm consistent detector performance and reliability over time, including background level assessments and check source utilization.

• Sources of Error and Correction Procedures
  IEC 61453 highlights critical error sources such as dead time corrections, counting geometry effects, temperature influences, and geometric correction factors. It emphasizes the importance of applying proper corrections to reduce measurement uncertainty.

• Detector Assemblies
  The standard defines components of a typical scintillation detector system - such as the scintillator material, photomultiplier tubes (or photodiodes), and associated electronics - providing a complete framework for calibration and testing.

Applications

IEC 61453:2007 is vital for professionals engaged in:

• Nuclear Industry and Environmental Monitoring
  Assay of radionuclides for safety, monitoring contamination, and compliance with regulatory requirements.

• Medical Physics and Radiology
  Calibration of gamma ray detectors used in diagnostic imaging, radiotherapy, and nuclear medicine.

• Industrial Radiography and Material Testing
  Quality assurance of radioactive sources and detector systems involved in nondestructive testing (NDT).

• Research and Development
  Standardizing measurements in nuclear science research involving gamma spectroscopy.

By adhering to IEC 61453’s calibration and testing procedures, users can ensure high measurement accuracy, detector reliability, and results that meet international traceability standards.

Related Standards

For comprehensive nuclear instrumentation practices and terminology, IEC 61453:2007 should be used alongside related standards including:

• IEC 61948-1:2001 – Provides definitions related to energy calibration in radiation detectors.
• IEC 60788:2004 – Deals with activity measurements and radioactivity data.
• IEC 62372:2006 – Defines detector assembly components and testing methods.
• ISO/IEC Directives – Guidelines for preparing and maintaining international standards.

Additionally, users are encouraged to consult the IEC Electropedia online dictionary for terms related to scintillation detectors, nuclear instrumentation, and gamma-ray measurements to maintain consistency in definitions and terminology.

---

By implementing IEC 61453:2007, industries and laboratories benefit from standardized, validated calibration and routine testing of scintillation gamma ray detector systems ensuring accurate assay of radionuclides for safety, compliance, and operational efficiency.

Standards Content (Sample)

IEC 61453
Edition 2.0 2007-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Nuclear instrumentation – Scintillation gamma ray detector systems for the
assay of radionuclides – Calibration and routine tests

Instrumentation nucléaire – Equipements avec détecteurs à scintillation de
rayonnement gamma, pour le dosage de radionucléides – Etalonnage et essais
individuels
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IEC 61453
Edition 2.0 2007-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Nuclear instrumentation – Scintillation gamma ray detector systems for the
assay of radionuclides – Calibration and routine tests

Instrumentation nucléaire – Equipements avec détecteurs à scintillation de
rayonnement gamma, pour le dosage de radionucléides – Etalonnage et essais
individuels
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
R
CODE PRIX
ICS 27.120 ISBN 2-8318-9258-9
– 2 – 61453 © IEC:2007
CONTENTS
FOREWORD.3

1 Scope.5
2 Terms, definitions, symbols and abbreviations.5
2.1 Terms and definitions .5
2.2 Symbols and abbreviations.8
3 Procedure .9
3.1 Total spectrum counting systems .9
3.1.1 General .9
3.1.2 System response calibration.9
3.1.3 Activity determination .10
3.1.4 Routine test.10
3.2 Single-channel analyzer counting systems .11
3.2.1 General .11
3.2.2 Energy calibration.11
3.2.3 Total absorption peak efficiency calibration .11
3.2.4 Activity determination .11
3.2.5 Routine test.12
3.3 Multichannel analyzer counting systems.13
3.3.1 General .13
3.3.2 Energy calibration.13
3.3.3 Total absorption peak efficiency calibration (see 5.10).13
3.3.4 Activity determination .14
3.3.5 Routine test.14
4 Sources of error and uncertainty.15
5 Precautions .15
5.1 Assay of a radionuclide for which no reference source is readily available .15
5.2 Assay of mixtures of radionuclides .16
5.3 Thin-window detectors.16
5.4 Count rates .16
5.5 Geometric correction factors .16
5.6 Counting statistics and range of measurement .16
5.7 Dead time corrections .16
5.8 Correction for decay during the counting period .17
5.9 Counting geometry .18
5.10 Total absorption peak efficiency versus energy function .18
5.11 Net count rate .18
5.12 Temperature effects .18

Bibliography.19

61453 © IEC:2007 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
NUCLEAR INSTRUMENTATION –
SCINTILLATION GAMMA RAY DETECTOR SYSTEMS
FOR THE ASSAY OF RADIONUCLIDES –
CALIBRATION AND ROUTINE TESTS
FOREWORD
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International Standard IEC 61453 has been prepared by IEC International Committee 45:
Nuclear instrumentation.
This second edition cancels and replaces the first edition published in 1997. It constitutes a
technical revision and an expansion of detector types considered.
The major change in comparison with the previous edition of IEC 61453 is an expansion of
detector types considered. Along with sodium iodide detector systems, this new edition
standardizes scintillation detector systems based on other inorganic scintillators for photon
measurements. Furthermore, Clause 2 has been updated.
The revision of the standard is intended to accomplish the following:
• to extend detector systems base from sodium iodide to inorganic scintillators for photon
measurements;
• to review the existing requirements and to update the terminology, definitions and
normative references.
– 4 – 61453 © IEC:2007
The text of this standard is based on the following documents:
FDIS Report on voting
45/645/FDIS 45/646/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
61453 © IEC:2007 – 5 –
NUCLEAR INSTRUMENTATION –
SCINTILLATION GAMMA RAY DETECTOR SYSTEMS
FOR THE ASSAY OF RADIONUCLIDES –
CALIBRATION AND ROUTINE TESTS
1 Scope
This International Standard specifies methods of calibration and routine tests of scintillation
detector systems for the measurement of gamma-ray energies and emission rates of
radionuclides and the assay of radioactivity.
This International Standard is applicable to scintillation detector systems based on inorganic
scintillators for photon measurements.
Typical applications include radionuclide identification and assay in various industrial,
environmental, and medical applications. The detector system consists of three major
components: a scintillating material that produces photons of light when ionizing radiation
interacts with it; one or more photomultipliers or photodiodes, optically coupled to the
scintillator, which convert the light photons to an amplified electrical pulse or pulses; and
associated electronic instrumentation which powers the photomultiplier and processes the
output signal.
Both energy calibration and efficiency calibration are covered. The following three techniques
are considered:
a) total spectrum counting (see 3.1) which employs a system that counts all pulses above a
low-energy threshold (see 5.1, 5.2 and 5.3);
b) single-channel analyzer (SCA) counting (see 3.2) which employs a system with a counting
channel established through upper and lower energy boundaries (see 5.1, 5.2, and 5.3);
c) multichannel analyzer counting (see 3.3) which employs a system in which multiple
counting windows are utilized. This technique allows measurements for which the
continuum under the total absorption peak may be subtracted without introducing
unacceptable error. In case of overlapping peaks in the spectrum, a multichannel analyzer
(MCA) with access to a peak deconvolution program is necessary. This case is not
covered by this standard.
2 Terms, definitions, symbols and abbreviations
2.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1.1
accuracy of measurement
closeness of the agreement between the result of a measurement and the conventionally true
value of the measurand
NOTE 1 "Accuracy" is a qualitative concept.
NOTE 2 The term precision should not be used for "accuracy".
[IEV 394-40-35]
– 6 – 61453 © IEC:2007
2.1.2
activity
A
quantitative indication of the radioactivity of an amount of radionuclide in a particular energy
state at a given time. Activity is determined as the quotient of dN by dt, where dN is the
expectation value of the number of spontaneous nuclear transitions from that energy state in
the time interval dt:
d N
A =
dt
–1
The unit of activity is the reciprocal second (s ). The special name of the unit of activity is the
becquerel (Bq), 1 Bq being equal to one transition per second. The earlier unit of activity was
the curie (Ci), 1 Ci being equal to 3,7 × 10 transitions per second
[IEC 60788:2004, rm-13-18]
2.1.3
assay of activity
determination of the activity of a radionuclide in a sample
2.1.4
assembly
a light protective chamber containing a housed scintillator, photomultiplier, photomultiplier
voltage divider
NOTE Assembly is used for testing of the housed scintillator.
[IEC 62372:2006, 3.1.4]
2.1.5
background level (of a measuring assembly)
signals of origin other than the radiation to be detected.
NOTE It may refer to:
a) signals caused by radiations from sources inside or outside the detector other than those of interest in the
measurement;
b) signals resulting from the short-comings of the electronic circuits of the detecting system and their power
supplies.
[IEV 394-39-08]
2.1.6
check source
radioactive source used to confirm the normal operation of measuring instruments
NOTE A source placed at a given distance from the detector producing a stable and reproducible indication.
[IEV 394-40-18]
2.1.7
resolving time correction
dead time correction
correction to be applied to the observed number of pulses in order to take into account the
number of pulses lost due to the resolving time or the dead time
[IEV 394-39-22]
61453 © IEC:2007 – 7 –
2.1.8
detector efficiency
ratio of the number of detected photons or particles to the number of photons or particles of
the same type which are incident on the detector in the same time interval
[IEV 394-38-17]
2.1.9
energy calibration
process of establishing a relation between the window setting of the pulse height analyzer and
the energy of the photons
[IEC 61948-1:2001, 3.5]
2.1.10
energy resolution
term used to characterize the ability of radiation detector to distinguish between photons of
different energies
NOTE The energy resolution can be expressed as the ratio of the peak full width at half maximum (FWHM) to
peak energy expressed as a percentage.
[IEC 61948-1:2001, 3.6]
2.1.11
Full Width at Half Maximum
FWHM
in a distribution curve comprising a single peak, the distance between the abscissa of two
points on the curve whose ordinates are half of the maximum ordinate of the peak
NOTE If the curve considered comprises several peaks, a full width at half maximum exists for each peak.
[IEC 62372:2006, 3.1.11]
2.1.12
live time
duration during which a detection assembly is sensitive to the input signal
[IEV 394-39-31]
2.1.13
net count rate
observed count rate (number of counts per unit time) corrected for dead time minus
background count rate
2.1.14
radiation detection assembly
assembly designed to produce a signal in response to incident ionizing radiation
NOTE 1 This signal carries information about physical properties of the radiation.
NOTE 2 One or more sub-assemblies may be included in the same unit.
[IEV 394-21-11]
2.1.15
reference source
radioactive secondary standard source for use in the calibration of the measuring instrument
[IEV 394-40-19]
– 8 – 61453 © IEC:2007
2.1.16
response (of a radiation measuring assembly)
ratio, under specified conditions, given by the relation:
v
R =
v
c
where
v is the value measured by the equipment or assembly under test;
v is the conventionally true value of this quantity
c
NOTE 1 The input signal to a measuring system may be called the stimulus; the output signal may be called the
response (IVM).
NOTE 2 Response can have several definitions. As an example, the definition of the response of a radiation
measuring assembly is given.
[IEV 394-40-21]
2.1.17
routine test
conformity test made on each individual item during or after manufacture
[IEV 394-40-03]
2.1.18
total absorption peak
portion of the spectral response curve corresponding to the total absorption of photon energy
in a radiation detector
NOTE This peak represents the total absorption of photon energy from all interactive processes, namely:
a) photoelectric absorption,
b) Compton effects, and
c) pair production.
[IEV 394-38-57]
2.2 Symbols and abbreviations
A the activity of a sample;
A the activity of a reference source;
r
C   the net count rate of a sample;
n
C   the net count rate of a reference source;
nr
G  the gamma-ray emission rate of a sample;
G  the gamma-ray emission rate of the gamma-ray of interest of energy E of a
E
sample;
G the gamma-ray emission rate of a reference source;
r
F multiplicative correction factor to correct for decay of the source during
b
counting;
F  multiplicative correction factor considering decay during a measurement;
m
ε  the total absorption peak efficiency;
λ  the radionuclide decay constant;
C  the observed count rate;
C dead time corrected count rate;
o
61453 © IEC:2007 – 9 –
P  the absolute emission probability (of the gamma rays of interest) per decay;
R the response;
T  the radionuclide half-life;
1/2
t  the counting time;
t the dead time;
d
S the standard deviation of a background;
B
v the value of a quantity measured by the equipment or assembly under test;
v the conventionally true value of this quantity;
c
dV the number of spontaneous nuclear transitions from that energy state;
dt the time interval;
FWHM full width at half maximum;
SCA single-channel analyzer;
MCA multichannel analyzer.
3 Procedure
3.1 Total spectrum counting systems
3.1.1 General
All instruments shall be installed and operated in accordance with the manufacturer's
instructions. The activity of a radionuclide can only be determined if the instrument has been
calibrated with a reference source (or simulated reference source) of the radionuclide being
assayed and in the absence of other radionuclides.
3.1.2 System response calibration
3.1.2.1 Set the lower-level discriminator to a value such that the following conditions are
satisfied:
a) the gamma rays of interest are being counted;
b) the system response is insensitive to small changes in discriminator setting;
c) any significant electronic noise is below the counting threshold and the upper-limit
discriminator is set to the highest possible setting.
3.1.2.2 For each radionuclide of interest, accumulate counts using a reference source in the
reproducible counting geometry desired (see 5.5). At least 10 000 total counts should be
accumulated (see 5.4; 5.6).
3.1.2.3 Correct for dead time as specified in 5.7.
3.1.2.4 Obtain the net count rate by subtracting the background level count rate from the
total count rate. The same instrument settings shall be used for both counts.
3.1.2.5 Correct for decay of the reference source activity from the time of calibration to the
time at which the count rate is measured (see 5.8).
3.1.2.6 Calculate the response R as follows:
C
nr
R = (1)
A
r
– 10 – 61453 © IEC:2007
where
C is the net count rate of the reference source (according to 3.1.2.4);
nr
A is the activity of the reference source (according to 3.1.2.5).
r
3.1.3 Activity determination
3.1.3.1 Using the instrument settings according to 3.1.2, place the sample to be measured in
the same counting geometry that was used for the system response calibration (see 5.5; 5.9).
3.1.3.2 Accumulate enough counts to obtain the desired statistical level of accuracy
(see 5.4; 5.6).
3.1.3.3 Correct the count rate for dead time as specified in 5.7.
3.1.3.4 Obtain the net count rate for the sample by subtracting the background level count
rate from the total count rate.
3.1.3.5 Calculate the activity A of the sample by
C
n
A = (2)
R
where
C is the net count rate of the sample (according to 3.1.3.4);
n
R is the response (according to 3.1.2.6).
3.1.4 Routine test
3.1.4.1 Reproducibility tests shall be performed by checking the system response calibration
at least once in every week of use with at least one long-lived radioactive checking source
with energies that span the region of interest. Correction for radioactive decay of the source
since its calibration shall be applied.
3.1.4.2 The response calibration of an idle system when returned to use shall be checked at
least semi-annually by using reference sources of radionuclides that span the energy region
of interest.
3.1.4.3 The background level of the system shall be measured immediately before and after
each batch of samples. The background level shall also be measured periodically, at least
once in every week of use.
For accurate assays of radioactive materials whose activities are only slightly above
background, the system background should be determined using a sufficient number of
background readings and using counting times of sufficient length so as to minimize the
uncertainty associated with the background count rate.
3.1.4.4 The results of all performance checks shall be recorded in such a way that
deviations from the norm will be readily observable. Appropriate action which could include
confirmation, repair and recalibration as required shall be taken when the measured values
fall outside of predetermined limits.

61453 © IEC:2007 – 11 –
3.2 Single-channel analyzer counting systems
3.2.1 General
All instruments shall be installed and operated in accordance with the manufacturer's
instructions.
3.2.2 Energy calibration
Establish the energy calibration of the system over the desired energy region at a fixed gain.
Using sources of known energy, determine the relationship between the gamma-ray energies
and the corresponding settings of the discriminator. Measure the count rate as a function of
the lower level discriminator setting at increments of not more than 2 % of the energy range of
interest. The window width should be constant and approximately equal to the increments of
the lower level discriminator setting. The centre of the window position corresponding to the
highest count rate may be assumed to be the centre of the total absorption peak. An improved
position can be found through function fit to the count rates around the maximum. The energy
calibration shall be determined for each amplifier gain and photomultiplier high-voltage setting
used. Radionuclides for which assays will be performed should be used for the energy
calibration. If that is not practical, radionuclides with gamma rays that span the energy region
of interest shall be used. It is recommended to use single- or double-line emitting
radionuclides for the energy calibration.
3.2.3 Total absorption peak efficiency calibration
3.2.3.1 The lower level discriminator and the window width shall be set to include the total
absorption peak(s) of interest.
3.2.3.2 For each radionuclide of interest, accumulate counts using a reference source in a
desired and reproducible counting geometry (see 5.5). At least 10 000 total counts should be
accumulated (see 5.4; 5.6).
3.2.3.3 Correct for dead time as specified in 5.7.
3.2.3.4 Obtain the net count rate by subtracting the background level count rate from the
total count rate. The same instrument settings shall be used for both counts.
3.2.3.5 Correct the reference source gamma-ray emission rate for decay from the time of
calibration to the time at which the count rate is measured (see 5.8).
3.2.3.6 Calculate the total absorption peak efficiency (ε) for each gamma-ray energy as
follows:
C
nr
ε =   (3)
G
r
If the reference source is calibrated with regard to activity, the gamma-ray emission rate is
given by
G = A × P    (4)
r r
3.2.4 Activity determination
3.2.4.1 Using the instrument settings of 3.2.3, place the sample to be measured in the same
counting geometry that was used for the efficiency calibration (see 5.5; 5.9).

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3.2.4.2 Accumulate enough counts to obtain the desired statistical level of accuracy
(see 5.4; 5.6).
3.2.4.3 Correct the count rate for dead time as specified in 5.7.
3.2.4.4 Obtain the net count rate for the sample by subtracting the background level count
rate from the total count rate (see 5.8).
3.2.4.5 Calculate the gamma-ray emission rate of the sample by
C
n
G = (5)
E
ε
When calculating an activity, the number of gamma rays emitted per decay is required, so that
G
E
A = (6)
P
3.2.5 Routine test
3.2.5.1 The system energy calibration shall be checked on every day of use with one or
more checking sources emitting gamma rays in the energy range of interest.
3.2.5.2 The energy resolution of the system shall be determined at the time of initial
installation and checked at least once in every month of use. The energy resolution should be
performed with a Cs source and a window width less than 1 % of the relevant energy span.
The window should be moved in 1 % increments from 10 % below the 661,6 keV peak to 10 %
above. The background level shall be determined below and above the peak and an
approximately linear baseline under the peak shall be calculated to correct all measured count
rates for the respective baseline contributions. FWHM shall be calculated by interpolation on
either side.
3.2.5.3 Reproducibility tests shall be performed by checking the efficiency calibration at
least once in every month of use with at least one long-lived radioactive checking source with
energies that span the region of interest. Correction for radioactive decay of the source since
its calibration shall be applied.
3.2.5.4 The efficiency calibration of an idle system when returned to use shall be checked at
least semi-annually by using reference sources of radionuclides that span the energy region
of interest.
3.2.5.5 The background level of the system shall be measured immediately before and after
each batch of samples. In addition, the background level shall also be measured periodically.
For accurate assays of radioactive materials whose activities are only slightly above
background, the system background should be determined using a sufficient number of
background readings and using counting times of sufficient length so as to minimize the
uncertainty associated with the background count rate.
3.2.5.6 The results of all performance checks shall be recorded in such a way that
deviations from the norm will be readily observable. Appropriate action, which could include
confirmation, repair and recalibration as required, shall be taken when the measured values
fall outside of predetermined limits.

61453 © IEC:2007 – 13 –
3.3 Multichannel analyzer counting systems
3.3.1 General
All instruments shall be installed and operated in accordance with the manufacturer's
instructions.
3.3.2 Energy calibration
Establish the energy calibration of the system over the desired energy region at a fixed gain.
Using sources of a known energy, record a spectrum containing total absorption peaks which
span the gamma-ray energy region of interest. Determine the channel numbers which
correspond to two gamma-ray energies that are near the extremes of the energy region of
interest. From those data, determine the slope and the intercept of the energy calibration
curve. For most applications, such a linear energy calibration curve will be adequate, except
when dealing with the low-energy regime. The energy calibration shall be determined for each
amplifier gain and photomultiplier high-voltage setting used.
3.3.3 Total absorption peak efficiency calibration (see 5.10)
3.3.3.1 Accumulate gamma-ray spectra using reference sources in a desired and
reproducible counting geometry (see 5.5). At least 10 000 total counts should be accumulated
in each total absorption gamma-ray peak of interest (see 5.4; 5.6).
3.3.3.2 Record the live time counting interval (see 5.4).
3.3.3.3 For each reference source, determine the net counts in the total absorption gamma-
ray peaks of interest (see 5.11).
3.3.3.4 Correct the reference source gamma-ray emission rate for decay from the time of
calibration to the time at which the count rate is measured (see 5.8).
3.3.3.5 Calculate the total absorption peak efficiency (ε) for each gamma-ray energy as
follows:
C
nr
ε = (7)
G
r
where
C is the net count rate in the total absorption peak (according to 3.3.3.2; 3.3.3.3);
nr
G is the gamma-ray emission rate of the reference source (according to 3.3.3.4).
r
If the reference source is calibrated in terms of activity, the gamma-ray emission rate is given
in accordance with equation (4) (see 3.2.3.6).
3.3.3.6 To obtain total absorption peak efficiency calibration data at energies for which
reference sources are not available, plot and fit an appropriate mathematical function to the
values for the total absorption peak efficiency (according to 3.3.3.5) versus gamma-ray
energy (see 5.12).
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3.3.4 Activity determination
3.3.4.1 Using the instrument settings of 3.3.3, place the sample to be measured in the same
counting geometry that was used for the efficiency calibration (see 5.5; 5.9).
3.3.4.2 Accumulate enough counts in the gamma-ray spectrum to obtain the desired
statistical level of accuracy (see 5.4; 5.6).
3.3.4.3 Record the live time counting interval (see 5.7; 5.4).
3.3.4.4 Determine the energy of the gamma rays present by the use of the energy calibration
data obtained according to 3.3.2.
3.3.4.5 Obtain the net count rate in each total absorption gamma-ray peak of interest (see
5.8; 5.11) by dividing the net counts by the live time.
3.3.4.6 Calculate the gamma-ray emission rate for each total absorption peak of interest as
follows:
C
n
G = (8)
ε
where
C is the net count rate in the total absorption peak (according to 3.3.4.5);
n
ε the total absorption peak efficiency (according to 3.3.3.5).
From the identified gamma-ray energies and other information available, decide which
radionuclides are present in the sample. When calculating the activity of a specific nuclide the
number of gamma-rays emitted per decay is required for each energy, equation (6) (see
3.2.4.5).
3.3.5 Routine test
3.3.5.1 The system energy calibration shall be checked on every day of use with one or
more checking sources emitting gamma rays in the energy range of interest.
3.3.5.2 The energy resolution of the system shall be determined at the time of initial
installation and checked at least once in every week of use.
3.3.5.3 Reproducibility tests shall be performed by checking the efficiency calibration at
least once in every month of use with at least one long-lived radioactive checking source with
energies that span the region of interest. Correction for radioactive decay of the source since
its calibration shall be applied.
3.3.5.4 The efficiency calibration of an idle system when returned to use shall be checked
at least semi-annually by using reference sources of radionuclides that span the energy
region of interest.
3.3.5.5 The background level of the system shall be measured immediately before and after
each batch of samples. The background level shall also be measured periodically, at least
daily.
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For accurate assays of radioactive materials whose activities are only slightly above
background, the system background should be determined using a sufficient number of
background readings and using counting times of sufficient length so as to minimize the
uncertainty associated with the background count rate.
3.3.5.6 The results of all performance checks shall be recorded in such a way that
deviations from the norm will be readily observable. Appropriate action, which could include
confirmation, repair and recalibration as required, shall be taken when the measured values
fall outside of predetermined limits.
4 Sources of error and uncertainty
Possible sources of error and uncertainty in inorganic scintillator measurements are listed in
a) to q).
a) Uncertainties in the calibration of the reference sources.
b) Error through deviation in the sample geometry from the standard geometry. This may
involve nearby material that can scatter gamma rays into the detector.
c) Error through variations in radiation background level (particularly for low-activity
measurements).
d) Error through the presence of radionuclide impurities or stray sources.
e) Error through differences in attenuation due to differences in container wall thickness or
material.
f) Error through non-uniformity of the radioactivity distribution in the sample.
g) Uncertainty through Compton continuum subtraction.
h) Error through timing, including errors in dead time correction.
i) Error through equipment malfunction.
j) Uncertainty through the counting of beta particles, conversion electrons, and
bremsstrahlung which are energetic enough to enter the scintillator and add to the
gamma-ray pulse-height spectrum.
k) Error through random photon summing at high count rates (see 5.4).
l) Error through photomultiplier tube gain drift as a function of time or count rate.
m) Error through gain shift caused by a changing magnetic field or change in the orientation
of the detector in a fixed magnetic field.
n) Error through gain shift caused by temperature change.
o) Error through detector activation by neutrons and charged particles.
p) Error through saturation or ringing of amplifiers or other system components.
q) Uncertainty in the decay correction.
5 Precautions
5.1 Assay of a radionuclide for which no reference source is readily available
A total-spectrum counting system or a single-channel analyzer counting system shall not be
used for quantitative determinations of radionuclides for which calibrated reference sources
are not available. Multichannel analyzer counting systems shall be used in such cases.

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5.2 Assay of mixtures of radionuclides
A total-spectrum counting system or a single-channel analyzer counting system shall not be
used for attempted quantification of the radionuclides contained within a mixture. In certain
special cases, multiple single-channel analyzer counting systems and multichannel analyzer
devices can be successfully used for the measurement of mixtures containing two
radionuclides. Such cases require careful procedures to ensure accurate results. However,
the use of MCAs is recommended.
5.3 Thin-window detectors
When working with a thin-window detector, it is necessary to be cautious about radionuclides
emitting conversion electrons, as these have energies close to that of the interesting gamma
rays. To avoid unquantifiable counting of conversion electrons in such detectors, insert a
sufficient amount of absorbing material between the source and the detector, then calibrate
the detector.
5.4 Count rates
Three scintillation detector systems are described: a total spectrum counting system, single-
channel analyzer (SCA) counting system and multichannel analyzer (MCA) counting system.
The performance of each system over a wide range of count rates is unique. In general, the
simplest system provides the highest rate capability and requires the fewest corrections.
System gain, stability, random coincidence losses and spectral shape may be counting rate
dependent.
5.5 Geometric correction factors
The dependence of the measurement on the geometric configuration and composition of the
sample container and other absorbers shall be taken into consideration in the calibration
procedure. Positioning of the sample containers within detector wells usually provides good
positional reproducibility. Positioning of sample containers on or above the surface of
detectors requires a method for reproducing the position. New correction factors and
calibrations shall be obtained when assaying radionuclides in containers of different size or
shape or position.
5.6 Counting statistics and range of measurement
It is recommended to measure more than 10 000 total counts from activities which are not
near the minimum detectable activity. More precisely, it is recommended to measure 3S /R
B
total counts where S represents the standard deviation of the background and R represents
B
the response of the system (this relationship takes into account only statistical fluctuations).
5.7 Dead time corrections
For a number of systems, there is internal dead time compensation. However, for those
systems that have no such compensation, the (non-extendable) dead time-corrected count rate,
C is given by
o
C
C = (9)
o
(1− C × t )
d
where
C is the observed count rate;

61453 © IEC:2007 – 17 –
t is the dead time which can be experimentally determined as specified below.
d
The measurement should be considered invalid if C exceeds C by more than 20 %. The
o
above expression for C gives, for example, a correction of 1 % for a dead time of 10 μs and a
o
count rate of 1 000 per second. With this method counting rates are measured using two
sources of the same radionuclide: first with source 1 (C ), second with source 2 (C ), third
1 2
with sources 1 and 2 together (C ), and fourth with no source present (C ).
12 b
The dead time t is determined using the following equations:
d
[ ]
x1− 1− z
t = (10)
d
y
x = C × C − C × C
(11)
1 2 b 12
y = C × C ×()C + C − C × C(C + C) (12)
1 2 12 b b 12 1 2
y(C + C − C − C )
1 2 12 b
z = (13)
x
When making the two-source measurements, it is important to maintain the exact position of
source 1 when introducing source 2, and similarly, when removing source 1, not to disturb
source 2. For multichannel analyzer systems, the live time feature is designed to compensate
for counting time lost during pulse processing, and a further correction for dead time losses is
usually not required, though pulse pile-up correction may be necessary.
When either source 1 or source 2 is not in use, it may be advisable in some cases to replace
the source that is not in use by an equivalent blank so that the scattering geometry is
unchanged.
5.8 Correction for decay during the counting period
5.8.1 If the value of a net count rate is determined by a measurement that spans a
significant fraction of a half-life, and the value is assigned to the beginning of the counting
period, a multiplicative correction factor F shall be applied:
b
λ × t
F =   (14)
b
−λ×t
(1− e )
where
...