Electronic Paramagnetic Resonance method for personal dosimetry and dating of archeological and paleontological samples

Author: Maia Topeshashvili
Co-authors: Eduard Chikvaidze
Keywords: free radicals, EPR dosimetry, in-vivo dosimetry, in-vitro dosimetry, nail, tooth enamel, Hydroxyapatite, ionization radiation
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With possibilities for radiation terrorism and intensified concerns about nuclear accidents since the recent Fukushima Daiichi event, the potential exposure of large numbers of individuals to radiation that could lead to acute clinical effects has become a major concern. For the medical community to cope with such an event and avoid overwhelming the medical care system, it is essential to identify not only individuals who have received clinically significant exposures and need medical intervention but also those who do not need treatment. The ability of electron paramagnetic resonance to measure radiation-induced paramagnetic species, which persist in certain tissues (e.g., teeth, fingernails, toenails, bone, and hair), has led this technique to become a prominent method for screening significantly exposed individuals. Although the technical requirements needed to develop this method for effective application in a radiation event are daunting, remarkable progress has been made. We will discuss three complimentary EPR dosimetry approaches—in vivo tooth dosimetry, in vivo nail dosimetry, and in vitro (ex vivo) nail dosimetry. These approaches vary somewhat in their utilities for dosimetry but are based on the same general phenomena: (1) ionizing radiation generates unpaired electrons (usually free radicals) in proportion to the absorbed dose; (2) EPR can selectively and sensitively detect and quantify the number of unpaired electrons; (3) in some tissues (e.g., teeth, nails, and bone) these free radicals are sufficiently stable to be measured by EPR long after irradiation (from weeks to years). While some recommended methods (e.g., dicentric chromosome assay and lymphocyte depletion rate) have demonstrated their usefulness for the clinical management of exposed individuals in small events, these methods cannot be effectively employed when many thousands of people must be evaluated and medical decisions made quickly. New methods are needed to estimate the dose for large events, like biodosimetry which will likely play an important role as it does not rely on a person being in close proximity to a conventional dosimeter at the time of the event. Instead, it uses each individual’s own tissues and their response to ionizing radiation, thereby assuring that the material for dosimetry will always be there to absorb the dose and be measured. At the same time the probes, that are close to, or part of, the exposed individual are of special interest to retrospective dosimetry. Hydroxyapatite (Ca10 (PO4)6(OH)2) contained in bones and teeth is a suitable probe for dose reconstruction because it contains stable radiation-induced radicals that are a diagnostic signature of radiation exposure. Electron paramagnetic resonance (also known as electron spin resonance, ESR) here also is an extremely sensitive method for the measurement of free radicals. Tooth enamel as a detector for in vivo dosimetry has been known for more than three decades. The usefulness of enamel for dosimetry results from its high content of hydroxyapatite . Carbonate impurities, which are incorporated into or attached to the surface of hydroxyapatite crystals during formation, are converted to CO2 - radicals through absorption of ionising radiation. The concentration of radicals increases with absorbed dose. The intensity of the resultant EPR absorption is a measure for the absorbed dose. The examples include the dose evaluation of: survivors of the atomic bomb explosions in Hiroshima and Nagasaki, nuclear workers in the South Urals, residents of the Techa river basin, the populations of contaminated areas in the Urals, the population living near the Chernobyl nuclear reactor and workers in the Chernobyl Sarcophagus. Finally, EPR dosimetry was applied to a population from an uncontaminated area in Russia demonstrating the potential to estimate the absorbed dose from natural background radiation. Tooth enamel is preferred in retrospective dosimetry because this tissue is completely formed in childhood and once formed, is never remodelled. Therefore, the accumulated concentration of radiation-induced radicals in the exposed enamel is preserved. At 25°C, a lifetime of 107 years was determined for the CO2 radicals in fossil tooth enamel. Hence, EPR dosimetry with tooth enamel is suitable for dose reconstruction after long periods of exposure and for many years after the exposure. However, the success to date should not lead to over-optimism; there remain several aspects of the method that require further investigation. Within the framework of joint EU/CIS projects, international comparisons on EPR tooth dosimetry were carried out. These comparisons were designed to check the consistency and reliability of EPR dose reconstruction among different laboratories and led to improvements of the capability of EPR dosimetry, where measurement of low doses in the range of 100 mGy was demonstrated. Today, EPR dosimetry with tooth enamel is a leading method for retrospective dosimetry and for dosimetry of individual radiation exposures.