Simulation of Absorbed Dose Distribution In Space Materials
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Simulation of Absorbed Dose Distribution In Space Materials Boris A. Briskman Karpov Institute of Physical Chemistry, Obninsk, 249033, RUSSIA ABSTRACT The problems of absorbed dose distribution simulation at on-ground space material tests are discussed. Several approaches to such simulation, oriented to increase the test adequacy and economy, are analyzed. Sometimes, it is possible to use quantitative criteria of absorbed dose distribution depending on the specific space vehicle orbit. The assessment of reliable simulation of the radiation spectrum may be made, for example, by introducing a special numerical characteristic of the depth dose profile in a material - depth dose criterion. For this purpose, it is recommended to use the ratio of the exponent index of the depth dose profile (µ) to the density of the material (ρ). In the simplest form, the depth dose profile can be represented as a sum of two exponents. The first depth dose profile applies to a near-the-surface layer of 5 to 10 µm in thickness, and the second to a layer of from 10 µm up to, as a minimum, 100 µm in thickness. The reference values of µ/ρ for typical spectra of ionizing radiation are calculated. INTRODUCTION On-ground radiation tests of space materials are to replace long-term and expensive in-flight tests under estimated conditions of operation (orbit, position on the spacecraft, flight duration etc). The test does not intend to reproduce exactly the ionizing radiation parameters for the specific orbit, and it is practically impossible. Laboratory reproduction of in-flight physical response of material, that is an indicator of its operational characteristic change, is the main idea of the on-ground test. With that end in view it is necessary to simulate correctly the space environment characteristics. The kind of radiation, its spectrum and intensity are the main simulated parameters of space environment ionizing radiation [1,2]. Here we will discuss only the problems of simulation of electron and proton spectra that are responsible for the charged particle damage of space materials. IONIZING RADIATION SPECTRUM EFFECT The influence of ionizing radiation spectrum on radiation degradation of polymeric materials can have an effect for two reasons: a) dependence of material radiolysis radiation-chemical yield on the value of linear energy transfer (LET) of radiation, and b) different depth distribution of an absorbed dose in the material. As the difference in LET values for actual operational spectra of the same radiation is relatively small (at most 2-3 times), we will take into account only the second reason. The abrupt character of absorbed dose profile is caused by the specificity of the energy spectrum of the space charged particles. Especial feature of interaction of space ionizing radiation with materials is a localization of the main part of absorbed
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energy in the near-surface thin layers. The dose decreases 7 orders of the value in 1g/cm2 near-surface layer. About 90% of absorbed energy is accumulated at up to 10micr
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