Measuring activity of 235, 238U, 232Th and 40K in geological materials using neutron activation analysis

Abstract

The radioactivity of the 235, 238U and 232Th isotope decay chains for geological samples can usually be assumed to be in equilibrium due to their age. 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, ICP-MS or passive gamma-ray spectrometry, all of which can be laborious and time consuming. In this research, we have used thermal and epithermal neutron activation analysis (NAA) of small sample sizes of various geological materials in order to ascertain these activities. By using NAA, we aim to obviate cumbersome sample preparation, the need for large samples and extended counting time. In addition to the decay chains of uranium and thorium, 40K was also determined using epithermal neutron activation analysis to determine total potassium content and then subtracting out its isotopic contribution.