Monte Carlo Simulations for Non-destructive Elemental Analysis of Large Samples by Neutron Activation Analysis

Neutron Activation Analysis (NAA) is an established nuclear analytical technique with applications in a broad range of scientific and technological fields. Typically, conventional NAA involves analysis of small material portions of 10 to 100 g in mass. However, analysis of samples of a larger mass implies a number of additional advantages such as (a) analysis of objects too precious to remove small parts from and (b) minimization of representative sampling problems of heterogeneous materials. In this work, a method to perform in depth, nondestructive, multi-element NAA of samples of large volume (up to 1 L) is presented. The Large Sample Neutron Activation Analysis (LSNAA) technique involves sample irradiation in the reactor’s thermal neutron column and subsequent measurement of the induced radioactivity in the sample employing a HPGe based spectrometry system. Correction algorithms, to compensate for the effects of (a) thermal neutron self-shielding within the sample, during sample irradiation and (b) source geometry and gamma ray attenuation by the sample material during gamma ray counting, have been developed. The correction methods were based on Monte Carlo simulations of both the irradiation and counting facilities using the MCNP computer code. Calculations of thermal neutron self-shielding and gamma-ray detector efficiency for large samples representing industrial and archaeological materials are presented. LSNAA compliments and significantly extends the analytical tools available for in-depth, non-destructive, multi-element analysis of materials too precious to damage for sampling purposes (whole object analysis), representative sampling of heterogeneous materials, or analysis of samples of arbitrary shape. Potential applications of the technique are environmental protection, industrial waste, advanced technological materials, as well as cultural heritage and authentication studies.