engleski

Thermal stability of radicals in gamma-irradiated L-alanine single crystals

Simple amino acid L-alanine has been the most common substance in electron paramagnetic resonance (EPR) dosimetry studied for the last 40 years. Following exposure to ionizing radiation, the EPR signal shows a linear relationship with the absorbed dose in relatively broad dose range from 0.5 Gy to 5 kGy, with long-term stability of the dosimetric signal. Although numerous publications regarding to L-alanine dosimetric properties (such as sensitivity and stability) were presented, the basic processes involved in generation and stabilization of the radiation induced radicals are still not completely understood as well as thermal annealing/fading behavior. The rate of fading increases with the relative humidity and illumination and also varies with the shape and possibly the composition of the dosimeters. It was noted that fading characteristics are very strongly influenced by crystallinity of the material. Generally, it is expected that larger crystal grains of sample could improve sensitivity of dosimeter. It is known that grinding of alanine powder results in increasing of the “ zero-dose” signal. Thus, the more detailed investigation on stability of paramagnetic centers in L-alanine is desirable in order to obtain optimal dosimetric properties. For this purpose, in this work the annealing effects of stable L-alanine radicals in crystalline state as well as in the polycrystalline matrix at various temperatures in the range from 379 to 476 K were investigated by using EPR spectroscopy. Obtained activation energy of radical decay process is E=12 538± 202 K (i.e. 104.3± 1.7 kJ/mol) with the frequency factor lnfo=24.1± 0.4 min-1. Lifetime of the radical is 162 years at 296 K. The obtained results confirm long term stability, but with faster decay rate than for polycrystalline samples.

alanine; thermal stability; epr

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