IEC 60544 1 Edition 3 0 2013 06 INTERNATIONAL STANDARD NORME INTERNATIONALE Electrical insulating materials – Determination of the effects of ionizing radiation – Part 1 Radiation interaction and dosi[.]
General
Radiation interacts with matter primarily by producing ions and electrically excited states of molecules, which can lead to the formation of free radicals Detection techniques for these short-lived intermediate species are outlined in Clause A.4 Additionally, radiation generates mobile electrons that can become trapped at low potential energy sites The production of ions and radicals results in permanent changes to the material's chemical, mechanical, and electrical properties, while trapped electrons cause temporary electrical performance changes.
Permanent changes
Irradiation of polymeric materials generates free radicals that lead to scission and cross-linking, altering the chemical structure and often resulting in mechanical property deterioration This mechanical decline can significantly affect electrical properties, with changes sometimes occurring before noticeable mechanical degradation For instance, variations in dissipation factor or permittivity can jeopardize the reliable operation of resonant circuits The degree of scission and cross-linking is influenced by factors such as absorbed dose, dose rate, material geometry, and environmental conditions during irradiation Additionally, the slow decay of free radicals may result in post-irradiation effects.
Environmental conditions and material geometry
Controlling and documenting environmental conditions and test specimen geometry is crucial when measuring radiation effects Key parameters include temperature, reactive medium, and mechanical and electrical stresses during irradiation The presence of air significantly impacts irradiation time, flux, and dose rate due to oxygen diffusion effects and hydroperoxide breakdown rates, both of which are time-dependent It is essential to manage conditions that affect oxygen diffusion and equilibrium concentrations in polymers, such as temperature, oxygen pressure, material geometry, and the duration of dose application.
Simulating the effects of simultaneous stresses, such as high-temperature radiation, through sequential stressing may yield different outcomes Additionally, the order of processes—whether the sample is irradiated first and then heat aged, or the other way around—can lead to varying results.
Post-irradiation effects
Post-irradiation effects in organic polymers can arise from the gradual decay of reactants, including residual free radicals It is essential to consider this behavior in evaluation procedures Testing should occur at specified intervals following irradiation, with specimens stored in a controlled laboratory environment Additionally, the interaction of oxygen with residual free radicals may lead to further degradation.
Temporary effects
Section 4.5.1 of IEC 60544 does not cover measurements during irradiation; however, it briefly addresses key aspects Temporary effects manifest mainly as alterations in electrical properties, particularly induced conductivity, occurring during and shortly after irradiation Therefore, measuring induced conductivity can serve as an evaluation property to assess temporary radiation effects, which are primarily dependent on dose rate.
Experience indicates that the induced conductivity is generally not directly proportional to the absorbed dose rate, but instead varies as \( Ḋ^\alpha \), where \( \alpha \) is less than one Therefore, the relationship describing radiation sensitivity can be expressed as \( \sigma_i = kḊ^\alpha \).
To determine k and α, at least two measurements are needed A further complication comes from the fact that k and α also depend on the integrated dose absorbed by the sample
Measuring induced conductivity is a delicate process, as photoelectrons and Compton electrons in electrode materials can disrupt the intrinsic induced current of the specimen Additionally, ionic currents from the ionized atmosphere may introduce measurement errors if not properly addressed Therefore, it is essential to establish experimental procedures that effectively eliminate these disturbances while maintaining simplicity.
To effectively characterize the sensitivity of materials to temporary effects, it is useful to employ a straightforward figure like the induced conductivity \$\sigma_i\$ or the ratio \$\sigma_i / \sigma_o\$, where \$\sigma_o\$ represents the dark conductivity measured under identical experimental conditions, normalized per unit dose rate.
5 Facilities for irradiation of material samples for evaluation of properties
General
Irradiation of material samples for evaluation of properties shall be performed at irradiation facilities that have undergone installation qualification, operational qualification and performance qualification, see e.g ISO 11137 [3]
Three principal types of radiation sources are used:
• gamma radiation from radionuclides such as 60 Co (1,25 MeV) and 137 Cs (0,66 MeV);
• X-rays generated from accelerated electrons
The design and characteristics of an irradiation facility significantly affect the distribution of absorbed dose in samples and the achievable absorbed dose range Key factors in designing such a facility include ensuring uniform absorbed dose distribution across the product and optimizing the efficient use of radiation energy.
Gamma-ray irradiators
Large capacity gamma radiation facilities commonly utilize cobalt-60 (\$^{60}Co\$) as their radiation source These sources are typically organized in individual capsules arranged in an array to optimize the irradiation volume The available dose rates depend on the distance from the sources to the samples, with typical rates ranging from 10 kGy/h (2.78 Gy/s) to 1 Gy/h (0.278 mGy/s) This range is particularly relevant for testing materials degradation.
Electron-beam irradiators
Electron beam irradiators use accelerators that generate electron beam in the energy-range of
Accelerating procedures for radiation resistance testing range from 300 keV to 10 MeV, with electro-static types (0.5 – 3 MeV) being the most commonly used In an electro-static accelerating system, thermo-electrons are emitted from a cathode and accelerated by a high voltage between electrodes The resulting electron beams are then electro-magnetically scanned in a scanning horn and extracted through a window.
Electron accelerators, often constructed with a thin titanium foil, operate safely and simply, as they emit no radiation when powered off, unlike 60 Co gamma irradiation facilities The dose rate, which can vary based on factors such as voltage, beam current, scan width, and the distance between the window and samples, typically reaches kGy/s, significantly surpassing that of gamma irradiators.
Penetration of the electron beam in samples shall be taken into account (see Clause A.3).
X-ray (Bremsstrahlung) irradiators
X-rays (or Bremsstrahlung) are created when accelerated electrons are slowed down in an absorbing material The fraction of kinetic energy of the electrons that is converted into X-rays
Higher atomic number materials, like tungsten, exhibit greater conversion efficiency as X-ray converters This efficiency improves with increased electron energy, reaching approximately 5% at 5 MeV and rising to about 12.5% at 10 MeV However, the low conversion efficiency at these energy levels has restricted the application of such irradiators The introduction of high-power electron accelerators has the potential to enhance their use.
5 MeV to 10 MeV has renewed interest in the use of X-rays for irradiation of products
In contrast to radionuclide sources, which emit nearly mono-energetic photons, X-ray sources emit a broad spectrum of photons from the maximum energy of the electrons to zero energy
An X-ray beam produced by 5 MeV electrons exhibits penetration properties similar to that of 60 Co radiation Additionally, the scanning and pulsing features of the X-ray beam are influenced by the characteristics of the originating electron beam.
General
It is necessary to ensure that the correct absorbed dose is applied during irradiation The dose shall be measured, and measurement systems have been developed for this purpose
The advancement of dosimetry systems, initially designed for personnel radiation protection and medical treatment, has paved the way for new systems tailored for higher doses used in material testing, which can range from a few kGy to over 100 kGy These new dosimetry systems ensure that dose measurements are traceable to national standards, with a clear understanding of uncertainty and the influence of various parameters.
National laboratories uphold absolute methods of dosimetry as national standards These dosimeters measure radiation doses through physical measurements, eliminating the need for calibration in a known radiation field.
Other dosimeters are calibrated against these national standard dosimeters, thereby providing measurement traceability to the national standard dosimeters
Various dosimeter systems are utilized in irradiation facilities and laboratories to measure dose distribution for facility characterization and to assess products and samples intended for irradiation These dosimeters play a crucial role in monitoring the irradiation process The choice of a dosimeter system is influenced by the specific measurement task and the characteristics of the dosimeter Comprehensive descriptions of dosimetry methods and dosimeter systems are available in several resources.
ISO/ASTM standards and guides [4 – 18] More details of several of these dosimetry systems are found in ICRU 80 [19].
Absolute dosimetry methods
Gamma-rays
Free-air ionization chambers are utilized for measuring X-ray exposure up to 3 MeV by quantifying the charge dQ generated in air and the mass dm of air where ionizing electrons are released These chambers are effective when the dose rate remains within manageable limits.
Calorimeters function by absorbing energy from their surrounding radiation field, storing this energy until it transforms into thermal energy The amount of heat generated is assessed by measuring the temperature increase of the calorimetric absorber.
Electron beams
In addition to calorimetric methods, measurement of electron current density has been used to measure electron charge or current per unit area of radiation fields of electron accelerators
This approach is not a dosimetric technique; however, it allows for the calibration of absorbed dose when the mean electron energy impacting the charge absorber of the densitometer and the relative depth-dose distribution within the same absorber material are known.
Dosimetry systems
Reference standard dosimetry systems
Reference standard dosimetry systems serve as calibration benchmarks for routine measurement dosimetry systems The uncertainty associated with these reference standards directly impacts the calibration accuracy of the systems they are used to calibrate, making it crucial for them to possess high metrological quality High metrological quality entails low uncertainty and traceability to relevant national or international standards, ensuring that the dosimeter's response remains unaffected by environmental factors.
The expanded uncertainty for measurements using a reference standard dosimetry system is generally around ± 3 % (k = 2), reflecting a 95 % confidence level for normally distributed data However, in specific applications, such as with electrons below 1 MeV, practical limitations may lead to increased uncertainty in these dosimetry systems.
Examples of reference standard dosimetry systems are given in Table 1
NOTE ASTM E 2628-09 “Standard practice for dosimetry for radiation processing” [22] is a valuable guideline concerning Table 1 and Table 2
Table 1 – Examples of reference standard dosimeters
Dosimeter Description Reference Dose range
Fricke solution Liquid solution of ferrous and ferric ions in 0,4 M sulphuric acid Measured by spectrophotometry
Pellet or film containing alanine
Measured by EPR spectroscopy of radiation induced radical
Dichromate Liquid solution of chromium ions in 0,1 M perchloric acid
Ceric-cerous sulphate Liquid solution of ceric and cerous ions in 0,4 M sulphuric acid Measured by spectrophotometry or potentiometry
(Classification dependent on solution composition and method of measurement)
Liquid solutions of various compositions containing chlorobenzene in ethanol Measured by titration
Routine dosimetry systems
A routine dosimetry system is classified based on its application for routine absorbed dose measurements, including dose mapping and process monitoring It consists of dosimeters, measurement equipment, and quality system documentation to ensure traceability to national or international standards The response of these dosimeters is often complexly influenced by environmental factors.
The expanded uncertainty achievable with measurements made using a routine dosimetry system is typically of the order of ± 6 % (k = 2)
Examples of routine dosimetry systems are given in Table 2 Dosimeters in Table 1 can also be used as routine systems
Table 2 – Examples of routine dosimeter systems
Dosimeter Description Reference Dose range
Calorimeter Assembly consisting of calorimetric body (absorber), thermal insulation, and temperature sensor with wiring
Cellulose triacetate Untinted cellulose triacetate (CTA) film
(classification dependent on solution composition and method of measurement)
Liquid solutions of various compositions containing chlorobenzene in ethanol Measured by spectrophotometry or oscillometry
LiF photo- fluorescent Lithium fluoride based photo-fluorescent film Measured by photo-stimulated luminescence
Humidity Ambient light Radiochromic film Specially prepared film containing dye precursors Measured by spectrophotometry
Humidity Ambient light Radiochromic liquid Specially prepared solution containing dye precursors
Radiochromic optical waveguide Specially prepared optical waveguide containing dye precursors Measured by spectrophotometry
A phosphor, alone, or incorporated in a material Measured by thermoluminescence
Measurement uncertainty
To be meaningful, a measurement of absorbed dose shall be accompanied by an estimate of uncertainty Components of uncertainty should be identified as belonging to one of two categories:
Type A — those evaluated by statistical methods, or
Type B — those evaluated by other means
Estimates of the expanded uncertainty of an absorbed dose measurement should be made with a coverage factor k = 2
The classification of uncertainties into Type A and Type B follows the methodology established by the International Organization for Standardization (ISO) in their 1995 publication, "Guide to the Expression of Uncertainty."
The characterization of measurement aims to enhance the understanding of how uncertainty statements are formulated, serving as a foundation for the international comparison of measurement results.
Dosimeter calibration
The response of dosimeters to radiation is significantly influenced by environmental factors such as temperature, humidity, and dose rate To ensure accurate measurements, it is essential to calibrate dosimeters under conditions that closely resemble their intended usage environment.
Calibration of dosimeters should be performed using one of two methods: either by irradiating dosimeters at the facility where they will be used, alongside transfer standard dosimeters from a national standards laboratory or an accredited calibration lab, or by conducting irradiations at a national standards laboratory or an accredited dosimetry calibration laboratory This approach generates a calibration curve based on a consistent set of influencing parameters Users must assess the impact of these parameters, which can be effectively done through a verification irradiation "in-plant" at specific doses.
Measurement traceability is defined in the International vocabulary of metrology [26] as follows:
“the property of a result of a measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons.”
Calibration is an important step in obtaining measurement traceability.
Dosimeter selection
The selection and use of a specific dosimetry system in a given application shall be justified taking into account at least the following:
The dosimetry system shall be calibrated in accordance with 6.3.4
The uncertainty associated with measurements made with the dosimetry system shall be established and documented All dose measurements shall be accompanied by an estimate of uncertainty
To ensure adherence to the minimum standards for the dosimetry system, it is essential to establish and maintain proper documentation Additionally, the user's quality system may exceed these basic requirements, providing a more comprehensive framework for compliance.
Tables 1 and 2 gives a non-exhaustive list of reference and routine dosimetry 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 dosimeter material;
Clause A.2 gives an example for calculation of absorbed dose
Irradiation facilities intended for assessing the performance of electrical insulating materials must be thoroughly characterized prior to use The specific parameters to be evaluated will vary based on the type of facility employed.
For a gamma irradiation facility, the following parameters shall be determined:
The dose rate distribution must be accurately measured within the facility designated for sample exposure to radiation This detailed mapping is essential to identify specific locations that correspond to particular dose rates.
– The ambient temperature within the radiation facility shall be measured while the sources are in their normal operating position
The duration of sample exposure must be accurately measured, with a measurement method capable of accounting for any interruptions in irradiation, such as the retraction of sources to facilitate sample removal.
For electron beam and X-ray facilities, the following parameters shall be determined:
– The dose as a function of the beam current, beam spot size and beam width shall be measured
– If a conveyor system is used to transport samples through the radiation field, the dose as a function of conveyor speed shall also be measured
NOTE There are a number of standards available for dosimetry and dose mapping specifically for use in radiation processing facilities [4 – 7, 27]
8 Dose mapping of samples for test
Charged particle equilibrium
For irradiation in gamma facilities it is recommend to provide secondary electron equilibrium in the irradiated sample leading to more uniform dose distribution throughout the sample, see
Depth-dose distribution (limitations)
The absorbed dose distribution in an irradiated specimen varies based on its thickness, density, and the energy of the incident radiation It is crucial to determine the acceptable variation in dose as radiation penetrates the specimen Common irradiation facilities utilize radiation sources with energy levels ranging from 0.5 MeV to 1.5 MeV For a point source, if a 25% limit is set for the difference in absorbed dose between the front and rear of the specimen, this establishes a maximum thickness for the specimen.
For radiation measurements, a thickness of 2.8 cm is required for 0.5 MeV and 5.0 cm for 1.5 MeV, assuming a unit density of 1 g/cm³ and unidirectional radiation It is important to note that these thicknesses will vary significantly for different source geometries, such as slab sources.
Irradiation testing of insulating materials necessitates long-term evaluation under low dose rate conditions, ideally conducted in a gamma irradiation facility It is crucial to consider the decay of the radionuclide used in the sources during these tests To ensure accuracy, dose rate mapping of gamma facilities should be performed at intervals that do not exceed the half-life of the radionuclide in use.
In electron beam and X-ray facilities, it is essential to monitor the characteristics of the electron beam during irradiation Any interruptions or changes in these characteristics must be documented, along with the actions taken in response.
Radiation chemical aspects in interaction and dosimetry
Whenever a material is irradiated from one side only with X- or γ-rays uncontaminated with secondary electrons, there is initially (in the first absorber) a build-up of energy deposition
As radiation penetrates a material, the absorbed dose decreases after a certain thickness This critical thickness, known as the charged-particle equilibrium thickness, depends on both the radiation energy and the electron density of the irradiated material Beyond this thickness, charged-particle equilibrium is achieved within the material.
Figure A.1 illustrates the relationship between energy deposition and thickness To achieve charged-particle equilibrium when irradiating a specimen from all sides, it is essential to encase the specimen in an absorber In scenarios involving highly scattered radiation, build-up may not be evident; however, utilizing built-up layers is advisable to establish clear irradiation conditions.
Figure A.2 illustrates the relationship between absorber thickness and energy for a material with an electron density of \(3.3 \times 10^{23} \, \text{cm}^{-3}\) In contrast, Figure A.3 depicts the thickness of water, or a material with equivalent electron density, as a function of energy, specifically for a defined level of attenuation under unidirectional irradiation.
NOTE The electron density n of any material can be evaluated from:
= ρ (A.1) where ρ is the density of material (g/cm 3 );
N A is the 6,023 × 10 23 mol -1 , Avogadro's constant;
M is the molar mass (g/mol);
Z i is the atomic number of element i; Σ i Z i is the total number of electrons per molecule.
Since 1/M (Σ i Z i ) is about 1/2 for elements up to Z = 17 (excluding H), for organic materials, this equation can be simplified to read:
Figures A.1 to A.3 are calculated with this approximation
As the electron density of the absorber rises above the calculated value, the curve in Figure A.2 shifts to the left, while it shifts to the right for materials with lower electron density Consequently, the equivalent thickness can be determined by taking the value from Figure A.2 and dividing it by the ratio of the absorber's electron density to 3.3 × 10²³ cm⁻³, which is the electron density of water.
For example, it is required to irradiate polytetrafluoroethylene (PTFE) film with 1,1 MeV photons Referring to Figure A.2, it is noted that 0,5 cm of material of electron density of
3,3 × 10 23 cm -3 is needed to ensure charged-particle equilibrium Therefore, this thickness of water shall surround the film
For PTFE with a density of ρ = 2,2 g/cm 3 one calculates:
This means that 0,25 cm of PTFE has to surround the film
Figure A.3 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
Higher-density materials cause the curves to shift left, while lower-density materials shift them right To determine the precise thickness for 10% or 25% attenuation in the specimen, divide the value from Figure A.3 by the ratio of the specimen's electron density.
The calculated curves, based on attenuation alone and neglecting build-up in thicker specimens, indicate the maximum attenuation for specific energy and thickness under unidirectional radiation In contrast, non-unidirectional radiation leads to increased attenuation.
E ner gy de po si tion
Figure A.1 – Absorbed dose as a function of thickness
To accurately analyze the curve, it is essential to obtain a sample of adequate thickness that either reaches or exceeds the maximum point, as the section of the curve to the left of the maximum is not well understood.
Figure A.2 – Absorber thickness for charged-particle equilibrium as a function of energy for a material with an electron density of 3,3 × 10 23 cm -3 (water)
Figure A.3 – Thickness of water (1 g/cm 3 ) as a function of photon energy for a given attenuation of unidirectional X-ray or γ-ray radiation
For irradiations using a photon source, the dosimeter may be considered as a cavity in the material interest and the interpretation of absorbed dose in materials is as follows:
When the sensitive region of a dosimeter is significantly thinner than the range of the highest energy secondary electrons, the majority of the energy deposited in the dosimeter and its surrounding material comes from secondary electrons generated outside the dosimeter, specifically in the equilibrium layer of material Consequently, the absorbed dose in the material, denoted as \$D_m\$, can be expressed as follows:
(S/ρ)m and (S/ρ) d is the mass collision stopping power for the surrounding material and dosimeter, respectively;
D d is the absorbed dose in the dosimeter
Values of mass collision stopping powers are given in Table A.1
When the sensitive region of a dosimeter is significantly thicker than the range of the highest energy secondary electrons, the majority of the energy deposited is due to secondary electrons generated within the dosimeter Consequently, the absorbed dose in the material can be expressed as:
(à en /ρ) m and (à en /ρ) d are the mass energy-absorption coefficients for the material and dosimeter, respectively.
Values of mass energy absorption coefficient are given in Table A.2
When the sensitive region of the dosimeter falls within the specified thickness limits, the equations can be integrated using suitable weighting factors to accurately represent the relative contributions of each term.
The collision stopping powers and energy absorption coefficients are energy dependent
Ratios of mass energy absorption coefficients can be plotted Similarly, ratios of mass collision stopping powers can be plotted
The ESTAR program provides essential calculations for stopping power, density effect parameters, range, and radiation yield for electrons across various materials Users can select a material and input specific energy values, which should be in MeV and range from 0.001 MeV to 10,000 MeV For further details, tables and figures related to stopping power and energy absorption can be accessed at the NIST websites.
For a photon energy spectrum including energies down to 50 keV, the ratios of mass energy absorption coefficient are essentially equal to unity for most polymer materials compared to water