R.B. Hayes

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

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.

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.

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.

A mathematical approach to optimal selection of dose values in the additive dose method of EPR dosimetry

Additive dose methods commonly used in electron paramagnetic resonance (EPR) dosimetry are time consuming and labor intensive. We have developed a mathematical approach for determining optimal spacing of applied doses and the number of spectra which should be taken at each dose level. Expected uncertainties in the data points are assumed to be normally distributed with a fixed standard deviation and the linearity of the dose response is also assumed. The optimum spacing and number of points necessary for minimal error can be estimated as can the likely error in the resulting dose estimate. When low doses are being estimated for enamel samples, the optimal spacing is shown to be a concentration of points near the zero dose value with fewer spectra taken at a single high dose value within the range of known linearity. Optimization of the analytical process results in increased accuracy and sample throughput.

Preparation-induced errors in EPR dosimetry of enamel: Pre- and post-crushing sensitivity

Polyakov et al. (1995) showed errors in dose estimation as a function of grain size for enamel grains given beta irradiation after crushing. We tested the effect of gamma irradiation applied to the specimens before and after crushing. We confirmed Polyakovs observations and found that post-crushing irradiation altered the slope of the dose-response curve of the hydroxyapatite signal and produced a grain-size-dependent offset. No changes in the slope of the dose-response curve were seen in enamel caps irradiated whole before crushing.

Accurate EPR radiosensitivity calibration using small sample masses

Abstract We demonstrate a procedure in retrospective EPR dosimetry which allows for virtually nondestructive sample evaluation in terms of sample irradiations. For this procedure to work, it is shown that corrections must be made for cavity response characteristics when using variable mass samples. Likewise, methods are employed to correct for empty tube signals, sample anisotropy and frequency drift while considering the effects of dose distribution optimization. A demonstration of the methods utility is given by comparing sample portions evaluated using both the described methodology and standard full sample additive dose techniques. The samples used in this study are tooth enamel from teeth removed during routine dental care. We show that by making all the recommended corrections, very small masses can be both accurately measured and correlated with measurements of other samples. Some issues relating to dose distribution optimization are also addressed.

An EPR intercomparison using teeth irradiated prior to crushing

An intercomparison of methods used for EPR dosimetry of enamel was made by the Center for Applied Dosimetry at the University of Utah and the Scientific Center for Radiation Medicine in the Ukraine (SCRM). The Utah group used the differential microwave power technique while the Ukrainian group employed a system of chemical preparation combined with a technique of subtraction of standard background signals. Because of time constraints, the results were reported before each laboratory had performed detailed background analyses. These results showed that both methods were in good agreement with the applied dose. Subsequent to this, the Utah group continued work on refining the differential power technique while SCRM investigated the differences between the inner and outer portions of the teeth. It was determined that the contribution of dental X-rays to the total dose was between 20 and 140 mGy.