Measuring the radioactivity of ²³⁵U, ²³⁸U, ²³²Th, and ⁴⁰K in geological matrices using instrumental neutron activation analysis

The radioactivity contributions of several daughter products of the 235,238 U and 232Th isotope decay chains within geological matrices can be assumed to be in equilibrium due to their age if they have remained undisturbed for an extended period of time. Similarly, one can assume that the isotopic mass proportions are equal to natural isotopic abundance. Current methods used to ascertain activity in these decay chains involve alpha particle spectrometry or ICP-MS, both of which can be laborious and time consuming. In this research, thermal and epithermal neutron activation analysis of small samples of various geological materials is used in order to ascertain activities. Through the use of NAA, cumbersome sample preparation and the need for large sample size and extended counting time are obviated. In addition to the decay chains of uranium and thorium, 40K, another large contributor to naturally occurring radioactivity, is determined using epithermal neutron activation analysis to determine total potassium content, and then calculating its isotopic contribution based on its natural isotopic abundance.