ESR investigation of joint use of dentin and tooth enamel to estimate photon and neutron dose components of a mixed field.

In the case of mixed photon and neutron field, estimation of photon and neutron dose components from the ESR signal of tooth enamel alone is impossible. To differentiate neutron and photon components using the method described in ICRU 26 for twin chambers, enamel and dentin sensitivities to photon and to neutron were investigated. Enamel and dentin relative sensitivities were, respectively, estimated at 0.03 +/- 0.02 and 0.14 +/- 0.10 for fission neutrons. Basing on this result, calculation of neutron and photon doses was performed in realistic case of criticality accident. Estimation of neutron and photon dose components was found in good agreement with reference dosimetry.

Study of materials for mixed field dosimetry by EPR spectroscopy.

EPR dose reconstruction after accidental photon exposure based on materials irradiated in the vicinity of the victim (sucrose, medicine tablets, etc.) was used successfully in several cases referenced in the literature. However, accidental exposure may also occur with a neutron component such as in the Tokai-Mura criticality accident. The aim of this work is to investigate the potentiality of EPR dosimetry for mixed photon and neutron field exposure with different organic materials already used for photon exposure (sucrose) or with potential dosimetric properties (ascorbic acid, sorbitol, glucose, galactose, fructose, lactose and mannose). To assess the neutron sensitivity, the materials were exposed to a mixed radiation field of an experimental reactor with different neutron to photon ratios. The relative neutron sensitivity was found to range from 12 to 43% according to the materials. The potentiality of these materials for mixed field EPR dosimetry is discussed.

Calibration of EPR signal dose response of tooth enamel to photons: experiment and Monte Carlo simulation.

The experimental energy dependence of the electron paramagnetic resonance (EPR) radiation-induced signal at irradiation by photons in the energy range of 13 keV-1.25 MeV was analysed in terms of the absorbed dose in human tooth enamel. The latter was calculated using a Monte Carlo simulation of the photon and electron transport. The dependence of the calculated absorbed dose on the sample thickness was analysed. No energy dependence of the EPR signal on the absorbed dose in enamel was verified in the range of 37 keV-1.25 MeV. At 13 and 20 keV the EPR signal dose response was reduced by 8% probably due to sample powdering. Dose-depth profiles in enamel samples irradiated by 1.25 MeV photons in polymethylmethacrylate and aluminium build-up materials were calculated. It was concluded that secondary electron equilibrium conditions are better fulfilled for irradiation in aluminium, which makes this material preferable for calibration.

Dose reconstruction by EPR spectroscopy of tooth enamel: application to the population of Zaborie village exposed to high radioactive contamination after the Chernobyl accident.

Individual irradiation doses were determined by electron paramagnetic resonance spectroscopy of the tooth enamel of the inhabitants of Zaborie, the most contaminated inhabited settlement not evacuated after the Chernobyl accident. Dose determination was performed using a specially developed automatic spectrum processing procedure. Spectrum processing was carried out in different operating modes, and average results were taken in order to reduce the contribution of uncertainty in dose determination caused by spectrum processing. The absorbed doses determined in enamel were corrected to take into account the contribution of natural background radiation and to determine the individual excess dose due to radioactive contamination of the territory. Individual excess doses are compared to calculated individualized doses to teeth, estimated using the local radioactive contamination levels, dose rates, and information concerning individual behavior. The individual excess doses measured by electron paramagnetic resonance spectroscopy and the calculated individualized doses are fully independent. Mean square variation between results of two methods was found to be 34 mGy, which is consistent with error estimation for both methods. This result can validate both the methodology of signal processing presented here when using electron paramagnetic resonance dosimetry of tooth enamel for low doses and the methodology of individualized dose calculation.

[Retrospective dose estimation in remote period after exposure using different biological methods]. / Retrospektivnaia otsenka doz v otdalennyi postraditsionnyi period raznymi biologicheskimi metodami.

Investigation of application of chromosome aberrations of lymphocytes in peripheral blood for biological dosimetry purposes in remote (up to 40 years) period after acute exposure to doses of 1 Gy and more was carried out. The comparative analysis of frequency of unstable and stable (using FISH and G-banding methods) aberrations was performed for 24 subjects accidentally exposed to radiation on nuclear submarines during 1961-1985. Statistically significant increasing of frequency of dicentrics and centric rings was determined in the exposed subjects in remote period after exposure to compare with controls. Their sum frequency in the exposed group varied depending on ARS heaviness from 0.1 to 1.0 aberrations per 100 cells. In control group it was from 0 to 0.2 correspondingly. Translocation frequency (complete + incomplete) fixed by FISH method (2, 4, and 12 chromosomes) varied within the limits of 0.2-16.0 for exposed subjects and 0.3-1.26 translocations per genome per 100 cells for controls. Some examined persons (5 subjects) exposed to accident in 1985 had results of analysis of unstable chromosome aberration in acute period after exposure that allow to estimate obtained doses by dicentrics frequency which having good correlation with ARS heaviness. Individual dosed using traslocation frequency were defined retrospectively in 11 from 21 exposed persons. They correlate with calculated physics doses and doses estimated by haematolotical parameters in acute period and also doses obtained by ESR spectroscopy of tooth enamel in remote period.

Elimination of the background signal in tooth enamel samples for EPR-dosimetry by means of physical-chemical treatment.

A method of elimination of the background EPR signal in tooth enamel is proposed. This method implies treatment of enamel powder by highly active reduction reagent hydrazine with subsequent washing out by ethanol-water solution. Such treatment results in reducing both the native background signal (which is assumed to be originated by the organic component) and the mechanical induced EPR signal in enamel. Testing of the efficiency of hydrazine treatment is made for different sizes of enamel powder. It is shown that the optimal results are obtained for a powder fraction of about 100-200 microm. The radiation-induced EPR signal in enamel is practically not changed after treatment by hydrazine.

EPR tooth enamel dosimetry: optimization of the automated spectra deconvolution routine.

A computer routine was developed for automatic deconvolution of electron paramagnetic resonance spectra of tooth enamel samples for individual radiation dose reconstruction in the low dose region. The deconvolution routine uses the non-linear least square fit of a model simulating a tooth enamel spectrum by superposition of derivative Gaussian functions to obtain the amplitude of the dosimetric radiation induced signal. The parameters of the model and of the routine were optimized on a dose response level using a criterion of the least standard deviation of the derived radiation induced signal amplitude from the regression line vs. the nominal doses for the series of spectra of samples irradiated in known doses in the range 0-500 mGy. It was found that for the series of spectra of the heterogeneous samples (every sample is prepared from different teeth of different persons), it is essential to vary in the least square fit the parameters describing the shape of the native background signal in order to obtain the best accuracy. In the case of the series of spectra of the homogeneous (pooled) samples, almost the same accuracy of the results was obtained using the procedures with varied and fixed background signal parameters. The standard error of the dose reconstruction by the optimized deconvolution procedure was estimated as about 30 mGy for heterogeneous samples and 20 mGy for homogeneous samples.

Application of EPR retrospective dosimetry for large-scale accidental situation.

Above 3000 tooth enamel samples, collected at population of radioactive contaminated territories after Chernobyl accident, the Chernobyl liquidators, the retired military of high radiation risk and the population of control radiation free territories were investigated by EPR spectroscopy method in order to obtain accumulated individual exposure doses. Results of EPR spectra measurements are stored in data bank; enamel samples are also stored in order to provide the possibility to repeat the measurements in future. Statistical analysis of results has allowed to detect the contribution into EPR signal in tooth enamel due to the action of the natural background radiation, and the radioactive contamination of territory. In general, the average doses of external exposure of the population obtained with EPR spectroscopy of teeth enamel are consistent with results based on other methods of direct and retrospective dosimetry. Essential exceeding of the individual doses above the average level within the population groups was observed for some persons. That gave the possibility to detect the individuals with overexposure, which were included into groups for medical monitoring.

Tooth enamel EPR dosimetry: sources of errors and their correction.

Some of the most important sources of systematic errors in dose determination using tooth enamel EPR spectroscopy and ways of reducing those errors are discussed. Enamel from the outside of the front teeth should not be used for dose determination because of induction of paramagnetic centers by solar light. The accuracy of the method in the low dose range is limited by variation in the shape of the EPR signal of unirradiated enamel, which can be described by an initial intrinsic signal and which varies for different samples with standard deviation of 20-30 mGy. The energy dependence of enamel sensitivity should be taken into account in the form of a correction factor. The value of this factor is estimated at 1.1-1.3 for real radiation fields in radiation contaminated territories. Variation in enamel sensitivity for different samples is shown to be within limits of 10-15% of the average value.

ESR and TL dosimetry systems: comparative measurements for human phantom.

Mixtures of small fragments of tooth enamel as well as thermoluminescence (TL) dosimeters were placed into the tissue-equivalent phantom of the human head with skeleton (approximately at the level of the jaws) and irradiated using 137Cs low dose-rate gamma therapeutic sources ('SELEKTRON' LDR 137Cs). Phantom, samples of teeth and TL detectors were irradiated behind water tank to produce scattered irradiation. The same irradiation with the same geometry was performed in air too. For gamma-spectrometry 137Cs sources with very low activity were used but with the same geometry as therapeutic sources. The absorbed dose in enamel was estimated with the help of ESR spectrometer 'ESP-300 E' (Brucker). The samples of tooth enamel were partially used for preliminary dose evaluation by ESR signal before starting of experiment. TL dosimetry was performed by TL reader model 8800 (HARSHAW) using TL dosimeters calibrated with 137Cs. The paper presents data obtained in comparative aspects.