Radiation damage in bioapatites: The ESR spectrum of irradiated dental enamel revisited

Decomposition of the laboratory gamma irradiation component of angular ESR spectra of fossil tooth enamel fragments

Spectrum decomposition of the angular measurements of fossil tooth enamel fragments using an automated simulated annealing (SA) procedure shows that the mix CO(2)(-) radicals generated by laboratory irradiation is significantly different to that of the natural sample. The naturally irradiated sample contains about 10% of non-oriented CO(2)(-) radicals and a mix of 35:65 orthorhombic to axial CO(2)(-) radicals. In contrast, laboratory irradiation generated about 40% of non-oriented radicals and a large amount of orthorhombic CO(2)(-) radicals, while we failed to detect any axial CO(2)(-) radicals. The results indicate that geological aging of the sample incurs various annealing and transfer processes; their precise nature is yet unknown. Nevertheless, the understanding of the formation and transfer processes that leads to the observed mix of CO(2)(-) radicals in fossil tooth enamel is essential for the reliable application of ESR dating.

Multi-frequency electron paramagnetic resonance study of irradiated human finger phalanxes

Electron paramagnetic resonance (EPR) is often used in dosimetry using biological samples such as teeth and bones. It is generally assumed that the radicals, formed after irradiation, are similar in both tissues as the mineral part of bone and tooth is carbonated hydroxyapatite. However, there is a lack of experimental evidence to support this assumption. The aim of the present study was to contribute to that field by studying powder and block samples of human finger phalanxes that were irradiated and analyzed by multi-frequency EPR. The results obtained from bones are different from the ones obtained in enamel by several respects: the ordering of the apatite crystallites is much smaller in bone, complicating the assignment of the observed CO2- radicals to a specific location, and one type of CO3(3-) radical was only found in enamel. Moreover, a major difference was found in the non-CO2- and non-CO3(3-) signals. The elucidation of the nature of these native signals (in bone and tooth enamel) still represents a big challenge.

Comparative X- and Q-band EPR study of radiation-induced radicals in tooth enamel

Human tooth enamel blocks and powders that were either unheated or heated prior to X irradiation at room temperature were investigated by means of Q-band electron paramagnetic resonance (EPR). It was found that the EPR spectra of unheated human tooth enamel consist mainly of two different anisotropic signals, as was suggested previously from an X-band study of analogous samples. In the present study, the two radical contributions could be differentiated convincingly by comparing the anisotropic Q-band spectra of heated and unheated enamel blocks. One type of is probably located in the bulk of the apatitic microcrystallites that constitute the enamel, and it appears in both heated and unheated samples. The other type is presumably located in an intercrystallite position and appears mainly in the unheated samples. Clear differences between g values in the Q-band spectra of heated and unheated enamel suggest that the radicals in the bulk exhibit larger g anisotropy than those in intercrystallite positions. Isotropic signals and contributions that may be from and radicals have also been detected. However, the present work focuses mainly on the signals and discusses potential and/or real difficulties that may be encountered in applications of EPR dosimetry using calcified tissues.

13C hyperfine interactions of CO2- in irradiated tooth enamel as studied by EPR

The EPR spectrum of tooth enamel caused by 13C hyperfine interactions of the CO2- radical were studied on gamma-irradiated powdered samples annealed for 40 min at different temperatures up to 250 degrees C. The lineshape and hyperfine splitting of the spectra were found to depend on the annealing temperature. Experimental spectra were compared with calculated ones assuming that EPR spectra are formed by two CO2- species--axial (rotating) and orthorhombic (braked) radicals. We assumed that the axial CO2- radicals are centers located in perfect areas of the hydroxyapatite crystals of tooth enamel whereas the orthorhombic CO2- radicals are rotating centers which are braked by defects. The thermal treatment of enamel samples leads to defective annealing and transformation of the orthorhombic centers into axial ones. This results in an increasing axial CO2- radical contribution to the EPR spectrum with increase of annealing temperature.

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.

ESR study of elephant tooth enamel from the Kärlich-Seeufer site in Germany

Enamel from 6 different positions in a well preserved elephant tooth from the Kärlich-Seeufer site in Germany has been irradiated up to 32 kGy. The X-band (v = 9.5 GHz) ESR spectra of two subsamples have been decomposed into three real components with Maximum Likelihood Common Factor Analysis (MLCFA). One of these components due to orthorhombic CO2- radicals is predominant. Dose response curves for the contributions of these MLCFA components and for different heights in the ESR spectra have been obtained and fitted with different models. Depending on the model, the equivalent dose for the preferably used height at g = 1.9973, due to CO2-, ranges from 70 to 130 Gy. Due to a very low uranium and thorium content in both enamel and dentine (< or = approximately 10 ppb) and to an important external y-attenuation, the ages fluctuate between 300 and 575 ka.

Paramagnetic resonance in tooth enamel created by ultraviolet light

Paramagnetic centres are present in human tooth enamel after irradiation with ultraviolet light. Their thermal stability and electron spin resonance spectrum coincide with those of the ion radicals created with gamma-rays are used in geological or archaeological dating and accident dosimetry. The long wavelength tail of their creation spectrum intersects with the short wavelength tail of the solar spectrum reaching Earth's surface, thus raising the question about the possible effect of solar irradiation on dating and dosimetry.

EPR dating CO2- sites in tooth enamel apatites by ENDOR and triple resonance

In this work we combine electron paramagnetic resonance (EPR), high-resolution electron nucleus double resonance (ENDOR) and general triple resonance (GTR) spectroscopies, to study the local environment of the CO2- groups created by ionizing radiation in fossil tooth enamel. We demonstrate that the CO2- groups occupy slightly modified phosphate sites in the hydroxyapatite lattice. In quaternary shark enamel we found these groups to be interacting with water molecules in the apatite channels. The absence of water molecules as first neighbors in mammalian samples indicate, however, that these molecules are not significantly responsible for the stabilization of CO2- dating centers in enamel.