Alpha and beta dose gradients in tooth enamel

New results describing gradients in effective alpha and beta doses within a layer of tooth enamel in planar geometry are presented. The alpha (track) dose to an enamel layer is calculated using an algorithm similar to that of Aitken (Aitken, M.J., 1987. Alpha dose to a thin layer. Ancient TL 5, 1–3.). The code for ROSY version 1.4 (Brennan, B.J., Rink, W.J., Rule, E.M., Schwarcz, H.P., Prestwich, W.V., 1999. The ROSY ESR dating software. Ancient TL, in press.) incorporates this algorithm. The approach allows for variation of the alpha track dose near the edges of the enamel, and we describe the gradient of the alpha track dose within 40 μm of each edge of the enamel for natural sources. In ESR or luminescence dating of naturally thin layers, for which stripping of the surface layer containing the alpha dose gradients may not be feasible, age estimates may change by as much as 5–10% when the detailed alpha dose calculation is included. Modern Monte Carlo-based results for the variation of beta dose for depths up to 2 mm are compared with ROSY results. For external irradiation by different sources, the attenuation of the ROSY dose estimate with depth is usually less rapid than that of the Monte Carlo-based estimate. The ROSY estimate of average beta dose to a layer in this case is between 5 and 18% higher than the Monte Carlo estimate.     

Monte Carlo determinations of the beta dose rate to tooth enamel

The results of Monte Carlo simulations of the beta dose rate to enamel are presented. The dose rates are the most comprehensive to date, incorporating the beta spectrum from 21 different radionuclides, all internal conversion and Auger electrons, the majority of the bremsstrahlung radiation, effects due to radon loss in the uranium series, and variations in the moisture content of the sediment. Applications to a new dating technique in archaeology using electron spin resonance and beta-gradient isochrons are discussed.     

Optically stimulated luminescence dating of Holocene alluvial fans,East Anatolian Fault System,Turkey

In this study, the optically stimulated luminescence dating technique was used to determine the time of deposition of alluvial sediment samples from the Türko?lu-Antakya segment of the East Anatolian Fault System (EAFS) in Turkey. The double-single aliquot regenerative dose protocol on fine grain samples was used to estimate equivalent doses (D_e). Annual dose rate was computed using elemental concentration of uranium (U) and thorium (Th) determined by using thick-source alpha counting and potassium (K) concentrations using X-ray fluorescence and/or atomic absorption spectroscopy. The environmental dose was measured in situ using α-Al₂O₃:C chips inside plastic tubes for a year. The two different bulk sediment samples collected from the Islahiye trench yielded ages of 4.54?±?0.28 and 2.91?±?0.23?ka. We also obtained a 2.60?±?0.18?ka age for the alluvial deposit in the K?ranyurdu trench and 2.31?±?0.14?ka age for an excavation area called Malzeme Oca??. These ages were consistent with the corresponding calibrated Carbon-14 (¹⁴C) ages of the region. The differences between the determined ages were insufficient to clearly distinguish the disturbance event from the effects of bioturbation, biological mixing, or other sources of D_e variation in the region. They provide a record of alluvial aggradation in the region and may determine undocumented historical earthquake events.     

Experimental determinations of beta attenuation in planar dose geometry and application to ESR dating of tooth enamel

Two experiments were performed to determine the distribution of beta dose in a target medium as a function of distance from a planar source (2π geometry). In Experiment 1 planar absorbers were used to simulate depth in the target medium, and the attenuated dose that penetrated the intervening absorbers was detected using thermoluminescence from CaSO₄:Tm. In experiment 2 pellets of powdered tooth enamel were situated in a cavity in solid tooth enamel. Dose response of the electron spin resonance (ESR) signals in the pellets allowed direct detection of dose distribution within the target medium. The dose distributions were found to agree more closely with Monte Carlo calculations and with one-group transport theory (Prestwich et al., 1997 and Brennan et al., 1997) than with approximations used widely in ESR dating applications (Grün, 1986). Although the latter were supported by experiments by Aitken et al. (1985), the results of experiment 1 provided data to suggest that departure from 2π geometry was unacceptably large in the Aitken et al. (1985) configuration. The beta doses obtained by one-group theory in typical target volumes of enamel were 17 to 40% lower than those obtained using the Grün approximation. It was concluded that one-group transport theory provides the best available routine tool for calculations of beta dose attenuation in near-planar target materials (e.g. tooth enamel and shell) irradiated by natural sources, although Monte Carlo methods are better and could eventually be introduced into the ROSY ESR dating program (Brennan et al., 1997). It was also concluded that a revision of previously published ESR ages using one-group transport for beta dosimetry will be needed. For cases where the U concentrations in enamel are generally low (less than a few ppm), previously published ESR ages on tooth enamel are systematically too young by 5–30%.     

Calculation of the parameters of the effects of uranium radiation on humans in emergencies at nuclear plants

Human radiation exposures from uranium via the skin or lungs during emergencies at nuclear plants are considered. The effective dose that can accumulate in humans and the dose coefficient of uranium are calculated. Models developed by the authors, which describe air pollution in a factory shop and uranium penetration through the skin are used. The effective doses accumulated in humans through the skin and via inhalation are compared. The effect of uranium on the skin is estimated and the skin layer most affected by radiation is determined.     

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.     

Monitoring environmental dose rate using Risø TL/OSL readers with built-in sources: recommendations for users

Measurements were carried out to quantify the dose rate delivered to samples due to leakage and cross-talk of the beta and alpha sources in various models of Risø TL/OSL readers; additionally the offset time was calculated. Highly sensitive -Al₂O₃:C and CaF₂:Mn dosemeters were used to investigate the impact on low environmental dose-rate assessment. The effect is shown to depend on the energy response of the detectors, the source strength and the reader design of the equipment. It was tested, if the scattered radiation of the closed source can be utilised to extend the dose range downwards to lower doses, as the dose rates of the built-in sources of Risø TL/OSL readers often exceed the practical values for ultra-low-dose measurements. The results and their consequences for environmental dose-rate assessment are discussed.     

A study of bremsstrahlung spectra and absorbed dose of beta in teeth enamel and dentin

The beta‐induced bremsstrahlung spectra produced by 113 pure beta nuclides in teeth enamel and dentin are computed. The computed spectral distributions are presented. The spectral shapes are primarily responsible for variations in the shapes of depth‐dose distributions. They are intended to provide a quick and convenient reference for spectral shapes and to give an indication of the wide variation in these shapes. The computed bremsstrahlung spectrum is used in the evaluation of bremsstrahlung dose in teeth enamel and dentin. The evaluated Beta bremsstrahlung dose as a function distance for the studied nuclides is also presented. The beta bremsstrahlung dose decreases with the increase of distance. Copyright © 2013 John Wiley & Sons, Ltd.     

Electron Spin Resonance (ESR) Dating of Calcareous Fault Gouge of the Ushikubi Fault, Central Japan

The ages of fault events of active faults have been estimated using electron spin resonance (ESR) signals of siliceous gouges. This technique of ESR method is limited by obtaining only ages that are greater than tens of millennia. So this study focuses on developing a new technique of using calcareous gouges to gain an insight into the ages of latest seismogenic event within the Holocene. For the first time, signal B of the ESR method has been used to estimate the age of the Ushikubi fault from calcareous gouge. This technique proved reliable because the mean age (1.9 ka) obtained agrees with previous works on indirect age determination of latest fault events by utilizing radiocarbon dating in the study area. However, the result from the ESR technique showed an increase relative to the age of 1 ka that was obtained by the radiocarbon dating method. This disparity may be due to a high dose rate value of 50 Gy/h of artificial irradiation that was used to determine the equivalent dose (ED). Moreover, isochronal experiment revealed that the gouge did not comprise pure carbonates but consisted of a mixture of calcite and quartz grains. A younger age value would have been obtained if a lower artificial irradiation dose rate and a relatively pure carbonate fault gouge were used in the ED determination.     

Synchrotron X-ray diffraction and scanning electron microscopy to understand enamel affected by metabolic disorder mucopolysaccharidosis

Mucopolysaccharidosis (MPS) is an inherited metabolic disorder that can affect the tooth structure leading to defects. Synchrotron X-ray diffraction being a state of the art technique has been used to determine the enamel crystallite orientation in deciduous enamel affected by Mucopolysaccharidosis Type I and Mucopolysaccharidosis Type IVA and comparing these with that of healthy deciduous enamel. Using this technique it was observed that there is a loss of texture in deciduous enamel affected by Mucopolysaccharidosis Type I and Mucopolysaccharidosis Type IVA when compared to the healthy deciduous enamel. Generally it was observed that the incisal surface of the deciduous teeth possessed a higher texture or preferred orientation of enamel crystallites and on progression towards the cervical region there was a decrease in the texture or preferred orientation of enamel crystallites. Scanning electron microscopy showed that the presence of a poorly calcified layer between the enamel and dentine at the enamel–dentine junction (EDJ) in MPS affected samples was likely to be responsible for rendering the tooth structure weak and prone to fracture as is often the case in MPS affected deciduous enamel.     

ISS protocol for EPR tooth dosimetry

The accuracy in Electron Paramagneetic Resonance (EPR) dose reconstruction with tooth enamel is affected by sample preparation, dosimetric signal amplitude evaluation and unknown dose estimate. Worldwide efforts in the field of EPR dose reconstruction with tooth enamel are focused on the optimization of the three mentioned steps in dose assessment. In the present work, the protocol implemented at ISS in the framework of the European Community Nuclear Fission Safety project “Dose Reconstruction” is presented. A combined mechanical–chemical procedure for ground enamel sample preparation is used. The signal intensity evaluation is carried out with powder spectra simulation program. Finally, the unknown dose is evaluated individually for each sample with the additive dose method. The unknown dose is obtained by subtracting a mean native dose from the back-extrapolated dose. As an example of the capability of the ISS protocol in unknown dose evaluation, the results obtained in the framework of the 2nd International Intercomparison on EPR tooth enamel dosimetry are reported.     

Dose response of hydrazine – Deproteinated tooth enamel under blue light stimulation

The beta dose response and Optically Stimulated Luminescence (OSL) signal stability characteristics of human tooth enamel deproteinated by hydrazine reagent under blue photon stimulation are reported. Removal of the protein organic component of tooth enamel resulted in a higher OSL sensitivity and slower fading of OSL signals. The effect of chemical sample preparation on the enamel sample sensitivity is discussed and further steps to make this deproteinization treatment suitable for in vitro dose reconstruction studies are suggested.     

The importance of U-series disequilibrium of sediments in luminescence dating: A case study at the Roc de Marsal Cave (Dordogne,France)

A multidisciplinary study of the Roc de Marsal Cave gave us the opportunity to re-examine the known problem posed by U-series disequilibria in sediments in the field of palaeodosimetric dating. Apart from burnt flints and heated limestones, the material being dated, THAT IS our principal concern here, is made of burnt sediment samples collected from a reddish layer that forms the very base of the Roc de Marsal Mousterian sequence (Couche B). U-series disequilibria of long life nuclides in sediments were observed: as a general trend, the activity of ²³⁸U is greater than that of ²²⁶Ra. Important discrepancies in age determination can occur according to the origin of the disequilibrium and its kinetics. To characterise these ancient modifications of radioactivity and constrain the range of possible variations of annual doses, a statistical study of ²³⁸U, ²²⁶Ra and ²³²Th activities, measured by gamma spectrometry, was carried out. It shows that the sediment was mainly enriched in mobile uranium. This conclusion is in accordance with the results of complementary measurements of ²³⁰Th and ²³⁴U performed by alpha spectrometry. On the kinetics side, two models of time-variation of U and daughter concentrations are compared, taking experimental data into account. A detailed presentation of the methodology and the method of calculation of dose rates, and particularly time-averaged dose rates in such a situation, is given.     

Systematic underestimation of the age of samples with saturating exponential behaviour and inhomogeneous dose distribution

In luminescence and ESR studies, a systematic underestimate of the (average) equivalent dose, and thus also the age, of a sample can occur when there is significant variation of the natural dose within the sample and some regions approach saturation. This is demonstrated explicitly for a material that exhibits a single-saturating-exponential growth of signal with dose. The result is valid for any geometry (e.g. a plain layer, spherical grain, etc.) and some illustrative cases are modelled, with the age bias exceeding 10% in extreme cases. If the dose distribution within the sample can be modelled accurately, it is possible to correct for the bias in the estimates of equivalent dose estimate and age. While quantifying the effect would be more difficult, similar systematic biases in dose and age estimates are likely in other situations more complex than the one modelled.     

Sorption kinetics of uranium-238 and neptunium-237 on a glacial sediment

Studies of uptake of radionuclides by natural materials have shown that sorption may occur via fast equilibrium exchange and/or slow kinetic processes, which can be described using a series of box models. This paper describes the use of such models to evaluate the solid-solution partitioning of ²³⁸U and ²³⁷Np on a clay rich sediment. Experimental data are obtained using the batch sorption technique, which are then analysed using 1, 2, and 3 box kinetic models. Uptake of ²³⁸U is initially rapid, with approximately 90% sorbed within the first 30 minutes. Sorption continues, but at a slower rate. Uptake of ²³⁷Np is initially slower than U, with approximately 30% sorbed within the first 30 minutes. Sorption again continues, at a slower rate. Analysis of the experimental data indicates that sorption can be described using 2 and 3 box kinetic models. The results demonstrate that sorption of uranium and neptunium on clay rich sediments occurs via equilibrium and kinetically controlled pathways, with neptunium being controlled by kinetics to a greater extent than uranium. The 2 or 3 box model can describe sorption of neptunium, uranium requires the 3 box model.     

Electron paramagnetic resonance radiation dosimetry in fingernails

There is now an increased need for accident dosimetry due to the increased risk of significant exposure to ionizing radiation from terrorism or accidents. In such scenarios, dose measurements should be made in individuals rapidly and with sufficient accuracy to enable effective triage. Electron paramagnetic resonance (EPR) is a physical method of high potential for meeting this need, providing direct measurements of the radiation-induced radicals, which are unambiguous signatures of exposure to ionizing radiation. For individual retrospective dosimetry, EPR in tooth enamel is a proven and effective technique when isolated teeth can be obtained. There are some promising developments that may make these measurements feasible without the need to remove the teeth, but their field applicability remains to be demonstrated. However, currently it is difficult under emergency conditions to obtain tooth enamel in sufficient amounts for accurate dose measurements. Since fingernails are much easier to sample, they can be used in potentially exposed populations to determine if they were exposed to life-threatening radiation doses. Unfortunately, only a few studies have been carried out on EPR radiation-induced signals in fingernails, and, while there are some promising aspects, the reported results were generally inconclusive. In this present paper, we report the results of a systematic investigation of the potential use of fingernails as retrospective radiation dosimeters.     

On the dose-rate estimate of carbonate-rich sediments for trapped charge dating

In a wide range of environmental conditions sediments are subject to changing water content and carbonate cementation during burial. Trapped charge dating of these carbonate-rich deposits requires the determination of a dose rate which is not constant during burial because sediments were subject to post-depositional geochemical alterations. The dose-rate model established in this study assumes linear increase of carbonate mass and linear decrease of water mass in pores between sediment particles during burial. Numerical modelling assesses the effect of carbonate and water on the infinite-matrix dose rate as a function of time. Sensitivity testing of the system indicated that water and carbonate content have the greatest effect on the resulting dose rate, followed by the timing of onset and completion of carbonate formation. As a consequence, a comprehensive re-calculation of the water correction factors was undertaken. It revealed a 5% lower value for the annual beta dose and a 10% lower value for the annual gamma dose compared to values formulated by Zimmerman [1971. Thermoluminescence dating using fine grains from pottery. Archaeometry 13, 29–52].

The dose-rate model was tested using samples from geologically well-constrained coastal sites. The differences between onset and final dose rate were up to 30% resulting in differences between modelled and conventional optical ages between 2% and 15% depending on the final (today's) water and carbonate content. The divergence of dates may be greater under certain conditions. The dose-rate model can be applied to a wide range of contexts similar to those considered in this case study.     

Comparative EPR study CO2− radicals in modern and fossil tooth enamel

Comparative EPR investigation of CO₂⁻ radicals in modern (γ-irradiated) and fossil samples of tooth enamel was performed. The samples studied were the enamel powders and plates, the latter demonstrating an orientation dependence of EPR spectra in an external magnetic field. It was found that the ratio between the axial and orthorhombic CO₂⁻ centers amounts appears to be different for modern and fossil enamels. This ratio can be estimated by modeling of EPR spectra lineshape of powders or, in the case of plates, from the orientation dependence of EPR spectra in an external magnetic field. It was assumed that the difference between modern and fossil enamels is caused by the transformation, in the course of time, of orthorhombic CO₂⁻ centers into axial ones. The equations that describe this process were deduced. Their solutions show that the ratio between the amounts of the axial and orthorhombic centers does not depend on the dose rate. This finding can be used for the development of the method to determine the fossil enamel age avoiding the determination of the annual dose.     

Spectroscopy of the gamma and X ray leakage radiation from the built-in sources of a Risø TL/OSL reader

In this work we evaluate the intensity and the spectral shape of the leakage radiation from the built-in beta and alpha sources of a Risø TL/OSL reader. LiF (TLD-100), fluorite-based pellets and Al₂O₃:C detectors were used in order to determine the dose rate delivered to the dosimeters when the sources are closed. The leakage spectra under both alpha and beta sources were registered with a CdTe semiconductor detector. The spectrum measured under the beta source shows the X-ray beam generated by the interaction of the beta particles with the lead used to shield the source. Besides, the 59.4 keV gamma ray from ²⁴¹Am was registered under the alpha source. Dose rates from 50 to 100 μGy/h were obtained for the dosimeter positions in the turntable under the beta and the alpha sources with the luminescent dosimeters.     

Method of H1(10) evaluation using a spherical-type energy-independent neutron dose monitor

A spherical-type neutron dose monitor has been developed for the use of area monitoring around the neutron producing fields. Considering the practical fields that neutrons coming from multiple directions, the study has been performed using MCNP simulation, in order to estimate the resultant direction of the sources and to find a suitable method of H¹(10) evaluation. This was done by irradiating the monitor using two neutron sources of different energies and intensities; placed at an angle between them. A parallel neutron beam of 50 cm radius with wide energy was used for the irradiation. The direction of the neutron sources was determined comparing the responses of 12 thermo-luminescent detectors (TLDs) of first layer between single and two directional sources. It was found that in most of the cases, the direction of the source was estimated to be the average angle or near the average angle of single sources. Using a linear combinational equation the method has been developed for evaluating the dose from the mean values of selected detectors of two consecutive depths of TLDs and the TLD of core depth. It was found that the ratio of the calculated to the expected dose was almost close to unity which indicates that the method can be used for evaluating the dose with good accuracy.