UC Berkeley

Neutron induced reactions on Chlorine, especially $^{35}$Cl, have wide relevance across the nuclear science spectrum. The $^{35}$Cl($n,p_0$)$^{35}$S channel is particularly vital to the design of Molten Chloride Fast Reactors (MCFR) as it impacts core reactivity through neutron loss in the Chlorine based carrier salt. However, until 2019, very little experimental data existed for the relevant energy range. Secondary $\gamma$-ray production data for the $^{35}$Cl($n,p$)$^{35}$S reaction are also needed for national security applications, including active neutron interrogation, yet there is currently no useful experimental data available. Gamma-ray data is also used in space exploration for detecting the isotopic makeup of extraterrestrial environments as well as in oil-well logging. To address these nuclear data needs, measurements of the $^{35}$Cl(n,x) reaction cross sections were conducted at Lawrence Berkeley National Laboratory’s (LBNL) 88-inch cyclotron.

The nuclear data evaluation process that produces the cross sections used for applications assumes a fixed total (n,x) cross section. The result is that an increase in one channel causes a corresponding decrease in one or more other evaluated channel(s). To address this aspect of nuclear data evaluation we performed a multi-component experiment whose goal was to measure all energetically possible reaction channels instead of using the more common single channel approach. The experiment consisted of three independent parts. First, the energy differential $^{35}$Cl($n,p_0$)$^{35}$S cross section was obtained using a CLYC detector as an active target. Second, energy-differential $\gamma$-ray production data for $^{35}$Cl($n,p\gamma$)$^{35}$S and $^{35}$Cl($n,n'\gamma$)$^{35}$Cl were obtained using the GENESIS array. This data was compared against (elsewhere produced) CoH$_3$ theoretical calculations to inform model predictions as to the relative strength of available reaction channels. Finally, energy-integral $^{35}$Cl(n,p) and $^{35}$Cl(n,${\alpha}$) cross sections from activation were compared against CoH$_3$. These allow verification of the inelastic channel strength obtained from the $\gamma$-ray data as well as determining the total $^{35}$Cl(n,${\alpha}$) channel strength which was not available from the other methods.

Our results identify a $\sim$50$\%$ reduction in the magnitude of the $^{35}$Cl($n,p_0$)$^{35}$S cross section compared to the ENDF/B-VIII.0 evaluation. This result is consistent with a 2020 measurement by Kuvin \textit{et. al.} Comparison of the $\gamma$-ray production data to CoH$_3$ model calculations suggest that the elastic and/or inelastic channels must increase to compensate for the reduction observed in the ($n,p_0$) channel.