Application of Alanine/Esr Spectrum Shape Change in Gamma Dosimetry

Abstract

Temperature and radiation are two of the major environmental elements exerting aging effects on the incontainment instrumentation and control cables connected to major components. The effect of radiation on cable material is related to the total absorbed dose, which is mainly dependent on gamma rays [1]. For validity of radiation measurement, at least two operation cycles are required for the installation period. Generally used personal dosimeters are not appropriate for this application due to their rapid signal fading feature and relatively poor linearity at a range of high dose. Alanine-electron spin resonance (ESR) dosimetry material, meanwhile, has been proved to be suitable for long term measurement on the basis of the following factors: small signal dependency on temperature ( 0.2%/K), limited signal fading ( 1%/year), and low influence of humidity, which can be easily overcome by hermetic sealing of capsules [2]. Based on the aforementioned advantages as a long term dosimetry material, in the present study dozens of pellet type alanine dosimeters were installed at selected positions in a nuclear power plant (NPP) for two operation cycles. ESR dosimety is known as a retrospective dosimetry method that can be applied in an emergency after an accident and is accepted as a secondary standard and transfer dosimeter for the comparison of dose quantity between laboratories [3][4][5]. Recently, it was found that the peaks of spectra are mainly due to three kinds of radicals induced by radiation [6][7][8].The central resonance peak amplitude of main peaks is used to estimate radiation dose quantity. The weak "satellite line" beside the central resonance peak has been interpreted as the combined effects of "spin flip" of neighboring hydrogen protons of the methyl group of the paramagnetic center and a R2 radical [9][10][11]. The ratio of the "satellite line and main central resonance peak" is frequently expressed as the "x/y ratio". A decrease of this ratio has been explained by less saturation resistance of the main peak amplitude, which is caused by a decrease of the spin-lattice relaxation time T1. Crystallographic defects and plastic deformation in the structure of the alanine lattice by thermal shock of high linear energy transfer (LET) radiation have been reported as the most likely causes of relaxation time change. The "x/y ratio" irradiated with mixed radiation, composed of two radiations with different LET values, was reported to be intermediate and is determined by the absolute population size of each radical produced by radiation with different LET values. In this paper, the "x/y ratio" of alanine dosimeters returned from the containment building of the NPP is lower than that of dosimeters exposed to gamma rays. Furthermore, the ratio constantly decreased in proportion to the dose quantity, which corresponds with the installation period in the NPP. These results showed good accordance with previously reported results. Application of the above features to estimate gamma dose in mixed radiation fields is ...

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