Freddy Callens

EPR dosimetry with tooth enamel: A review.

When tooth enamel is exposed to ionizing radiation, radicals are formed, which can be detected using electron paramagnetic resonance (EPR) techniques. EPR dosimetry using tooth enamel is based on the (presumed) correlation between the intensity or amplitude of some of the radiation-induced signals with the dose absorbed in the enamel. In the present paper a critical review is given of this widely applied dosimetric method. The first part of the paper is fairly fundamental and deals with the main properties of tooth enamel and some of its model systems (e.g., synthetic apatites). Considerable attention is also paid to the numerous radiation-induced and native EPR signals and the radicals responsible for them. The relevant methods for EPR detection, identification and spectrum analyzing are reviewed from a general point of view. Finally, the needs for solid-state modelling and studies of the linearity of the dose response are investigated. The second part is devoted to the practical implementation of EPR dosimetry using enamel. It concerns specific problems of preparation of samples, their irradiation and spectrum acquisition. It also describes how the dosimetric signal intensity and dose can be retrieved from the EPR spectra. Special attention is paid to the energy dependence of the EPR response and to sources of uncertainties. Results of and problems encountered in international intercomparisons and epidemiological studies are also dealt with. In the final section the future of EPR dosimetry with tooth enamel is analyzed.

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

The contribution of CO3(3-) and CO2- to the ESR spectrum near g = 2 of powdered human tooth enamel.

SummaryThe ESR spectrum near g=2 of powdered human tooth enamel from upper central incisors and lower canines was studied as a function of microwave power, irradiation, and storage time. The results clearly demonstrate that the ESR spectrum is composite with at least five paramagnetic species contributing to the signal. The main stable component is assigned to CO2−. Two other components arise from CO33− radicals, one of which is demonstrated to be the same center as is present on a phosphate site in sodium- and carbonate-containing calciumapatite.

AN EPR STUDY OF INTACT AND POWDERED HUMAN TOOTH ENAMEL DRIED AT 400C

An EPR study of human tooth enamel dried at 400°C is presented. Enamel blocks as well as powdered samples were investigated. The discussion deals mainly with three different spectral components, i.e., a CO33-and two different CO2-signals. Using the anisotropic enamel block spectra, a convincing differentiation between the latter two radicals was possible. The first CO2-signal shows no dependence on the orientation of the enamel blocks, disappears from the spectrum upon heating, and was assigned to a surface radical. The second CO2-component is mainly responsible for the angular variation of the enamel blocks and is assigned to a bulk position. For the CO33-ion, the (pseudo) angular variation of its isolated spectrum is presented and discussed. By means of the results presented in this study, earlier interpretation problems are considerably reduced.

EPR of carbonate derived radicals: Applications in dosimetry, dating and detection of irradiated food

After exposure of biological (tooth enamel, bone, …) and synthetic apatites to ionizing radiation, the so-called “asymmetric EPR signal nearg = 2” is formed. Although this signal is being used in EPR dosimetry, dating and detection of irradiated food for many years already, its composite character and the precise nature of the radicals contributing to the spectrum are still insufficiently known and/or recognized. For some fifteen years already, the EPR group in Ghent is gaining extensive experience on the radicals present in calcified tissues and model systems like synthetic apatites, calcites and single crystals doped with carbonate. It will be shown that the majority of radicals in calcified tissues are carbonate derived, e.g., CO2−, CO3−, CO33− while also phosphate derived radicals like PO42− and oxygen species (O−, O3−) have been identified with EPR and/or ENDOR. For the EPR applications mentioned above, the most important type of radicals is CO2t- (g values ranging from 2.0035 to 1.9970). A second type of radicals which is very intriguing but still badly known, exhibits a spectrum atg values around 2.0045. It is very apparent in tooth enamel below doses of 1 Gy, it has been observed in certain fossil teeth in a very prominent way and also in irradiated food containing bone (e.g., frog legs). It will be shown that the organic origin of this signal can be questioned. The importance of other radicals like CO33t- and CO3t- for EPR applications will also be discussed.

Effect of carbonate content on the ESR spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

SummaryThe ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using13C-enriched samples, four different carbonate-derived radicals and a surface O− ion could be identified. Isotropic CO3− and CO2− ions are present at a B site in the apatite lattice, and anisotropic CO3− and CO2− radicals are located at the surface of the crystallites. Only the isotropic ESR signals increase with increasing carbonate content. The anisotropic signal ascribed to a surface CO2− radical is mainly responsible for the so-called asymmetric ESR signal near g=2. It is argued that this surface signal may still be composite and caused by several very similar CO2− ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.

A decomposition study of the EPR spectrum of irradiated sucrose

In general, the EPR spectra of irradiated sugars are very complex because of their multicomponent character. In this study we applied a multivariate statistical method called MLCFA, maximum likelihood common factor analysis, and it predicted at least six components contributing to the total EPR spectrum of irradiated sucrose. Three dominant components have already been isolated in an irradiated sucrose single crystal using electron nuclear double resonance (ENDOR) and ENDOR induced EPR (EI-EPR). Results of EPR simulations based on the ENDOR data are in a reliable agreement with the experimental EPR spectra of irradiated sucrose single crystals.

EPR spectrum deconvolution and dose assessment of fossil tooth enamel using maximum likelihood common factor analysis

In order to determine the components which give rise to the EPR spectrum around g = 2 we have applied Maximum Likelihood Common Factor Analysis (MLCFA) on the EPR spectra of enamel sample 1126 which has previously been analysed by continuous wave and pulsed EPR as well as EPR microscopy. MLCFA yielded agreeing results on three sets of X-band spectra and the following components were identified: an orthorhombic component attributed to CO2-, an axial component (CO3(3-)), as well as four isotropic components, three of which could be attributed to SO2-, a tumbling CO2- and a central line of a dimethyl radical. The X-band results were confirmed by analysis of Q-band spectra where three additional isotropic lines were found, however, these three components could not be attributed to known radicals. The orthorhombic component was used to establish dose response curves for the assessment of the past radiation dose, D(E). The results appear to be more reliable than those based on conventional peak-to-peak EPR intensity measurements or simple Gaussian deconvolution methods.

Adsorption of carbonate-derived molecules on the surface of carbonate-containing apatites

Carbonated apatites synthesized at low temperatures were dried until constant weight at 25 °C. Two 99%13C-enriched samples were examined with EPR after X-irradiation. The observed spectra are interpreted in terms of five paramagnetic radicals, i.e. on O–, two CO–3 and two CO–2 radicals. As a result of the 13C hyperfine coupling, practically no signals were found in the region around g= 2, where EPR signals originating from 12C-containing radicals (commonly called 12C signals) are to be expected. Afterwards, the apatite powders were spread out in order to ensure a good contact of the apatite surface with the surrounding atmosphere. The samples were again X-irradiated and re-examined with EPR. Strong 12C signals were detected and attributed to surface radicals. Two different paramagnetic species were tentatively identified as CO–2 and CO– radicals adsorbed on the apatite surface. The g values for these radicals are g1= 2.0030, g2= 2.0015, g3= 1.9973 for the CO–2 radical and g1= 2.0058, g2= 2.0041, g3= 2.0023 for the assumed CO– radical.

31 P and 1H powder ENDOR and molecular orbital study of a CO33– ion in X-irradiated carbonate containing hydroxyapatites

An X-irradiated synthetic carbonate-containing apatite powder is examined with EPR and ENDOR. At low microwave powers, the room-temperature EPR spectrum contains a major contribution of a signal with g values: gx= 2.0045, gy= 2.0034 and gz= 2.0014. In a related 13C-enriched sample, the radical was shown to exhibit a hyperfine interaction with one carbon nucleus. The 13C hyperfine tensor values are: Ax= 263 MHz, Ay= 263 MHz and Az= 423MHz. The radical is assigned to a CO33– molecular ion. It is demonstrated by means of CNDO/II and INDO calculations that by lowering the symmetry of the CO33– ion from C3v to Cs, an orthorhombic g tensor can be obtained. However, the deviation from axial symmetry for the 13C hyperfine tensor is so small that it is not measurable on a powder specimen. The thus-calculated spin-Hamiltonian parameters are in very good qualitative and quantitative agreement with the experimental ones, adding strong evidence for the assignment of the observed signal to a CO33– radical.At low temperatures, both 31P and 1H ENDOR spectra are recorded for different settings of the magnetic field (i.e. when the magnetic field is swept through the EPR CO33– spectrum). By a careful analysis of the ENDOR powder spectra using computer simulations based on the ‘orientation-selection’ principle, a detailed model for the CO33– ion could be proposed. In this way, it is established unambiguously that the CO33– ion substitutes for a phosphate group in the hydroxyapatite lattice, with a vacancy on the nearest hydroxy-group site. In addition, some deductions can be made about the substitution mechanism according to which the precursor of the CO33– radical (i.e. a carbonate ion) is incorporated into the apatitic lattice.