EPR Dosimetry Of Tooth Enamel Research Articles

The use of double-integral of experimental EPR spectra for tooth enamel EPR dosimetry.

The presence of background signal in tooth enamel EPR dosimetry is the most challenging situation to overcome. This becomes even more important because it obscures the radiation-induced signal, especially at radiation doses below 1Gy. In order to overcome this problem, subtraction of the unirradiated sample signal from the irradiated one and the simulation methods are the most widely used methods in the literature. These methods have their own difficulties. Obtaining the double integral of the experimental EPR spectra (first derivative) of tooth enamel and its advantages are presented in the present work. This method offers the opportunity to handle the whole experimental spectrum, both background, and radiation-induced signals, without performing any subtraction or simulation operations, and to overcome the signal-to-noise effects.

Tooth enamel EPR dosimetry of neutrons: Enhancement of the apparent sensitivity at irradiation in the human head phantom

Induction of the EPR signal in tooth enamel at irradiation by the neutrons produced by a generator with maximal energy of 0.8 MeV in the air and in the human head phantom was investigated. Neutron and photon tissue absorbed doses in the mixed radiation field were determined experimentally and calculated using the Monte-Carlo method. It is shown that the EPR signal response of enamel to neutrons relative to the tissue dose is ( 2 ± 2 ) % of the appropriate value for 60Co gamma radiation. At irradiation in the human head phantom, the EPR signal response relatively to the input neutron tissue dose near the surface of the phantom (apparent EPR sensitivity to neutrons) increases to ( 14 ± 1 ) % . The observed EPR signal enhancement is caused by absorption in enamel of the secondary photons produced at H ( n , γ ) H 2 , E γ = 2.23 MeV reaction in the material of the phantom.

Tooth Enamel EPR Dosimetry: Optimization of EPR Spectra Recording Parameters and Effect of Sample Mass on Spectral Sensitivity

In order to improve the accuracy of the tooth enamel EPR dosimetry method, EPR spectra recording conditions were optimized. The uncertainty of dose determination was obtained as the mean square deviation of doses, determined with the use of a spectra deconvolution program, from the nominal doses for ten enamel samples irradiated in the range from 0 to 500 mGy. The spectra were recorded at different microwave powers and accumulation times. It was shown that minimal uncertainty is achieved at the microwave power of about 2 mW for a used spectrometer JEOL JES-FA100. It was found that a limit of the accumulation time exists beyond which uncertainty reduction is ineffective. At an established total time of measurement, reduced uncertainty is obtained by averaging the experimental doses determined from recorded spectra following intermittent sample shaking and sample tube rotation, rather than from one spectrum recorded at longer accumulation time. The effect of sample mass on the spectrometer's sensitivity was investigated in order to find out how to make appropriate corrections.

EPR-dosimetry with carious teeth

The effect of caries in EPR dosimetry of tooth enamel (in the dose range of 0–1 Gy) was investigated. The enamel of each tooth was divided into carious, non-carious and intermediate portions. The EPR signals of enamel at g = 2.0018 (dosimetric) and g = 2.0045 (native) were examined. The intensity of the dosimetric signal was the same for all three portions, while that of the native signal was higher for carious portions than for non-carious and intermediate portions. Reconstruction of the laboratory applied doses was done using all portions. Reasonable correlation between nominal and reconstructed doses was found in most cases. The effect of alkali treatment on the native and dosimetric signals of enamel was also tested. Reduction of the native signal intensity, particularly in the carious portions, was found to be the only significant effect. This resulted in a slight improvement in the accuracy of the reconstructed doses.

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.

The Individual Form of the Background Signal in EPR Dosimetry of Tooth Enamel

The dependence of the calculated individual absorbed dose on the form factor, degree of anisotropy, and the model line width of the background component of the EPR spectrum of tooth enamel established. It is shown that an incorrect choice of the model line form for the background component can lead to inadequate evaluation of reconstructed load dose.

Achievable Precision and Accuracy in EPR Dosimetry of Tooth Enamel

The utility of combining various methodologies is examined to remove systematically or correct for the major instrumental effects which introduce uncertainties in EPR dosimetry of tooth enamel. The result is a relatively high precision method that can be successfully transferred between laboratories. Only an ideal case was considered in this study whereby the actual native signal prior to irradiation was used as the empirical zero in each individual dose response. To prove its general value, the overall methodology was applied to seven teeth of differing quality, prepared using different methodologies, and measured in three laboratories using different EPR spectrometers. Using this approach to characterise the utility of both the methodology and instrumentation resulted in 5 ± 5 mGy (1s) spectral intensity reproducibility at the 100 mGy dose level for the seven samples analysed. Similarly, at the 1100 mGy level, spectral intensity reproducibility was found to be equivalent to 13 ± 5 mGy (1s) for the same seven samples. Assuming linearity of dose response in the 0-1100 mGy dose range and using dose responses at 100 mGy and 1100 mGy levels, gave an overall extrapolated x intercept (dose estimate) offset of -13 ± 19 mGy (1s)

Automated Spectral Manipulation and Data Analysis for EPR Dosimetry of Teeth

A method for automating the spectral manipulation and data analysis procedures for EPR dosimetry of teeth is presented. The method is shown to correlate with conventional spectral peak-to-peak values to within 10 mGy for the reconstructed doses. Paired difference t-test data show that definitive systematic differences exist but that these effects on the reconstructed dose accuracy and precision are negligible for typical applications in tooth enamel EPR dosimetry. The algorithm is written in Hypercard© script for Macintosh computers but could be -implemented on other platforms.

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 on EPR signals with g-factors of 2.0045 and g ⊥=2.0018, g ||=1.9975 was studied. Eight fractions, ranging in size from <75 μm to 2 mm, were prepared from one tooth. Two cases were investigated: crushing of a non-irradiated sample and crushing of a previously irradiated sample (6 Gy from a 60Co gamma ray source). In the non-irradiated case, the intensity of the native signal at 2.0045 increased by approximately 1.75 times as the grain size decreased from maximum to minimum. A small decrease (<8%) in radiation sensitivity (change in signal intensity per unit of irradiation) 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 case (grain sizes <75 μm) showed that additional errors were less than 10%. Due to the presence of transient signals, 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 is proposed which explains the above phenomena. Recommendations about how to use this interpretation in retrospective EPR dosimetry are given.

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

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 90Sr 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 137Cs 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.