Sources of uncertainties in prompt gamma activation analysis

Two prompt gamma-ray activation analysis (PGAA) facilities at the NIST Center for Neutron Research have been used routinely to perform elemental analyses of a variety of materials. Results from these analyses are usually expressed as mass fraction values with expanded uncertainties. The expanded uncertainty consists of the combined uncertainty multiplied by the appropriate coverage factor (k) required to achieve a 95% confidence interval. The combined uncertainty includes the uncertainties associated with preparation, irradiation, and gamma-ray spectrometry of samples and standards, and corrections for gamma-rays from the background or blanks where necessary. To determine the combined uncertainty, each component of uncertainty associated with each variable and constant in the basic measurement equation is evaluated. In this paper we present the PGAA measurement equation, a description of the potential sources of uncertainty for each component of the equation, and three examples of uncertainty evaluation. The examples are for determination of H in standard reference material (SRM) 2454, hydrogen in titanium alloy using the cold neutron PGAA facility, Cd in SRM 2702 Inorganics in Marine Sediment using the original thermal neutron PGAA facility, and N in SRM 3244 Ephedra-Containing Protein Powder using the recently designed thermal neutron PGAA facility.