Indirect (n,γ) cross sections of thorium cycle nuclei using the surrogate method

Abstract

Indirect neutron capture ($n$,$\ensuremath{\gamma}$) cross sections have been extracted for the key thorium cycle nuclei ${}^{232}$Th, ${}^{231}$Pa, and ${}^{230}$Th using the surrogate reaction method. Final nucleus $\ensuremath{\gamma}$-decay probabilities were measured between the neutron binding energy and around 1 MeV above it using the ${}^{232}$Th(d,p)${}^{233}$Th, ${}^{232}$Th(${}^{3}$He,t)${}^{232}$Pa, and ${}^{232}$Th(${}^{3}$He,$\ensuremath{\alpha}$)${}^{231}$Th reactions in experiments with the CACTUS $\ensuremath{\gamma}$-detector array and Silicon Ring charged-particle detectors at the Oslo Cyclotron Laboratory. Because the neutron capture cross section for ${}^{232}$Th is already well known from direct measurements a comparison with these results provides a stringent test of the applicability of the surrogate method in the actinide region for indirect ($n$,$\ensuremath{\gamma}$) cross-section measurements. In addition, a new technique for correcting measured $\ensuremath{\gamma}$-ray decay probabilities below the neutron emission energy threshold is proposed and used. We find good agreement between indirect and direct ($n$,$\ensuremath{\gamma}$) cross-section measurements in the range 500 keV--1 MeV, but large discrepancies outside this range. Explanations for the observed differences are proposed.