The effect of dose on light-sensitivity of radicals in alanine EPR dosimeters

In this work we present a study of light-induced effects on free radicals and their transformations in gamma-irradiated pure L-alanine and in commercially available alanine detectors: rods, pellets and films. Samples irradiated to doses from 2 Gy to 4000 kGy were exposed to light from a fluorescent lamp and to ordinary daylight. The observed changes in EPR spectra of the samples were analyzed with regard to their intensity and shape. The shape analysis was based on numerical decomposition of the measured spectra into model spectra reflecting contributions of R1, R2 and R3 radical populations in the samples. The illumination of alanine dosimeters resulted in significant decrease of the central EPR line and was accompanied by distinct variations in the shape of EPR spectra. The rate of light-induced decay in spectra amplitude was found to be dependent on dose of ionizing radiation--the sensitivity to light was decreasing with increase in dose in all detectors in the 2-5x10(5) Gy dose range. The exposure of gamma-irradiated (to 300 Gy) alanine to normal, diffused daylight resulted in decay of the signal amplitude at rate about 0.5% per week. It was shown that decay in the R1 component was responsible for the observed reduction of the spectra amplitude. The observed increase in R2 contributions in samples exposed to light confirmed a hypothesis of R-->R2 radical transformations promoted by visible light. The reported effects indicate a necessity of protection of irradiated dosimeters from their prolonged exposure to light.

Lessons of the 3rd international intercomparison on EPR dosimetry with teeth: similarities and differences of two successful techniques

Despite the considerable improvement in accuracy in comparison with previous intercomparison programmes, the outcome of the recent 3rd International Intercomparison on EPR Tooth Dosimetry has demonstrated that performance of various protocols practised in different laboratories significantly varies. SCRM and MUG took part in this intercomparison with their own versions of EPR dosimetry protocols, demonstrating the good correlation between reconstructed and nominal doses (best result for SCRM and fourth best for MUG) and the lowest both absolute and relative mean deviations from the nominal doses. Although the general results of the 3rd Intercomparison are being discussed elsewhere in this issue by Wieser et al., this presentation is focused on the discussion of the common features of the two techniques, which may have an effect on good performance in dose reconstruction. In addition to the mthods of analysis of the intercomparison results, as used in Wieser et al., SCRM and MUG studied the influence of an additional factor--the selection of the standard of the native signal--on the quality of the dose reconstruction.

The Third International Intercomparison on EPR Tooth Dosimetry: part 2, final analysis

The objective of the Third International Intercomparison on EPR Tooth Dosimetry was to evaluate laboratories performing tooth enamel dosimetry <300 mGy. Final analysis of results included a correlation analysis between features of laboratory dose reconstruction protocols and dosimetry performance. Applicability of electron paramagnetic resonance (EPR) tooth dosimetry at low dose was shown at two applied dose levels of 79 and 176 mGy. Most (9 of 12) laboratories reported the dose to be within 50 mGy of the delivered dose of 79 mGy, and 10 of 12 laboratories reported the dose to be within 100 mGy of the delivered dose of 176 mGy. At the high-dose tested (704 mGy) agreement within 25% of the delivered dose was found in 10 laboratories. Features of EPR dose reconstruction protocols that affect dosimetry performance were found to be magnetic field modulation amplitude in EPR spectrum recording, EPR signal model in spectrum deconvolution and duration of latency period for tooth enamel samples after preparation.

The 3rd international intercomparison on EPR tooth dosimetry: Part 1, general analysis

The objective of the 3rd International Intercomparison on Electron Paramagnetic Resonance (EPR) Tooth Dosimetry was the evaluation of laboratories performing tooth enamel dosimetry below 300 mGy. Participants had to reconstruct the absorbed dose in tooth enamel from 11 molars, which were cut into two halves. One half of each tooth was irradiated in a 60Co beam to doses in the ranges of 30-100 mGy (5 samples), 100-300 mGy (5 samples), and 300-900 mGy (1 sample). Fourteen international laboratories participated in this intercomparison programme. A first analysis of the results and an overview of the essential features of methods applied in different laboratories are presented. The relative standard deviation of results of all methods was better than 27% for applied doses in the range of 79-704 mGy. In the analysis of the unirradiated tooth halves 8% of the samples were identified as outliers with additional absorbed dose above background dose.

EPR study of light illumination effects on radicals in gamma-irradiated L-alanine

Exposure of gamma-irradiated L-alanine samples to sunlight and to light from a regular, fluorescent lamp resulted in significant changes in their EPR resonance patterns, both to spectral shapes and intensities. The experimental EPR spectra were numerically decomposed into three components reflecting contributions of three different radicals (R1-R3) generated by ionizing radiation in alanine. The light exposure caused a decay of the measured EPR signal intensity. For similar light intensities and exposure times the decay was much more pronounced in samples illuminated by sunlight than in samples illuminated by the fluorescent lamp. In both cases light-induced decay of R1 radicals was observed. Sunlight illumination resulted in a moderate decay of R2 radicals and in a doubling of the R3 radical population. On the other hand, fluorescent light caused a significant increase of R2 radicals and did not change the amount of R3 radicals. A quantitative analysis of the variations of the three radical contributions to the total EPR spectra upon fluorescent light exposure suggests a net R1-->R2 free radical transformation. These effects of light on the alanine dosimetric signal should be taken into account in dosimetry protocols, assuring protection of alanine dosimeters from extended exposure to light.