E.H. Haskell

The second international intercomparison on EPR tooth dosimetry

Eighteen international EPR laboratories participated in the second intercomparison programme. Each participant had to prepare enamel samples and evaluate the absorbed dose from molars that were irradiated in vitro in the range 0-1000 mGy. The objective of the programme was to bring together all methods which are currently applied by different laboratories for EPR dose reconstruction and to demonstrate the present state of dosimetry. An overview of the essential features of the different methods is presented. The current accuracy of EPR tooth enamel dosimetry under defined conditions of irradiation is evaluated

The First International Intercomparison of EPR-dosimetry with Teeth: First Results

Intercomparison of EPR-dosimetric techniques using tooth enamel had been performed in order to check whether the results produced by different laboratories are consistent and accurate. Participants were supposed to evaluate doses applied to pulverized enamel samples, using routine techniques from their laboratories. The intercomparison has demonstrated a great variety of methods used for dose reconstruction. Peculiarities of experimental approaches are discussed systematically in terms of procedure for recording the EPR-spectra, determination of the amplitude of the radiation induced signal, determination of the dose, and error propagation.

Influence of crushing and additive irradiation procedures on EPR dosimetry of tooth enamel

The effect of the crushing and additive dose procedures used in EPR dosimetry of enamel was studied on the signals with g-factors of 2. 0045 and g, = 2.0018, g. = 1.9975. Eight fractions, ranging in size from <75 micrometers to 2 mm, were prepared from one tooth. Two cases were investigated: crushing of a non-irradiated sample and of a sample previously irradiated (6 Gy from `Co gamma ray source). In the non-irradiated study, the intensity of the native signal at 2.0045 in by circa 1.75 times as the grain size decreased from maximum to minimum. A small in radiation sensitivity (< 8%) was also observed with decreasing grain size. In the irradiated samples, crushing resulted in slight variations of reconstructed doses from expected values, but the worst possible case (grain sizes < 75 micron) showed that additional errors were less than 10%. The radiation sensitivity of enamel measured immediately after exposure is underestimated. It increases by about 15% in the first month. Based on the decomposition of the observed spectra, a new interpretation of transient signals 1108 is proposed which explains the above phenomena. Recommendations about how to use this interpretation in retrospective EPR dosimetry are given.

Comparison of sample preparation and signal evaluation methods for EPR analysis of tooth enamel.

In dose reconstruction by EPR dosimetry with teeth various methods are applied to prepare tooth enamel samples and to evaluate the dosimetric signal. A comparison of seven frequently used methods in EPR dosimetry with tooth enamel was performed. The participating Institutes have applied their own procedure to prepare tooth enamel samples and to evaluate the dosimetric signal. The precision of the EPR measurement and the dependence of the estimated dosimetric signal with irradiation up to 1000 mGy were compared. The obtained results are consistent among the different methods. The reproducibility of the dosimetric signal and its estimated relation with the absorbed dose was found to be very close for the applied methods with one possible exception.

Anisotropy effects of EPR signals and mechanisms of mass transfer in tooth enamel and bones

Peculiarities of the internal construction of tooth enamel and bones that cause anisotropy effects and mass transfer in these objects are described. It is shown that the composition of the mineral component of teeth and bones depends on a mechanical-electrical mechanism, which pumps ions into nanocrystals. Decrease in the efficiency of the mechanical-electrical mechanism results in demineralization of enamel and bones, which progresses most rapidly at a disease of the biomineral or under special conditions, such as in space flights. Effects of signal anisotropy in the practice of retrospective EPR dosimetry are discussed.

Thermoluminescence measurements of gamma-ray doses attributable to fallout from the Nevada test site using building bricks as natural dosimeters.

During the 1950s, the U.S. Government conducted an intensive atmospheric nuclear testing program in Nevada. Fallout from these atmospheric tests was measured throughout the U.S. with some of the heaviest concentrations to populated areas falling east of the test site in Washington County, UT. External exposures from 6.5 x 10(-4) C kg-1 to 26 x 10(-4) C kg-1 (2.5-5.0 R) were reported for this region. This study provides an independent measurement of fallout radiation doses to selected communities in Utah using a thermoluminescence technique originally developed for the dating of ancient pottery. The application of the predose thermoluminescence technique to fallout dosimetry is described. A mean dose of 38 +/- 15 mGy (4.4 +/- 1.7 R), attributed to fallout radiation, was measured in quartz grains extracted from the outer centimeter of bricks removed from six communities in Washington and Kane Counties in Utah.

Effect of mechanically induced background signal on EPR dosimetry of tooth enamel

Abstract The effect of the mechanically induced background ESR signal whose Lande factor is g = 2.0038, width = 0.791 mT, on absorbed dose estimation using the additive method was studied. The intensity and width of this signal increases with decreasing grain size. It was found to be thermally stable and sensitive to 90 Sr radiation. The latter phenomenon should lead to its increasing contribution to the radiation-induced hydroxyapatite signal at g ⊥ = 2.0018 at irradiation with higher doses. However, it was found that the interference between mechanically induced and the hydroxyapatite signals may be interpreted as either ‘negative’ for larger grain size or ‘positive’ for finer grain size. This feature in turn leads to under and overestimation of the hydroxyapatite signal, respectively, and is apparently caused by the inverse relationship between the signal width and grain size. Enamel samples were irradiated with 44, 88, 220, 440, 660 and 880 mGy from a 137 Cs gamma ray source. It was determined that 220 mGy was the lowest absorbed dose that could be reliably detected, while doses as low as 44 mGy could tentatively be identified.

Issues in the reconstruction of environmental doses on the basis of thermoluminescence measurements in the Techa riverside

The potential of thermoluminescence measurements of bricks from the contaminated area of the Techa river valley, Southern Urals, Russia, for reconstructing external exposures of affected population groups has been studied. Thermoluminescence dating of background samples was used to evaluate the age of old buildings available on the river banks. The anthropogenic gamma dose accrued in exposed samples is determined by subtracting the natural radiation background dose for the corresponding age from the accumulated dose measured by thermoluminescence. For a site in the upper Techa river region, where the levels of external exposures were extremely high, the depth-dose distribution in bricks and the dependence of accidental dose on the height of the sampling position were determined. For the same site, Monte Carlo simulations of radiation transport were performed for different source configurations corresponding to the situation before and after the construction of a reservoir on the river and evacuation of the population in 1956. A comparison of the results provides an understanding of the features of the measured depth-dose distributions and height dependencies in terms of the source configurations and shows that bricks from the higher sampling positions are likely to have accrued a larger fraction of anthropogenic dose from the time before the construction of the reservoir. The applicability of the thermoluminescent dosimetry method to environmental dose reconstruction in the middle Techa region, where the external exposure was relatively low, was also investigated.

Improved accuracy of EPR dosimetry using a constant rotation goniometer

Grains of sufficiently small diameter will theoretically result in an isotropic EPR response which is independent of sample orientation. In practice, use of such small grains may present problems of altered sensitivity and newly induced paramagnetic centers. Additionally, the effect of anisotropy is increased with increasing microwave power. This places limits on the effectiveness of the differential power method which allows determination of cumulative dose without the need for a zero background signal. The use of a constant rotation goniometer reduces the effect of anisotropy, making measurements at all microwave powers, and particularly the higher powers, significantly more accurate. Since the organic and the mineralized components of the EPR signal are both effected by anisotropy, this procedure may well result in reduced limits of detection.

Technique for increasing reproducibility in EPR dosimetry of tooth enamel

Variations in several electron paramagnetic resonance (EPR) spectral parameters of importance in retrospective dosimetry of tooth enamel were measured using instrumentation and methodologies designed to maximize measurement reproducibility. Measurements were made using accessory EPR equipment that can be readily constructed with little more than a variable power supply, a geared down motor and some high-purity CaO. Spectroscopic precision is considerably enhanced due to elimination of sample anisotropies by scanning samples throughout their angular range. Frequency drifting incurred by sample reorientation and instrument/sample warm-up is effectively eliminated using an in-cavity :CaO standard. This work shows that for highly anisotropic samples, spectral characteristic parameters (peak-to-peak amplitudes, signal component widths and positions) are reproduced with equal or comparable precision to the case where standard EPR configurations are used while measuring approximately isotropic samples.