Measuring apparent dose rate factors using beta and gamma rays, and alpha efficiency for precise thermoluminescence dating of calcite

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In addition to the conventional ¹⁴C and Th/U dating methods, thermoluminescence (TL) dating has been applied to calcite, but has been less popular partly because the luminescence responses for different types of radiation are unclear. To report more reliable TL ages for calcite, the fundamental characteristics of its response to radiation exposure were investigated and related to chemical composition. Relative TL factors for calcite after beta and gamma irradiation normalized with quartz, hereafter termed the beta and gamma factors, were measured as 0.19–0.34 and 0.16–0.33, respectively. These lower values than for quartz may be caused by differences in common substitution elements in calcite (²⁰Ca, ²⁵Mn, and ²⁶Fe) versus quartz (³Li, ¹¹Na, ¹³Al, and ¹⁴Si), and the interaction between mediums with different atomic numbers and radiation energies. The beta factor is higher than the gamma factor for some samples. These samples show relatively higher concentrations in lighter elements (up to Ba); thus, the concentration of minor elements may cause differing behavior between beta and gamma rays. The gamma factor may depend on Mn concentration; however, the elements most affecting the beta factor remain unknown. The accumulated dose from alpha rays is affected by sample thickness because of the spatial energy density around the center of the alpha track and luminescence detection range. Thus, for accurate alpha efficiency measurements, evaluation of the effective alpha ray range and luminescence detection thickness is important. The alpha efficiency against the gamma factor, known as the k–value, increases with Mn concentration. Previous studies have suggested that the alpha efficiency is lower than beta and gamma efficiency because the ionization density produced by alpha particles is so great that the thermoluminescence traps in the tracks’ central core become saturated. This leads to a much greater proportion of the ionized electrons being wasted compared with beta and gamma radiation. Thus, we concluded that luminescence traps increase with increasing Mn concentrations.