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

Tungsten-188 (t1/2 = 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 ±â€¯1.8 b for thermal and I0 = 476 ±â€¯25 b for resonance integrals with σ0/I0 = 12.6 ±â€¯0.4) in two short irradiations, longer irradiations (1-10 d) were performed to obtain a value of 6.5 ±â€¯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 (t1/2 = 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.