Self Attenuation of Gamma Rays in Titanite, Zircon and Apatite

Several of the gamma-emitting U-Th series, cosmogenic and anthropogenic radionuclides (²¹⁰Pb, ²³⁴Th, ²²⁶Ra, ²²⁸Ra, ⁷Be, ¹³⁷Cs, etc) have been widely utilized as tracers and chronometers in environmental studies. Precise measurements of these nuclides using gamma-ray spectrometry in environmental matrices require that the proper correction factors for self- and external-absorption be applied. In this study, we examine factors associated with absorption and self attenuation of gamma-rays of ²¹⁰Pb (46.5 keV), ²³⁴Th (63 keV), ²²⁶Ra (via ²¹⁴Pb and ²¹⁴Bi, 351.9 and 609 keV) and ²²⁸Ra (via ²²⁸Ac, 338.3 and 911.2 keV) using a well-type germanium gamma-ray detector. Samples of three naturally occurring minerals (titanite, apatite and zircon) were separated into 5 size fractions (<63 μm, 63-125 μm, 125-250 μm, 250-500 μm, and >500 μm) and analyzed for ²¹⁰Pb, ²³⁴Th, ²²⁶Ra, and ²²⁸Ra. We also analyzed two synthetic silica standards (RGU-1, RGTH-1) that have a relatively uniform grain size of 63 μm. These minerals were chosen based on their varying chemical compositions and densities. Chosen samples are of an age that isotopes of ²³⁸U and ²³²Th are expected to be in secular equilibrium with their daughter products. However, the measured activity ratios between members of the family vary widely. In the case of titanite, the ²¹⁰Pb/²²⁶Ra ratios in 5 size fractions varied between 0.44×0.03 and 0.53×0.03, while in apatite it varied between 0.54×0.03 and 0.67×0.04, without applying any self- and external-absorption correction factors. Using the attenuation coefficients of constituent elements at different energies, we estimate the attenuation coefficient for each of these 4 minerals and determine the self- and external-absorption correction factors. The self- and external-absorption corrected activities agree with the expected activities in these minerals. Our data suggests that variations in the activity levels are dependent on chemical composition, density, and grain-size fraction. The ratios of activities in the < 63 μm size-fraction to that in the > 500 μm size-fraction for gamma energies between 46 and 609 keV varied between 1.67 and 1.75 for titanite, while this ratio varied between 5.23 and 6.99 for apatite. These results demonstrate that self- and external-absorption for apatite is much stronger than for titanite. We will present our method of quantification of the self- and external-absorption corrections for a suite of gamma-ray energies for titanite, apatite, and zircon.