Effect of spectrum processing procedure on the linearity of EPR dose reconstruction in tooth enamel

Electron Paramagnetic Resonance (EPR) spectroscopy with tooth enamel is a widely used method of dosimetry. The accuracy of EPR tooth dosimetry depends on the spectrum processing procedure, the quality of which, in its turn, relies on instrumental noise and the signals from impurities. This is especially important in low-dose evaluation. The current paper suggests a method to estimate the accuracy of a specific spectrum processing procedure. The method is based on reconstruction of the radiation-induced signal (RIS) from a simulated spectrum with known RIS intensity. The Monte Carlo method was used for the simulations. The model of impurity and noise signals represents a composite residual spectrum (CRS) obtained by subtraction of the reconstructed RIS and the native background signal (BGS) from enamel spectra measured in HMGU (Neuherberg, Germany) and IMP (Yekaterinburg, Russia). The simulated spectra were deconvoluted using a standard procedure. The method provides an opportunity to compare the simulated “true” RIS with reconstructed values. Two modifications of the EPR method were considered: namely, with and without the use of the reference Mn²⁺ signals. It was observed that the spectrum processing procedure induces a nonlinear dose response of the reconstructed EPR amplitude when the height of the true RIS is comparable with the amplitudes of noise-like random splashes of CRS. The area of nonlinearity is below the limit of detection (DL). The use of reference Mn²⁺ signals can reduce the range of nonlinearity. However, the impact of the intensities of CRS random signals on nonlinearity is two times higher than the one observed when the reference signals were not used. The reproducibility of the software response is also dependent on both the amplitude of the CRS and the use of a reference signal, and it is also two times more sensitive to the amplitude of the CRS. In most EPR studies, all of the data are used, even those for which the dose value is lower than the DL. This study shows that low doses evaluated with the help of linear dose–response can be significantly overestimated. It is recommended that linear dose response calibration curves be constructed using only data above the DL. Data below the DL should be interpreted cautiously.     

Feasibility of Q-Band EPR Dosimetry in Biopsy Samples of Dental Enamel, Dentine and Bone

Electron paramagnetic resonance (EPR) dosimetry of tooth enamel in X-band has been established as a suitable method for individual reconstruction of doses 0.1 Gy and higher. The objective was to demonstrate the feasibility of using Q-band EPR in small biopsy tooth enamel samples to provide accurate measurements of radiation doses. Q-band spectra of small (<10 mg) irradiated samples of dentine and bone were studied to investigate the possibility of using Q-band EPR for dose measurements in those materials if there are limited amounts of enamel available, and there is no time for the chemical sample preparation required for accurate X-band measurements in dental enamel. Our results have shown that Q-band provides accurate measurements of radiation doses higher than 0.5 Gy in tooth enamel biopsy samples as small as 2 mg. Q-band EPR spectra in powdered dentine and bone demonstrated significantly higher resolution and sensitivity than in conventional X-band measurements.     

Interlaboratory comparison of tooth enamel dosimetry on Semipalatinsk region: Part 1, general view

For intercomparison of methods of dose determination using electron paramagnetic resonance (EPR) spectroscopy of tooth enamel, the same sets of enamel samples were analyzed in different laboratories using similar recording parameters. The sets of samples included calibration samples irradiated in known doses, test samples irradiated to doses unknown to the participants and accidental dose samples prepared from teeth of humans affected by radioactive fallout from nuclear tests in the Semipalatinsk Nuclear Test Site in Kazakhstan. The test samples were analyzed to determine the differences in the resulting doses using different spectrometers and different spectra processing methods. The accidental dose samples were analyzed in order to test the precision of doses determined by EPR spectroscopy and to obtain more accurate values by averaging the results from different laboratories.     

Deconvolution of complex EPR spectrum of tooth enamel into three components: native,dosimetric and mechanical

A software was developed on the base of non-linear simulation, which allowed the deconvolution of EPR spectra of tooth enamel into three components: native, radiation- and mechanically induced. The software was designed for the reconstruction of individual absorbed doses by EPR spectra of tooth enamel using the method of additive irradiation of samples. It has been demonstrated with the help of this program that the presence of mechanically induced paramagnetic centers in enamel samples led to an excessive individual absorbed dose reconstructed by EPR spectra of tooth enamel.     

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.     

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.     

Radiation dose reconstruction and nuclear-explosion dating in the East-Kazakhstan region

The radioecological situation in the East-Kazakhstan region was studied by electron paramagnetic resonance (EPR), radiography and nuclear spectroscopy methods. The eastern part of this region borders the Semipalatinsk nuclear test site. Radiation doses for 33 residents of this region were measured by EPR dosimetry in tooth enamel. It was found that for 25% of the residents the measured radiation doses do not exceed the background level. The rest of the doses exceed the background level by a factor of 2–4 on the average. A new method of nuclear explosion dating was developed on the basis of the EPR measurements of the free radical concentration in annual tree rings. Their maximal concentration corresponds to the years when nuclear explosions were executed. The obtained results correlate well with the commonly accepted radiography method. Both methods show a maximum of radionuclide levels in the years of nuclear testing. The plutonium-239 content in residents hair samples was found to be equal to (0.8±0.2)·10^?9 g/kg and with activity of 1.9±0.4 Bq/kg. This is by a factor of 20 higher than the permitted content for the population.     

A method for identification of interfering signals in EPR dating of tooth enamel

In this paper, a method for identifying interfering EPR signals in fossil tooth enamel is presented. This method consists of optimizing EPR parameters to enhance spectral resolution of the fossil signals followed by a g-factor and intensity normalized subtraction of a high-dosed, modern tooth enamel sample spectrum. By scanning the modern tooth enamel sample with the same parameters as the fossil sample, the difference spectrum of the fossil and modern irradiated tooth enamel samples can resolve numerous hyperfine and other interfering signals. This method was successfully applied to a fossil sample studied elsewhere (Grün, R., 1998a. Ancient TL 16, 51–55; Grün, R., Clapp, R., 1996. Ancient TL 14, 1–5; Martin Jonas, 1997. Ph.D. Thesis, Cambridge University; Jonas, M., Grün, R., 1997. Radiat. Meas. 27, 49–58; Vanhaelewyn et al., 2000. Appl. Radiat. Isot. 52, 1317–1326). This sample has shown discrepancies in dose estimations obtained from the power absorption curve versus those obtained from the first derivative spectra (Grün, R., 1998b. Radiat. Meas. 29, 177–193). The reason for this, and other discrepancies, are accounted for by the signals resolved using the method presented here.     

Spectral analysis of paramagnetic centers induced in human tooth enamel by x-rays and gamma radiation

Based on study of spectral and relaxation characteristics, we have established that paramagnetic centers induced in tooth enamel by x-rays and gamma radiation are identical in nature. We show that for the same exposure dose, the intensity of the electron paramagnetic resonance (EPR) signal induced by x-radiation with effective energy 34 keV is about an order of magnitude higher than the amplitude of the signal induced by gamma radiation. We have identified a three-fold attenuation of the EPR signal along the path of the x-radiation from the buccal to the lingual side of a tooth, which is evidence that the individual had undergone diagnostic x-ray examination of the dentition or skull. We have shown that the x-ray exposure doses reconstructed from the EPR spectra are an order of magnitude higher than the applied doses, while the dose loads due to gamma radiation are equal to the applied doses. The data obtained indicate that for adequate reconstruction of individual absorbed doses from EPR spectra of tooth enamel in the population subjected to the combined effect of x-radiation and accidental external gamma radiation as a result of the disaster at the Chernobyl nuclear power plant, we need to take into account the contribution to the dose load from diagnostic x-rays in examination of the teeth, jaw, or skull.     

EPR spectroscopy of radiation-induced centers in tooth enamel and its application for retrospective dosimetry in chernobyl case

A specific EPR tooth enamel dosimetry is discussed which appears to have genuine utility in retrospective dosimetry. The sample preparation technology, digital form of the native background and radiation-induced signals, and their separation from the experimentally observed spectrum are discussed. The possibility of dose estimation from the single initial EPR spectrum of randomly irradiated teeth and the uncertainty of such evaluation are discussed. The method has been used for dose reconstruction of a group of people irradiated due to the Chernobyl accident, and some results are presented.     

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.     

Radiation Dosimetry Using Alanine and Electron Paramagnetic Resonance (EPR) Spectroscopy: A New Look at an Old Topic

The detection and quantification by electron paramagnetic resonance (EPR) spectroscopy of stable radicals formed in alanine by exposure to γ-radiation is used as a secondary standard for radiation dosimetry measurements, even though the EPR signal is actually derived from >1 radical with different spectral properties. For high radiation doses, microwave power saturation and spectral linewidths are both dependent on the received dose, and result in non-linear calibration curves. Furthermore, using a high-sensitivity microwave cavity, the power at which EPR signal saturation commences is ~0.3–0.4 mW for samples with irradiation doses ≤10 kGy; these values are an order of magnitude lower than those normally used in alanine dosimetry. In addition, the central peak of the first derivative spectrum, the height of which is commonly used in dosimetry measurements, is the most susceptible to microwave power saturation. Therefore, for high-level dosimetry we now recommend that analyses be performed under non-saturating conditions, and that the spectral acquisition parameters should be determined with a standard irradiated to ≤10 kGy to eliminate any intensity problems associated with variable saturation characteristics. At low radiation doses, variations in spectral saturation characteristics are negligible, and partially saturating conditions along with modulation amplitudes much higher than those normally used can reliably produce improved signal-to-noise ratios and allow extension of the methodology to practical working limits of ~0.1–0.2 Gy.     

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.     

Verification of radiotherapy doses by EPR dosimetry in patients' teeth

The aim of this work was to verify applicability of electron paramagnetic resonance (EPR) ex vivo dosimetry in teeth enamel for determination of doses absorbed by patients during radiotherapy with radiation fields covering head regions and to examine with what accuracy the doses predicted by radiotherapy treatment plan (RTP) can be confirmed by doses measured ex post by the EPR method. The doses were determined in 22 enamel samples obtained from 11 patients who, after their radiotherapy treatment underwent extraction of teeth due to medical reasons. The delivered doses were determined by measuring EPR signals in enamel samples from the extracted teeth; magnitude of these signals is proportional to concentration of stable free radicals induced by radiation in the hydroxyapatite content of enamel. The measured doses were compared with doses planned in the teeth locations by RTP systems. The relation between the measured (D_m) and the planned (D_p) doses can be described as a linear function: D_m = s·D_p + b, with the slope s = 0.93 ± 0.03 and the intercept b = 0.67 ± 1.26. The deviations between the measured and calculated doses were in the (−12.6%, +1.9%) range with the average deviation of – 4.6%. It is concluded, than more accurate measurements, achievable when using a higher calibration dose than in the present study, are necessary to confirm or to deny the observed bias between the measured and planned doses.     

Analysis of EPR tooth enamel spectra exposed to combined radiation and mechanical effects

In the EPR spectra of tooth enamel samples exposed to sequential radiation and mechanical effect, the intensity of the signal in the spectra of tooth enamel samples exposed to sequential mechanical and radiation effects exceeded the amplitude of a signal in enamel samples that were only exposed to radiation. The increased dosimetric signal can be explained by superposition of mechanically and radiation-induced signals. The contribution of the mechanically induced component to the individual dose load reconstructed by EPR-spectra of tooth enamel has been evaluated.     

EPR dosimeter material properties of potassium tartrate hemihydrate

The ability of potassium tartrate hemihydrate as a radiation sensitive material for electron paramagnetic resonance (EPR) dosimetry was investigated. The samples were subjected to different doses, in the range of 1–9 Gy of ⁶⁰Co gamma rays at room temperature. The EPR spectra were investigated through variation of signal intensity with respect to absorbed dose, magnetic field modulation amplitude, microwave power and time stability. The results indicate that the sensitivity of potassium tartrate hemihydrate is about 30% higher than that of alanine. However, the EPR signal is timely less stable within the first two weeks after irradiation.     

EPR dosimetry in chemically treated fingernails

By using EPR measurements of radiation-induced radicals it is possible to utilize human fingernails to estimate radiation dose after-the-fact. One of the potentially limiting factors in this approach is the presence of artifacts due to mechanically induced EPR signals (MIS) caused by mechanical stress during the collection and preparation of the samples and the so-called background (non-radiation) signal (BKS). The MIS and BKS have spectral parameters (shape, g-factor and linewidth) that overlap with the radiation-induced signal (RIS) and therefore, if not taken into account properly, could result in a considerable overestimation of the dose. We have investigated the use of different treatments of fingernails with chemical reagents to reduce the MIS and BKS. The most promising chemical treatment (20 min with 0.1 M dithiothreitol aqueous solution) reduced the contribution of MIS and BKS to the total intensity of EPR signal of irradiated fingernails by a factor of 10. This makes it potentially feasible to measure doses as low as 1 Gy almost immediately after irradiation. However, the chemical treatment reduces the intensity of the RIS and modifies dose dependence. This can be compensated by use of an appropriate calibration curve for assessment of dose. On the basis of obtained results it appears feasible to develop a field-deployable protocol that could use EPR measurements of samples of fingernails to assist in the triage of individuals with potential exposure to clinically significant doses of radiation.     

Potential use of wallboard (drywall) for EPR retrospective dosimetry

Concern regarding the possibility of criminal or terrorist use of nuclear materials has led to an interest in developing the capability to measure radiation dose in a variety of natural and manufactured materials. Electron paramagnetic resonance (EPR) measurements of radiation dose following a radiological incident may aid in screening affected populations (triage) and in reconstruction of doses following accidents. One such EPR dosimeter is wallboard (drywall), a common construction material composed largely of gypsum (calcium sulphate dihydrate). We have identified the CO₃⁻ and SO₃⁻ dose-sensitive lines in drywall and developed a measurement protocol using the intensity of CO₃⁻ line. Proper background subtraction is a major difficulty, and we demonstrate a procedure based on alignment of a contaminant Mn²⁺ line. As a proof-of-concept, a wallboard panel was irradiated with a ⁶⁰Co source, and a two-dimensional map of the absorbed dose was measured. While most aliquots yielded reasonably accurate doses, a spatially contiguous region of apparent dose-insensitivity in one panel was identified.     

Alterations in enamel remineralization in vitro induced by blue light

Blue light, especially from LED devices, is a very frequently used tool in dental procedures. However, the investigations of its effects on dental enamel are focused primarily on enamel demineralization and fluoride retention. Despite the fact that this spectral region can inhibit enamel demineralization, the effects of the irradiation on demineralized enamel are not known. For this reason, we evaluated the effects of blue LED on remineralization of dental enamel. Artificial lesions were formed in bovine dental enamel blocks by immersing the samples in undersaturated acetate buffer. The lesions were irradiated with blue LED (455 nm, 1.38 W/cm², 13.75 J/cm², and 10 s) and remineralization was induced by pH-cycling process. Cross-sectional hardness was used to asses mineral changes after remineralization. Non-irradiated enamel lesions presented higher mineral content than irradiated ones. Furthermore, the mineral content of irradiated group was not significantly different from the lesion samples that were not submitted to the remineralization process. Results obtained in the present study show that the blue light is not innocuous for the dental enamel and inhibition of its remineralization can occur.     

Post-annealing effects on EPR response of irradiated nano-structure hydroxyapatite

In this work, the nano-structure hydroxyapatite was synthesized via the hydrolysis method. The produced powders were thermally treatment at different temperatures from 400 to 1200°C. The morphological and chemical analyses were carried out using the Fourier transmission infrared spectroscopy, transmission electron microscopy, and X-ray diffraction system. Then, the samples were irradiated at different absorbed doses from 1 to 80 kGy using ⁶⁰Co γ -ray. Electron paramagnetic resonance (EPR) responses of the samples were measured at room temperature in air. Subsequently, the variations of EPR signal intensities were constructed as the peak-to-peak signal amplitude and results were compared with those of non-annealed samples. The results show that the EPR responses of non-annealed samples are higher rather than other samples and also are saturated at higher doses in comparison with the others.