EN 60544-1:1994

SLOVENSKI STANDARD
01-junij-1998
Electrical insulating materials - Determination of the effects of ionising radiation -
Part 1: Radiation interaction and dosimetry (IEC 60544-1:1994)
Electrical insulating materials - Determination of the effects of ionizing radiation -- Part 1:
Radiation interaction and dosimetry
Elektroisolierstoffe - Bestimmung der Wirkung ionisierender Strahlung -- Teil 1: Einfluß
der Strahlenwirkung und Dosimetrie
Matériaux isolants électriques - Détermination des effets des rayonnements ionisants --
Partie 1: Interaction des rayonnements et dosimétrie
Ta slovenski standard je istoveten z: EN 60544-1:1994
ICS:
29.035.01 Izolacijski materiali na Insulating materials in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

NORME
CEI
INTERNATIONALE IEC
544-1
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
1994-04
Matériaux isolants électriques —
Détermination des effets des rayonnements
ionisants —
Partie 1:
Interaction des
rayonnements et dosimétrie
Electrical insulating materials —
Determination of the effects of ionizing
radiation —
Part 1:
Radiation interaction and dosimetry
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3 -
544-1 © IEC:1994 -
CONTENTS
Page
FOREWORD 5
INTRODUCTION 7
Clause
1 Scope and object 9
2 Normative references 9
Definitions 3 9
4 Aspects to be considered in evaluating the radiation resistance of
insulating materials 11
4.1 Evaluation of the radiation field 11
4.2 Evaluation of absorbed dose and absorbed dose rate 13
4.3 13 Radiation-induced changes and their evaluation
5 Dosimetry methods 17
5.1 General 17
5.2 Absolute methods 17
5.2.1 Gamma-rays 17
5.2.2 Electron beams 19
5.3 Relative methods 21
5.4 Recommended methods for measuring absorbed dose 21
6 Calculation of absorbed dose from X- or gamma-radiation
General 23
6.1
6.2 Calculation of the absorbed dose from a measurement of exposure 23
6.3 Calculation of absorbed dose in one material from that in another 25
6.4 Depth-dose distribution (limitations) 25
7 Dose estimation methods for electron radiation 27
7.1 General 27
7.2 Recommended procedures for electron-beam dosimetry
7.3 Electron-beam irradiation 31
7.4 Methods for measuring depth-dose distributions 31
Tables 37
Figures 45
Annexes
A Charged-particle equilibrium thickness 51
B Derivation of numerical factors fi 59
Bibliography C 60
544-1 ©IEC:1994 - 5 -
INTERNATIONAL ELECTROTECHNICAL COMMISSION
ELECTRICAL INSULATING MATERIALS —
DETERMINATION OF THE EFFECTS OF IONIZING RADIATION —
Part 1: Radiation interaction and dosimetry
FOREWORD
The IEC (International Electrotechnical Commission) is a worldwide organization for standardization
1)
comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to
promote international cooperation on all questions concerning standardization in the electrical and
electronic fields. To this end and in addition to other activities, the IEC publishes International Standards.
Their preparation is entrusted to technical committees; any IEC National Committee interested in
the subject dealt with may participate in this preparatory work. International, governmental and
non-governmental organizations liaising with the IEC also participate in this preparation. The IEC
collaborates closely with the International Organization for Standardization (ISO) in accordance with
conditions determined by agreement between the two organizations.
The formal decisions or agreements of the IEC on technical matters, prepared by technical committees on
2)
which all the National Committees having a special interest therein are represented, express, as nearly as
possible, an international consensus of opinion on the subjects dealt with.
They have the form of recommendations for international use published in the form of standards, technical
3)
reports or guides and they are accepted by the National Committees in that sense.
In order to promote international unification, IEC National Committees undertake to apply IEC International
4)
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
International Standard IEC 544-1 has been prepared by sub-committee 15B: Endurance
tests, of IEC technical committee 15: Insulating materials.
This second edition cancels and replaces the first edition published in 1977 and
constitutes a technical revision.
The text of this standard is based on the following documents:
Report on Voting
DIS
15B(CO)93
15B(CO)91
Full information on the voting for the approval of this standard can be found in the report
on voting indicated in the above table.
Electrical insulating
IEC 544 consists of the following parts, under the general title:
materials - Determination of the effects of ionizing radiation.
- Part 1: 1994, Radiation interaction and dosimetry
- Part 2: 1991, Procedures for irradiation and test
- Part 4: 1985, Classification system for service in radiation environments
Annexes A and B form an integral part of this standard.
Annex C is for information only.

-
544-1 © IEC:1994 7 -
INTRODUCTION
The establishment of suitable criteria for the evaluation of the radiation resistance of
insulating materials is very complex, since such criteria depend upon the conditions under
which the materials are used. For instance, if an insulated cable is to be flexed during a
refuelling operation in a reactor, the service life will be that time during which the cable
receives a radiation dose sufficient to reduce to a specified value one or more of the
relevant mechanical properties. Temperature of operation, composition of the surrounding
atmosphere, and the time interval during which the total dose is received (dose rate or
flux) are important factors which also determine the rate and mechanisms of chemical
changes. In some applications, temporary changes may be the limiting factor.
Firstly, it is necessary to define the radiation fields in which materials are exposed and the
radiation dose subsequently absorbed by the material. Secondly, it is necessary to
establish procedures for testing the mechanical and electrical properties of materials,
which will define the radiation degradation, and link those properties with application
requirements in order to provide an appropriate classification system.
This part of IEC 544 is the introductory part in a series dealing with the effect of ionizing
radiation on insulating materials. Part 2 of IEC 544 describes procedures for maintaining
different types of exposure conditions during the irradiation. It also specifies the controls
that shall be maintained over these conditions so that desired performances can be
obtained. Further, it defines certain important irradiation conditions and specifies the test
procedures to be used for property-change determinations and the corresponding
end-point criteria. Part 3 (IEC 544-3: 1979) has been incorporated into the second edition
of IEC 544-2. Part 4 of IEC 544 defines a classification system to categorize the radiation
endurance of insulating materials. It provides a set of parameters characterizing the
suitability for radiation service. It is a guide for the selection and indexing of insulating
materials and for material specification.

544-1 - 9 -
© IEC:1994
ELECTRICAL INSULATING MATERIALS —
DETERMINATION OF THE EFFECTS OF IONIZING RADIATION —
Part 1: Radiation interaction and dosimetry
1 Scope and object
This pa rt
of IEC 544 deals broadly with the aspects to be considered in evaluating the
effects of ionizing radiation on all types of organic insulating materials. It also provides, for
X-rays, y-rays, and electrons, a guide to dosimetry terminology, methods of determining
exposure and absorbed dose, and methods of calculating absorbed dose.
2 Normative references
The following normative documents contain provisions which, through reference in this
text, constitute provisions of this part of IEC 544. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and pa
rties to
agreements based on this pa
rt of IEC 544 are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. Members
of IEC and ISO maintain registers of currently valid International Standards.
IEC 544-2: 1991,
Guide for determining the effects of ionizing radiation on insulating
materials - Part 2: Procedures for irradiation and test
IEC 544-4: 1985,
Guide for determining the effects of ionizing radiation on insulating
materials - Part 4: Classification system for service in radiation environments
3 Definitions [151*
For the purposes of this part of IEC 544, the following definitions apply.
3.1 exposure (X): Exposure is the measure of an electromagnetic radiation field (X- or
y-radiation) to which a material is exposed. The exposure is the quotient obtained by
dividing dQ by dm, where dQ is the absolute value of the total charge of the ions of one
sign produced in the air when all of the electrons (and positrons) liberated by photons in
air of mass dm are completely stopped in air:
dQ
X=
dm
SI
The
unit of exposure is the coulomb (C) per kilogram: C/kg. The old unit is the roentgen
R: 1 R = 2,58 x 10
-4 C/kg.
* The numbers in square brackets refer to the bibliography given in annex C.

544-1 © IEC:1994 - 11 -
The exposure thus describes the effect of an electromagnetic field on matter in terms of
the ionization that the radiation produces in a standard reference material, air.
3.2
electron charge fluence (Q'): The quotient obtained by dividing dQ by dA, where
dQ is the electron charge impinging during the time
t on the area dA:
- dQ
dA
3.3
electron current density (j): The quotient obtained by dividing dQ' by dt, where dQ'
is the electron charge fluence during the time interval dt:
dQ' d2Q
l = -
dt dA dt
3.4 absorbed dose (D): Measure of the energy imparted to the irradiated material,
regardless of the nature of the radiation field. The absorbed dose
D is the quotient
obtained by dividing
a by dm where da is the mean energy imparted by ionizing radiation
to matter of mass dm:
de
D-
dm
The SI
unit is the gray (Gy). The old unit is the rad:
1 Gy = 1 J • kg -1 (= 10
2 rad)
Since this definition does not specify the absorbing material, the gray can be used only
with reference to a specific material. The absorbed dose is determined in part by the
composition of the irradiated material. When exposed to the same radiation field,
therefore, different materials usually receive different absorbed doses.
3.5 absorbed dose rate (b):
The quotient obtained by dividing dD by dt, where dD is the
increment of absorbed dose in the time interval dt:
dD
D-
dt
The SI
unit of absorbed dose rate is the gray per second:
1 Gy • s- = 1 W • kg- (= 102 rad s
-1 = 0,36 Mrad h-1)
4 Aspects to be considered in evaluating the radiation resistance of
insulating materials
4.1
Evaluation of the radiation field
For various types of radiation, the radiation field is described in different ways.

-13 -
544-1 © IEC:1994
4.1.1 An electromagnetic radiation field may be described in terms of photon flux density
and energy distribution. However, for X- and y-rays up to 3
MeV it is customary to
characterize the field in terms of its ionizing effect on air. For this purpose, the quantity
"exposure" is used.
4.1.2 A particle field is usually characterized in terms of the current density (fluence
rate). When the particles have a distribution of energies, as for electron beams, additional
information concerning the energy spectrum is required.
4.1.3 In all cases, the objective is to characterize the radiation field in such a way that
the absorbed dose and dose rate in any material placed in the field may be calculated.
When different materials are exposed to the same fluence of photons or particles, they
may absorb different amounts of energy. The first objective is thus to describe standard
methods and procedures for measuring the characteristics of the radiation fields to which
insulating materials have to be exposed. Clause 5 meets this objective by presenting a list
of radiation dosimetry techniques with the relevant references.
4.2
Evaluation of absorbed dose and absorbed dose rate
Techniques have been perfected to obtain - from measurements with radiation detectors
such as ionization chambers, calorimeters, and chemical dosimeters - the data for
calculating the absorbed dose or absorbed dose rate for a material under irradiation.
Clause 5 deals with the reliable and conventional techniques of such measurements.
Clause 6 contains the material- and energy-dependent factors to be used in the
calculation of absorbed dose or absorbed dose rate in other materials of interest from the
measured data for X- and y-radiation, while clause 7 gives dose estimation methods for
electron radiation.
4.3 Radiation-induced changes and their evaluation
Although the various types of radiation interact with matter in different ways, the primary
process is the production of ions and electrically excited states of molecules which, in
turn, may lead to the formation of free radicals. Radiation-generated mobile electrons,
which become trapped at sites of low potential energy, are also produced. The first
phenomenon leads to permanent chemical, mechanical, and electrical changes of the
material; the second results in temporary electrical changes in performance [10].
4.3.1
Permanent changes
In polymeric materials, the formation of free radicals during irradiation leads to scission
and cross-linking processes that modify the chemical structure of the insulation, generally
leading to deterioration of the mechanical properties. This mechanical deterioration
frequently gives rise to significant electrical property changes. However, impo rtant
electrical property changes sometimes occur before mechanical degradation is serious.
For example, a change in dissipation factor or in permittivity might become serious for the
reliable functioning of a resonant circuit. The extent of scission and cross-linking
processes depends on the absorbed dose, the absorbed dose rate, the material geometry,
and the environmental conditions present during the irradiation. Because the free radicals
sometimes decay slowly, there may also be post-irradiation effects.

544-1 © IEC:1994 - 15 -
4.3.1.1 Environmental conditions and material geometry
Environmental conditions and test specimen geometry shall be well controlled and
documented during the measurement of radiation effects. Important environmental
parameters include temperature, reactive medium, and mechanical and electrical stresses
present during the irradiation. If air is present, the irradiation time (flux and dose rate) has
also been demonstrated to be a very impo ant experimental parameter because of oxygen
rt
diffusion effects and hydroperoxide breakdown rate constants. Both factors are time
dependent. The conditions that influence oxygen diffusion and equilibrium concentrations
in the polymer shall be controlled. These include: temperature, oxygen pressure, material
geometry, and the time during which the dose is applied.
If the effect of simultaneous stresses, e.g. radiation at high temperature, is simulated by
sequential stressing, other results are to be expected. Further, there can be differences in
results if the sample is first irradiated and then heat aged or vice versa.
4.3.1.2 Post-irradiation effects
In organic polymers, there may be post-irradiation effects due to the gradual decay of
various reactants, such as residual free radicals. Due allowance should be made for this
type of behaviour in any evaluation procedure. The tests should be made at recorded
intervals after irradiation, maintaining specimen storage in a standard laboratory
atmosphere. The reaction of oxygen with residual free radicals can cause further
degradation.
4.3.2 Temporary effects
4.3.2.1 Performing measurements during irradiation is not within the scope of this pa rt
of IEC 544. Despite this, some basic aspects will be discussed briefly. The temporary
effects appear primarily as changes in electrical properties such as induced conductivity,
both during and for some time after irradiation. Hence, measurement of the induced
conductivity could be used as an evaluation property to determine the temporary radiation
effects. These effects are primarily dose-rate dependent.
4.3.2.2 Experience has shown that the induced conductivity is usually not quite
proportional to the absorbed dose rate D, Da, where a is smaller than unity.
but varies as
Hence, the radiation sensitivity is described by the relation:
a = kDa
To determine k and a, at least two measurements are needed. A further complication
comes from the fact that k and a also depend on the integrated dose absorbed by the sample.
4.3.2.3 The measurement of the induced conductivity is actually quite delicate, since
photoelectrons and Compton electrons in the electrode materials will tend to perturb the
intrinsic induced current of the specimen. Ionic currents through the ionized atmosphere
will also introduce errors in the measurement if they are not eliminated. Experimental
procedures eliminating most of the disturbing effects, while remaining relatively simple,
should be defined.
544-1 ©IEC:1994 - 17 -
4.3.2.4 It would be convenient to use a simple figure such as the induced conductivity ai
or ai/aa, its ratio to the dark conductivity a
o measured in the same experimental
conditions, per unit dose rate to characterize the sensitivity of the materials to temporary
effects.
5 Dosimetry methods
5.1 General
Absolute methods are those that will provide a determination of the exposure, current
density, or the absorbed dose by means of physical measurements that do not depend on
a calibration of the instrument in a known radiation field. This definition does not directly
imply the accuracy of an absolute method; but through the results of much research on the
instrumental techniques and the fundamentals of the radiation-induced reactions, there
are absolute methods, such as calorimetry, that are widely considered as primary
dosimetry standards. These procedures are not regularly used in studies of radiation
effects but are available in national and international standards laboratories for calibration
of radiation sources. For photon sources, the calibration accuracy is within 2 % to 3 %.
These methods can be used as reliable standards of comparison between different
laboratories.
In addition to the absolute standard methods, there are many other dosimeters that have
been calibrated against them and which have become extensively utilized as relative
dosimeters for measuring the absorbed dose [21]. These are based on a wide variety of
measurable chemical reactions or energy transformations resulting from the energy
imparted to the dosimetry material as a result of the interaction with the radiation field. A
large number of radiation sensors such as plastic films and inorganic solids have been
used as dosimeters; these are relatively easy to handle and provide easy-to-analyse
responses. In many cases, there are definite advantages in using them where the required
accuracy is less stringent.
5.2 Absolute methods
5.2.1
Gamma-rays
5.2.1.1 Free-air ionization chambers are used to measure exposure X up to 3 MeV. That
is, they are designed to measure the quantity of charge dQ produced in air and the mass
dm of air where the ionizing electrons are liberated.
5.2.1.2 Cavity ionization chambers are radiation detectors that can be used to measure
exposure if D
is not too high and equilibrium conditions are ensured [9]. If a cavity
ionization chamber is used to measure the absorbed dose in a particular medium, both
wall and gas should be matched to this medium. Two materials may be said to be matched
for a particular type of radiation if the absorption of this radiation leads to the same flux
density and energy distribution of secondary ionizing particles in both media.

544-1 ©IEC:1994 –19 –
5.2.1.3 Calorimeters operate by absorbing energy from the radiation field in which they
are placed; they retain this energy until it is converted to thermal energy and this heat
quantity is evaluated by measuring the rise in temperature of the system [4]. The heat
capacity of the system may be calibrated electrically by measuring the amount of electrical
power input required to produce the same temperature rise as the radiation. In some
systems, the conversion of energy into chemical form by exochemical or endochemical
reactions has been noted to produce slight deviations for which corrections can be made.
However, since the conversion of absorbed radiation energy to heat establishes a system
that measures energy deposition almost independent of radiation quality, the calorimeter
constitutes an absolute method against which other standard methods have been
calibrated.
5.2.2 Electron beams
5.2.2.1 Dosimetric or radiometric methods will provide a determination of absorbed dose
or electron fluence by means of physical measurements without any calibration. There are
two absolute methods that are considered as dosimetric or radiometric references: the one
is calorimetry, and the other electron current densitometry. These absolute methods are
used mainly for calibration of routine dosimeters, and it is generally difficult to determine
depth-dose distribution without the use of routine film dosimeters.
5.2.2.2 The calorimeters are used to measure absorbed dose or energy fluence.
Measurement of the absorbed energy per unit area of the target enables the calibration of
absorbed dose and of a routine dosimeter by integrating a relative depth-dose distribution,
if the relative depth-dose distribution in the same target material is given by using the
routine dosimeters with high spatial resolution. Simple quasi-adiabatic methods can be
used as a partial-absorption type [25] and a total-absorption type calorimeter.
5.2.2.3 The electron current density measurement is a radiometric method [33] to
measure electron charge or current per unit area of radiation fields of electron
accelerators. This method is not a dosimetric method, but enables the calibration of
absorbed dose, if the mean electron energy impinging on the charge absorber of the
densitometer and the relative depth-dose distribution in the same absorber material are
given by using a routine dosimeter. The Faraday cup has been widely used for charge
measurement of electron beams, but it is not suitable for giving accurate values of
electron charge or current per unit area for broad electron beams with wide angular
distribution. A simplified method using an assembly of graphite charge absorbers without a
vacuum chamber is useful for scattered broad beams from electron accelerators [33], [35].
The effective absorbing area is accurately defined by special arrangement of the absorber
assembly and electron backscattering correction in consideration of oblique incidence.
The influence of ionized charge of the surrounding air can be avoided by minimizing
extraneous electric field formation around the centre absorber.

544-1 © IEC:1994 - 21 -
5.3 Relative methods
5.3.1 Chemical conversion dosimetry is based on the principle that ce rtain reactions take
place upon irradiation to an extent directly proportional to the absorbed dose. The ferrous
sulfate (Fricke dosimeter) method is a well-defined standard of this type and is the most
reliable. It is widely used and suitable for comparisons between different laboratories [4],
[9], [1]. Other systems are very impo rtant because of their utility in extending the range of
the ferrous sulfate method. The amino acid type (alanine) dosimeter is recommended by
IAEA as a transfer standard [28], [29], [20]. Plastics, dyed or undyed, are also suitable as
chemical dosimeters [18], [22], [23], [34], [32].
5.3.2 Other relative methods based on physical effects are photoluminescence or
thermoluminescence
[3], [26], [8].
5.3.3 Effective utilization of relative dosimeters is limited to conditions similar to those
under which they have been calibrated, if the dosimeters show variations in response to
environmental conditions (temperature and humidity before and after irradiation,
atmosphere, light, etc.), dose rate, and radiation spectrum. Other imprecisions are due to
instability before and after irradiation, batch-to-batch variation, non-linearity in the
response characteristics, size variation, impurity or chemical effects, etc.
5.3.4 In some cases calibration constants or calibration curves obtained with y-rays can
be applied for electron beams if the dosimetry characteristics do not depend on the
difference of irradiation parameters between -y-rays and electron beams, such as electron
energy spectrum, dose rate, irradiation time, temperature during irradiation, etc.
5.4 Recommended methods for measuring absorbed dose
5.4.1 Table 1 gives a non-exhaustive list of absolute and secondary methods with some
of their characteristics, such as:
-
range of absorbed doses and absorbed dose rates;
- influence of the radiation energy;
-
influence of temperature or humidity;
- material and thickness of film or target;
- type of read-out;
- observations of practical interest;
- bibliographical references.
5.4.2 A difficulty inherent in the dosimetry at high absorbed doses or high absorbed dose
rates, such as is customary with the irradiation of insulating materials, arises from
possible radiation effects or damage produced in pa rts of the dosimeters (e.g. damage to
insulators of ionization chambers).
Special tests and procedures may be required to avoid disturbances of this type. More
complete reviews of most of the methods listed can be found in [4] and in the references
in table 1.
544-1 ©IEC:1994 - 23 -
6 Calculation of absorbed dose from X- or gamma-radiation
General
6.1
The absorbed dose is the parameter for use in relating the effects of radiation in insulating
materials.
This recommended practice presents a technique for calculating the absorbed dose in a
material from the knowledge of the X- or y-radiation field and the composition of the
material [1], [2]. From the absorbed dose in one material, the absorbed dose in any other
material exposed to the same radiation field may be calculated.
6.2
Calculation of the absorbed dose from a measurement of exposure [13], [14]
6.2.1 Absorbed dose has become the basis for comparison of effects of different
radiations and it has become necessary to determine the absorbed dose deposited in the
irradiated material. The exposure measured in free air at the location of the specimen may
then be utilized as the basic information from which the absorbed dose is to be calculated.
The following subclauses contain the formulae to be used in this calculation. Tables 2
and 3 supply the necessary numerical factors and give examples for calculation.
6.2.2 The absorbed dose Dm
in a material m is calculated with the equation:
= fmX Dm (1)
where the numerical factor
fm is the quotient of absorbed dose per unit exposure. To
calculate fm, the composition of the material and the numerical factors f for the included
i
elements are needed according to the equation:
(2)
where
a. is the mass fraction of element i in the material
f
is the absorbed dose per unit exposure for element i.
6.2.3 Table 2 gives values of f
in J/C for photon energies between 0,1 MeV and 3,0 MeV
for the elements listed (see annex B for the derivation of f values). The values of f are
valid only under conditions of charged-particle equilibrium (see annex A for explanation).
Table 3 gives the values of fm calculated with equation (2) for some of the commonly used
materials for photon energy levels of 1 MeV and 0,1 MeV.
6.2.4 For various organic compounds irradiated at photon energies in the range between
MeV
0,5 and 1,5 MeV, equation (2) for fm may be approximated as:
fm = (32,9a H - 1,94a F - 1,55a
ci - 1,16aP + 33,7) J/C
544-1 © IEC:1994 _ 25 _
6.3
Calculation of absorbed dose in one material from that in another
6.3.1 The numerical factors are also applicable for the comparison of absorbed dose in
different media without reference to the exposure, provided the latter is held constant.
From equation (B.2) in annex B, it is seen that, for constant exposure, the ratio between
the fm
values for any two media equals the ratio between their mass energy-absorption
coefficients hence the ratio equals the ratio between the absorbed doses. The
(µen/p)m;
absorbed dose in medium 1, D 1 D2 in
, can be calculated from the absorbed dose
medium 2 using the numerical factors fi and f2 by the equation:
fi
D2
D1 = f (3)
6.3.2 When chemical dosimetry is employed, the measured chemical change may be
converted directly to the absorbed dose. By utilizing the ratio of m
values, one can
calculate the absorbed dose in any material from a knowledge of the absorbed dose in a
chemical dosimeter when the incident photon energy is known (within the limitations
stated in 6.4).
6.3.3 For example, if one measures an absorbed dose in the Fricke dosimeter to be
5 Gy for a 1 h exposure in a fi0Co irradiator and it is desired to determine the
=
DFricke
absorbed dose in a specimen of polyethylene in the same irradiator for 1 h, one proceeds
as follows.
The Fricke dosimeter has mass fractions of a = 0,88, and as = 0,013,
H = 0,11; ao
respectively. Table 2 gives the fi values for 1,0 MeV photon energy. Substituting the ai
and fi values in equation (2), one obtains ( 37,4 J/C. From table 3, fPE = 38,2 J/C
Fricke =
and with equation (3) the absorbed dose in polyethylene is equal to:
tPE x D =1,02x5Gy=5,1 Gy
Fricke
DPE —
fFricke
6.3.4 In the same way, the numerical factors in table 3 may be used to convert the
absorbed dose in any material listed to the absorbed dose in any other material listed
provided the energy of the incident radiation is either 0,1 MeV or between 0,5 MeV
and 1,5 MeV.
6.4 Depth-dose distribution (limitations)
6.4.1 Since the absorbed dose distribution through the specimen being irradiated will
vary and is a function of the thickness of the specimen, its density, and the energy of the
incident radiation, it is necessary to decide how much variation in dose one is willing to
tolerate as the radiation penetrates the specimen. The most commonly used irradiation
facilities have radiation sources in the energy range of 0,5 MeV to 1,5 MeV. If, for a point
source, one arbitrarily sets a limit of 25 % for the difference between the absorbed dose at
the front and rear of the specimen (25
% attenuation through the specimen), then the
specimen thickness is limited to 2,8 cm for 0,5 MeV and 5,0 cm for 1,5 MeV radiation,
assuming no build-up, a specimen of unit density equal to 1 g/cm
3 and unidirectional
radiation (see annex A). For other source geometries (e.g. slab sources) these
thicknesses will be significantly different.

544-1 ©IEC:1994 - 27 -
6.4.2 Figure A.3, in annex A, is a plot of thickness as a function of energy of a sample for
10 % and 25 % attenuation through a specimen of unit density equal to 1 g/cm
3 for
unidirectional radiation. The curves will shift to the left for higher-density material and to
the right for lower-density material. The accurate thickness for 10 % or 25 % attenuation in
the specimen will be the value obtained from figure A.3 divided by the ratio of the electron
density of the specimen to 3,3 x 1023 g-1 .
Since the curves are calculated on the basis of
attenuation only, and build-up in the thicker specimens is neglected, the curves represent
a maximum attenuation for a given energy and thickness, and for unidirectional radiation.
Non-unidirectional radiation results in larger attenuation.
7 Dose estimation methods for electron radiation
7.1 General
7.1.1 In this clause, dose estimation methods for electron beams are recommended
mainly for an electron energy range of several hundred keV to several
MeV and for a dose
range of kGy to MGy.
7.1.2 The radiation field of an electron beam is usually characterized in terms of electron
energy spectrum and electron current density, which is the electron charge impinging on a
unit area of the irradiated plane per unit time. In the usual operation of an electron
accelerator with a beam scanner, the instantaneous electron current density at the point of
interest in the irradiated plane changes periodically depending on the scanning frequency.
Practically, the radiation field is characterized in terms of the mean energy of electrons
transmitted through the beam window and the air gap, and of the electron current density
averaged over a scanning period. The mean electron energy at the surface of the sample,
Em
, is evaluated by the following equation:
Em =Eo - DEW - AEa
(4)
where E0
is the initial electron energy before incidence on the beam window, and DEw
and DEa
are the mean energy losses of the transmitted electrons in the beam window and
the air gap, respectively. Each energy loss is roughly equal to the product of the collision
stopping power and the thickness expressed in g/cm 2
, or roughly evaluated by equation
(6) when the thickness is much smaller than the electron range.
7.1.3 The distribution of the mean electron current density in the radiation field deter-
mines approximately the lateral distribution of mean absorbed dose rate in the material.
The lateral distribution in the direction normal to the scanning axis of the radiation field
usually has a Gaussian profile. The full width at half maximum of the Gaussian distribution
depends on the electron energy, the atomic number and the thickness of the beam
window, and the air-gap distance from the beam window to the specimen to be irradiated.
For stationary irradiation, the lateral dose uniformity in the material is determined mainly
by the distribution of the mean electron current density. Irradiation using a conveyor
system moving with constant velocity provides a uniform lateral dose distribution in the
material, if the scanned beam intensity is uniform in the direction of the scanning axis.

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7.1.4 For stationary irradiation, the dose rate averaged over the scanning period is
usually estimated to be the mean dose rate. Two factors complicate its estimation: i)
overlapping of the beam, ii) the movement of the sample on a conveyor system.
7.1.5 Dose uniformity throughout the material is determined by the depth-dose
distribution, which is usually independent of the lateral dose distribution in the slab layer
of the material. A typical depth-dose distribution in a homogeneous material obtained with
electron accelerators is shown in figure 1. The depth-dose distribution is characterized by
two depth regions: a dose build-up region and a dose declining region. The useful range
RU
roughly increases linearly with electron energy when it is higher than 1 MeV, but the
dose variation with depth is much larger in electron-beam irradiations than in y-ray
irradiations. The ratio of the surface dose to the peak dose and the useful range depend
on several irradiation parameters such as electron energy, atomic composition of the
material, thickness of the beam window, the air-gap distance, etc. In typical irradiating
conditions, the ratio is 0,6 - 0,8 for electron energy higher than 1 MeV.
7.1.6 The depth-dose distribution in the material per unit electron fluence can be
calculated as a pa rt
of energy deposition function, 1(z), in a three-layer slab absorber
(beam window, air layer, and the specimen) [31], [17], when a slab-layer material is moved
through the radiation field in the direction perpendicular to the direction of the beam
scanning, assuming that a monoenergetic and plane-parallel electron beam impinges
normally on the beam window. An example of calculated results of l(z') (z' = total depth in
three layers) for a slab layer of polyethylene exposed to 1 MeV electrons is shown in
figure 2. The difference of I(z') between the three layers is not equivalent to the difference
of absorbed dose. Figure 3 shows a comparison of relative depth-dose distributions for
typical insulating materials which were measured for the same irradiation conditions. The
difference between typical organic insulators is not negligible, which is due to the
difference of the mass collision stopping power [7] and the mass multiple scattering
power [16] that depends mainly on the content of hydrogen and the effective atomic
number .
7.2
Recommended procedures for electron-beam dosimetry
Table 1 gives a list of absolute and relative methods with some of their main
characteristics. A recommended check list for dosimetry procedures includes the
following [12]:
I) electron energy in relation to thickness of the specimen;
2) dose range to be covered in the test;
3) mean dose rate in relation to temperature rise in the specimen and irradiation time
allowed for the test;
4) dose uniformity within the specimen required for the test:
a)
limit allowed for dose uniformity within the thickness,
b) limit allowed for lateral dose uniformity within the specimen;
5) irradiation method, taking into consideration the number of specimens, their size,
the temperature rise, and the dose uniformity in the specimen (stationary or scanned
irradiation);
544-1 ©IEC:1994 - 31 -
6) accuracy and precision limit required in the dosimetry;
7) selection of dosimeters using the following criteria:
a) measurable dose range,
b) thickness of dosimeter giving a defined spatial resolution of the dose reading,
c) precision or reproducibility at specified dose levels,
d) variation of response within the expected dose-rate ranges,
e) limited variation of response during and after irradiation with environmental
conditions (effect of light, temperature, humidity, gases, storage),
f)
permanence of reading or stability of dose indication,
g) availability of well-developed and proven standard measurement procedure,
h) simplicity of handling and read-out procedure,
i) availability of dose reader for exclusive use of the dosimeter,
j) reproducibility between different batches,
k) cost;
8) other irradiation parameters (beam current, scan width, air-gap distance, conveyor
speed, backplate for irradiation, temperature, humidity, instantaneous dose rate,
backscattering effect, oblique electron incidence, charge accumulation in the insulator,
etc.).
7.3 Electron-beam irradiation
7.3.1 One-sided electron-beam irradiation of a specimen of the insulating materials is
usually conducted in two sample arrangements, shown in figure 4, taking into
consideration the dose variation within the specimen due to the depth-dose distribution: a)
specimen and a backplate of the equivalent material; and b) specimen sandwiched
between two plates of the equivalent material. The total thickness of the irradiation set-up
should be greater than the electron range. Two-sided irradiation is conducted for
specimens which are thick compared with the electron range.
7.3.2 Acceleration voltage is basically chosen to satisfy the requirement of dose
uniformity within the specimen thickness. Beam current and other irradiating conditions
such as air-gap distance, beam-scanning parameters, mechanical scanning parameters of
the conveyor system, etc. are usually chosen to minimize temperature rise in the
specimen during irradiation and to optimize the radiation utilization efficiency.
7.4 Methods for measuring depth-dose distributions
7.4.1 Measurement of depth-dose distributions in the insulating materials of interest is
the most typical practice in electron-beam dosimetry. There are two basic methods for
depth-dose measurement using film dosimeters in a stack of insulating materials and
wedge-shaped insulating materials, each with a few variations as shown in figure 5.

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7.4.2 In the uniform stack method (see figure 5a), dosimeter films themselves are
stacked as the equivalent insulating materials up to a thickness greater than the electron
range. This method gives the depth-dose distribution in the dosimeter material, and can be
applied for materials with a composition similar to the film. This may be the only method to
measure the depth-dose distribution for low-energy electron beams (< 300 keV).
7.4.3 In the alternate stack method (see figure 5b), dosimeter films and slab layers of an
equivalent insulating material that has a composition similar to the film are stacked
alternately. This method may be applied for relatively high-energy electron beams.
7.4.4 In the shift insertion method (see figure 5c), small chips of dosimeter films are
inserted into the stack of equivalent insulating materials in such a way that the dosimeter
films do not overlap.
7.4.5 When the stacked insulating materials are uniformly exposed to electron beams in
the lateral direction, the absorbed dose D
i in the insulating materials of interest is given by
the formula:
(S/p)col,i
Dd
Di = f • Dd =
(5)
(S/p)col,d
where Dd
is the absorbed dose in the dosimeter material, and are the
(S/p)col,i (S/p)col,d
electron mass collision stopping power of the insulating materials and the dosimeter
material, respectively, averaged over the approximate electron energy spectrum for the
two materials [24].
Tables 4 and 5 show the basic properties of some important insulating materials and of
other materials, and the mass stopping power for electrons in the materials. The electron
energy spectrum for the inserted dosimeter films depends on the depth in the stacked insu-
lating materials and generally it is not easy to evaluate the spectrum at each depth. How-
ever, the ratio f is only slightly energy dependent over the energy range of interest in
radiation resistance tests of insulating materials as shown in table 5. Rough estimation of
the average energy of the electron spectrum in the dosimeter material of interest provides
sufficient information to evaluate the ratio f with appropriate accuracy. The mean electron
energy, En
, as a function of depth in typical water-equivalent insulating materials, is
roughly estimated by the formula:
En = Em (1 - z/ Rex) (6)
where Em is the energy of incident electrons
...