ISO 6980:1996
(Main)Reference beta radiations for calibrating dosemeters and dose-rate meters and for determining their response as a function of beta-radiation energy
Reference beta radiations for calibrating dosemeters and dose-rate meters and for determining their response as a function of beta-radiation energy
Rayonnements bêta de référence pour l'étalonnage des dosimètres et des débitmètres et pour la détermination de leur réponse en fonction de l'énergie bêta
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INTERNATDNAL
STANDARD
Second edition
1996-I O-l 5
Reference beta radiations for calibrating
dosimeters and dose-rate meters and for
determining their response as a function of
beta-radiation energy
Rayonnements b&a de rhfhence pour Malonnage des dosim&res et
dbbitmirtres et pour la determination de leur Gponse en fonction de
I ‘hergie b& ta
Reference number
IS0 6980: 1996(E)
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IS0 6980: 1996(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard IS0 6980 was prepared by Technical Committee
ISO/TC 85, Nuclear energy, subcommitte SC 2, Radiation protection.
This second edition cancels and replaces the first edition (IS0 69809 984),
of which it constitutes a technical revision.
Annexes A, B and C form an integral part of this International Standard.
Annexes D and E are for information only.
0 IS0 1996
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii
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INTERNATIONAL STANDARD @ IS0 IS0 6980: 1996(E)
Reference beta radiations for calibrating dosimeters and dose-rate
meters and for determining their response as a function of beta-
radiation energy
1 Scope
This International Standard specifies the requirements for reference beta radiations produced by radionuclide
sources to be used for the calibration of protection level dosimeters and dose-rate meters’), and for the
determination of their response as a function of beta particle energy. It gives the characteristics of radionuclides
which have been used to produce reference beta radiations, gives examples of suitable source constructions and
describes methods for the measurement of the residual maximum beta particle energy and the dose-equivalent
rate at a depth of 7 mg cm-2 in the ICRU sphere. The energy range involved lies between 66 keV2) and 3,6 MeV
and the absorbed dose rates are in the range from about 10 ~SV. h-1 to at least IO Sv h-1. In addition, for some
sources variations of the absorbed dose rate as a function of the angle of incidence are given.
This International Standard proposes two series of reference beta radiations from which the radiation necessary for
determining the characteristics (calibration and energy response) of an instrument should be selected.
Series 1 reference radiations are produced by radionuclide sources used with beam-flattening filters designed to
give uniform dose rates over a large area at a specified distance. The proposed sources of 9oSr + 9OY, 204TI and
147Pm produce maximum dose rates of approximately 5 mSv h-1.
Series 2 reference radiations are produced without the use of beam-flattening filters, which allows planar sources
of large area and a range of source-to-calibration plane distances to be used. Close to the sources, only relatively
small areas of uniform dose rate are produced, but this Series has the advantage of extending the energy and dose
rate ranges beyond those of Series 1. The radionuclides used are those of Series 1 with the addition of the
. radionuclides 1% and 106Ru + 106Rh; these sources produce dose rates of up to IO Sv h-1.
2 Definitions
For the purposes of this International Standard, the following definitions apply.
2.1 absorbed dose, D: Quotient of dE by dm, where dE is the mean energy imparted by ionizing radiation to
matter of mass dm.
dE
D=-
dm
The SI unit of absorbed dose is joule per kilogram (J. kg--I) and has been given the special name gray (Gy):
1 Gy = 1 J . kg-1
1) These also include personal dosimeters.
2) This lower limit of the energies to be considered represents the energy of beta particles able to reach the sensitive layer of
the skin which is situated nominally 7 mg-cm -2 below the skin surface according to ICRP 60[ll.
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0 IS0
IS0 6980: 1996(E)
2.2 absorbed dose rate, 0: Quotient of dD by dt, where dD is the increment of absorbed dose in the time
interval dt.
dD
&-
dt
The SI unit of absorbed dose rate is the gray per second (Gy~l). Units of absorbed dose rate are any quotient of
the gray or its decimal multiples or submultiples by an appropriate unit of time (e.g. mGy. h-1).
2.3 dose equivalent, H: At a point in an irradiated medium, the product of the absorbed dose D and the quality
factor Q at this point.
H=DQ
NOTE - For beta, X and gamma radiation, Q may be taken as equal to unity for external radiation.
The SI unit of dose equivalent is joule per kilogram (J . kg-l) and has been given the specia name sievert (Sv):
1 Sv=l J-kg-1
2.4 dose-equivalent rate, I? : Quotient of dH by dt, where dH is the increment of dose equivalent in the time
interval dt.
dH
=-
ti
dt
The SI unit of dose-equivalent rate is the sievert per second (Sv s-l). Units of dose-equivalent rate are any quotient
of the sievert or its decimal multiples and a suitable unit of time (e.g. mSv h-1).
2.5 specification of the dose-equivalent quantities to
fields: This International Standard specifies reference beta radiation fields for which the following two dose-
equivalent quantities have been defined by ICRU 51 (reference [2] in annex E) for practical measurements. These
quantities are H&0,07), the personal dose equivalent, and H'(O,O7, L!), the directional dose equivalent.
In defining these quantities it is useful to stipulate a radiation field that is derived from the actual radiation field.
the fiuence and its
The term “expanded” is used to characterize the derived radiation field. in the expanded field,
angular and energy distribution are the same throughout the volume of interest as in the actual field at the point of
reference.
personal dose equivalent, H&0,07): Dose equivalent in soft tissue below a specified po nt on the body at a
2.51
depth of 0,07 mm.
25.2 directional dose equivalent, H'(O,O7, 0): Dose equivalent, at a point in a radiation field, that would be
produced by the corresponding expanded field in the ICRU sphere at a depth of 0,07 mm, on a radius in a specified
direction, 0.
NOTES
The ICRU sphere is a tissue-equivalent sphere of diameter 30 cm and density 1 g.cm-3.
2 An instrument which determines the dose equivalent at a depth of O,O7 mm in a plane slab of tissue-equivalent material will
adequately determine H’(0,07, Q) for weakly penetrating radiation if the slab surface is perpendicular to the specified direction
LJ and the radiation field is uniform over the entrance face of the instrument.
in most cases in practice H&0,07) can be considered as equal to H'(047, 12) in beta particle fields where the
fluence and its angular and energy distributions have the same values within a sufficiently large volume. When the
term “dose” or “dose rate” is used in this international Standard, no distinction is made between these dose-
equivalent quantities. As the range of the beta particles is small compared with the diameter of the ICRU sphere,
the curvature of this sphere is only of marginal influence, and depth dose curves in the sphere and a semi-infinitely
extended slab phantom may be regarded as the same. Expanded field conditions, at least throughout a sufficiently
large volume around the dosimeter on the phantom, shall be approximated for every calibration (see reference [3]
in annex E).
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@ IS0 IS0 6980: 1996(E)
2.6 total mass stopping power, S/p, (of a material for charged particles): Quotient of dE by pd2, where dE is
the energy lost by a charged particle in traversing a distance dl in a material of density p.
S IdE
-=-
P P*dl
m
The SI unit of mass stopping power is joule square metre per kilogra (J.rn2. kg-‘). E may be expressed in
hence S/p may be expressed in electronvolt square m etres pe r kilog ram (eV .rn2 - kg-l).
electronvolts (eV) and
NOTES
1 S is the total linear stopping power.
2 For energies at which nuclear interactions can be neglected, the total mass stopping power is
$=f[%)c-i ‘;(Z) rad
where
= SC01 is the linear collision stopping power;
kWd4,,l
= &ad is the linear radiative stopping power.
(dE/d&d
2.7 tissue: Material with a density of 1 g cm-3 and the following composition in terms of mass fraction for soft
tissue (see reference [2] in annex E):
0: 76,2% H: lO,l%
c: ll,l% N: 2,6%
Trace elements are generally not considered important for dosimetric purposes and have been ignored.
2.8 tissue equivalence: Property possessed by a material when the collision mass stopping power and the
scattering properties of the material equal those of soft tissue. The density of the tissue-equivalent material is
taken to be 1 gem -3 (see annex A; more tissue substitutes are given by ICRU 44141).
NOTE - In practice, tissue equival ence can only exist over a limited range of energie s for a particular type of radiation,
dependent upon the material utilized, unless th e at0 mic composition is the same as that of e.
tissu
maximum energy of a beta particle spectrum, Em,,: Highest value of the Ei max values, characteristic
2.9
of the particular nuclide listed in table 1. A number of radionuclides emit one or several continuous spectra of beta
particles with energies ranging from zero up to maximum values Ei Max, i = 1, 2.
I
2.10 residual maximum beta particle energy, Eres: Maximum energy of the beta particle spectrum from all
beta particle decay branches of a radionuclide at the calibration distance. E,,, is less than the corresponding Emax
as the spectrum is modified by absorption and scattering in the source material itself, the source holder, the source
encapsulation and other media between the source and the calibration position.
2.11 residual maximum beta particle range, &: Range in an absorbing material of a beta particle
spectrum of residual maximum energy, E,,,.
3 Requirements for reference beta radiations at the calibration distance
31 .
Energy of the reference radiations
The energy of the reference radiation is defined to be equal to E,,, (see 2.10 and 5.1.2).
3.2 Shape of the beta particle spectrum
The beta particle spectrum of the reference radiation should ideally result from one beta decay branch from one
radionuclide. In practice, the emission of more than one branch is acceptable provided that all the main branches
have similar energies, Emax, within + 20 %. In other cases, the lower energy branches shall be attenuated by the
source encapsulation or by additional filtration to reduce their beta emission rates to less than IO % of the
emission rate from the main branch.
3
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IS0 6980: 1996(E)
3.3 Uniformity of the dose rate
The dose rate at the calibration distance shall be as uniform as possible over the area of the detector. Since
available sources for Series 1 reference radiations (see 52.1) cannot at present produce high absorbed dose rates
with good uniformity for large radiation field diameters, a further series (Series 2) of reference beta radiations is
proposed (see 5.22). Beta radiations are considered to be uniform over a certain radiation field diameter, if the
2 300 keV and by not more than + 10 % for E,,, < 300 keV
dose rate does not vary by more than + 5 % for E,,,
(see 52.1).
NOTE - 1 eV=1,602xlO-‘9J
3.4 Photon contamination
The photon dose rate contributing to H&0,07) (see 2.5.1) due to contamination of the reference radiation by
gamma, X-ray and bremsstrahlung radiation shall be less than 5 % of the beta particle dose rate recorded by the
detector under calibration.
3.5 Variation of the beta particle emission rate with time
The beta particle emission rate decreases with time due to the radioactive decay of the beta particle source. The
half-life of a radionuclide should be as long as possible, preferably longer than one year. The half-lives of the
recommended sources are given in table 1.
4 Radionuclides suitable for reference beta radiations
Table 1 gives the characteristics of beta-emitting radionuclides of a suitable energy range. Beta-emitting
radionuclides shall be selected from those listed in table 1. These radionuclides emit a continuous spectrum of beta
characteristic of the particular nuclide.
particles with energies ranging from zero up to a maximum value, E,,,,
a radionuclide normally requires encapsulation and that the
Note that, to be used as a practical source,
encapsulating material will produce bremsstrahlung and characteristic X-rays.
- Beta radionuclide data
Table 1
Approximate Maximum energy Photon radiations
half-life of spectrum, Emax emitted’)
Radionuclide
d MeV
14C 2 093 000 0,156 None
‘47Pm 957 0,225 y : 0,121 MeV (0,Ol %)
Sm X-rays: 5,6 keV to 7,2 keV
39,5 keV to 46,6 keV
204TI 1 381 0,763 Hg X-rays: 9,9 keV to 13,8 keV
68,9 keV to 82,5 keV
gosr + 9OY 10 483 2,274 None
‘OaRu + ‘OaRh 372,6 3,54 ‘OsRh-y* 0,512 MeV (21%)
0,622 MkV (11 % doublet)
I,05 MeV (I,5 % doublet)
1 ,I3 MeV (0,5 % doublet)
I,55 MeV (0,2 %)
1) The values given in this column are for information only.
5 Source characteristics and their measurement
5.1 Fundamental characteristics of reference sources
5.1 .l Construction of reference sources
The construction of the reference sources shall have the following characteristics to meet the requirements of
clause 3:
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@ IS0 IS0 6980: 1996(E)
a) The chemical form of the radionuclide shall be stable with time over the range of temperatures and humidities
at which it will be used and stored.
b) The construction and encapsulation shall be sufficiently robust and stable to withstand normal use without
damage to the source and leakage of the radioactivity, but shall allow E,,, to exceed the minimum values
recommended in table 2.
5.12 Measurement of characteristics of the reference radiations
at the calibration distance shall be equal to or exceed the values
The values of the residual maximum energy Eres
given in table 2.
Table 2 - Minimum value of E,,, at calibration distance
Source E
res
I
‘4C 0,09
‘47Pm 0,13
0,53
*04TI
WSr + 9OY I,80
‘06Ru + ‘OsRh 2,80
The purpose in setting a lower limit to E,,, is to prevent the use of sources which have excessive self absorption
and/or window absorption.
The residual maximum energy, E,,,, in megaelectronvolts at the calibration distance shall be calculated from the
following relationship (see reference [5] in annex E):
where R,,, is the residual maximum beta particle range in milligrams per square centimetre (mg cm-z).
R shall be measured by a suitable detector (thin window ionization, chamber, Geiger Mtiller counter, beta-
r-es
sensitive phosphor, etc.) which shall be positioned at the calibration distance with its entrance window facing the
source, and various thicknesses of absorber shall be placed immediately in front of the detector. The absorber shall
be one of the materials polymethyl methacrylates), polystyrene, polyethylene, polyethylene terephthalated) or an
equivalent. The thickness of the detector window used for these measurements shall be taken into account in the
measurement of R,-es.
If the source uses a beam-flattening filter, i.e. is a Series 1 reference radiation (see 5.2.1), this filter shall be in
position for the measurement of R,,,.
The signal from the detector shall be determined as a function of absorber thicknes s and a plot shall be made of
abso rber thickness i n mil ligrams per square centimetre.
the logarithm of sign al versus
R is defined as the intersection of the extrapolated linear portion of the measured signal versus thickness graph
res
with the lower level signal due to the residual photon background.
E may also be determined by a beta particle spectrometer employing, for example, Si (Li) semiconductor
res
detectors. Figure 1 shows an example of measured beta particle spectra for the radiations given in table 2: series 1
reference beta radiations (table 3) and series 2 reference beta radiations (W and 106Ru + IOWh, calibration
distances 3 mm and 18 mm) measured at the calibration distances with effectively windowless uncooled Si (Li)
semiconductor detectors. The measured flux densities Q?E are normalized to the same maximum value @Emax but
not corrected for instrumental resolution or detector backscattering loss (see reference [6] in annex E). The
(Wr + 9OY) spectrum is produced by 9OY beta particles only due to the heavy encapsulation of the source (type 1 in
table C.l of annex C), whereas two components are evident in the (106Ru + 106Rh) spectrum. A survey of a number
of theoretical beta particle spectra is given in reference [7] in annex E.
Perspex, Lucite, Plexiglas are examples of commercial names for this plastic.
3)
Melinex, Mylar, Hostaphan are examples of commercial names for this plastic.
4)
conveni ence of users
This information is given for the of this International Standard and does not const itute an endorsement
by IS0 of the product named . uivalent products may be used if they can be shown
to lead to the same results
Eq
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IS0 6980: 1996(E)
I-
t
%
E
&
2
0,5 -
Beta particle energy, OteV
Figure 1 - Examples of beta particle spectra for series 1 and series 2 reference beta radiations
5.1.3 Beta contamination
The radionuclide sources shall be of adequate radiochemical purity. It is difficult to check for the presence of beta-
particle-emitting impurities but their presence may be inferred from the detection of their associated photon
radiation, if a
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