Measurement of neutron capture cross section of 187W for production of 188W

Abstract

Tungsten-188 (t_1/2 = 69.4 d) is routinely produced by double neutron capture using highly enriched ¹⁸⁶W target, ¹⁸⁶W(n,γ)¹⁸⁷W(n,γ)¹⁸⁸W reaction, at the ORNL 85 MWt High Flux Isotope Reactor. While the thermal neutron cross section for the first reaction, ¹⁸⁶W(n,γ)¹⁸⁷W, is well known, the single reported 64 b cross-section for the second reaction, ¹⁸⁷W(n,γ)¹⁸⁸W, cannot be validated by experimental results that yield lower than expected activities of ¹⁸⁸W. In this study, we report a new value for the thermal neutron capture cross section of ¹⁸⁷W. After confirming the neutron capture cross section of ¹⁸⁶W (σ⁰ = 37.8 ± 1.8 b for thermal and I⁰ = 476 ± 25 b for resonance integrals with σ⁰/I⁰ = 12.6 ± 0.4) in two short irradiations, longer irradiations (1-10 d) were performed to obtain a value of 6.5 ± 0.8 b for the σ⁰ of ¹⁸⁷W, which is lower than the adopted value by a factor of 10. Due to the short half-life of ¹⁸⁷W (t_1/2 = 23.7 d), the σ⁰ for ¹⁸⁷W was obtained empirically by comparing the ¹⁸⁸W experimental yields with the theoretical yields generated by code IsoChain and varying the ¹⁸⁷W cross section while keeping all other parameters constant.

Keywords: Empirical measurement; High flux isotope reactor (HFIR); IsoChain; Neutron capture cross-section; Neutron induced nuclear reaction; Tungsten-187; Tungsten-188.