Investigation of self-attenuation of Pb (46 keV) gamma ray in sediment, certified reference material and high-density minerals: Implication to precise measurement of Pb.

High resolution gamma spectrometry is one of the most widely used techniques in the measurements of environmental level Pb in sediment from coastal and freshwater environments and such measurements are needed to establish Pb chronology for the past 100-150 years. Precise measurement of Pb in sediment and soil requires appropriate self-absorption correction for its low-energy (46.5 keV) gamma radiation due to differences in the matrix between the sample and standard used to calibrate the instrument. Here we report a method that involves precise determination of Pb activity in sediment sample by calibrating the HPGe well detector with RGU-1-IAEA Certified Reference Material for well-defined geometries. A comparison of the Pb activity obtained from gamma-ray spectrometry with that obtained from alpha spectrometry via Po, using Po as yield tracer, indicates good agreement. We propose an empirical relation between the absolute efficiencies and packing densities of sample in a well-defined geometry (cylindrical counting vial) by affecting the count rate of Pb and its progenitor, Ra (via Pb and Bi). The effects of self-attenuation of 46.5 keV (Pb), for naturally-occurring high-density minerals (apatite, titanite, monazite, and cerite) are evaluated. Specific activity of Pb on apatite measured by alpha and gamma spectrometry are compared. This study is relevant and useful for precise measurements of gamma-emitting environmental radionuclides such as Pb, Be, Cs as well as Ra.