Computational evaluation of a two-source neutron irradiator for low-flux neutron activation applications

Abstract

The viability of a neutron irradiator facility (NIF) with two legacy plutonium-239 beryllium (²³⁹PuBe) neutron sources that are housed polyethylene moderator blocks was evaluated for neutron activation analysis (NAA) applications using Particle and Heavy Ion Transport Code System (PHITS) simulation. Calculations show that the NIF design with 6 cm thick irradiator volume provides the optimal thermal neutron flux of $6.57\times {10}^3$ n/cm²s, which corresponds to 60 % of the total flux. The study also identified 20 optimal elements for low-flux NAA, of which, Eu-151 was found to have the highest sensitivity ($6.28\times {10}^2$ per unit flux and volume), while I-127 has the lowest ($1.09\times {10}^{-2}$ per unit neutron flux and volume). Finally, the induced activity of the selected nuclides after 24-h irradiation was calculated. Among the 20 optimal nuclides, Au-198 has the highest saturation activity of $2.32\times {10}^4$ Bq while the lowest saturation activity is obtained by Er-171 with $1.84\times {10}^2$ Bq. This study serves as a foundation for guiding the practical application of NAA using the NIF being developed by Philippine Nuclear Research Insitute (PNRI) and highlighting areas for further validation and refinement.