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

Abstract

Indirect neutron capture (n,γ) 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 gamma decay probabilities were measured between the neutron binding energy and around 1MeV above it using the 232 Th(d,p) 233 Th, 232 Th( 3 He,t) 232 Pa and 232 Th( 3 He,α) 231 Th reactions in experiments with the Cactus gamma-detector array and Silicon Ring charged particle detectors at the Oslo Cyclotron Laboratory. Since 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,γ) cross section measurements. In addition, a new technique for correcting measured gamma ray decay probabilities below the neutron emission energy threshold is proposed and used. We find good agreement between indirect and direct (n,γ) cross section measurements in the range 500 keV -1 MeV, but large discrepancies outside this range. Explanations for the observed differences are proposed. PACS number(s): 24.87.+y, 24.10.-i, 25.45.-z, 25.55.-e

Novel techniques for constraining neutron-capture rates relevant for r-process heavy-element nucleosynthesis

“…The procedure now is to calculate σ CN (E n ) within the Hauser-Feshbach theory [85] and measure P γ determined from the ratio between particle-γ coincident and single-particle spectrum. Figure 13 shows the 232 Th(n, γ) cross section [262] extracted by use of the surrogate 233 Th(d, p) reaction in the Weisskopf-Ewing limit. The compound nucleus formation cross section was obtained from the TALYS optical model [73,74] calculations with input parameters adjusted to accurately reproduce the experimental total cross sections.…”

Investigation of theU238(d,p)surrogate reaction via the simultaneous measurement ofγ-decay and fission pro

“…Consequently, only neutrons with lower kinetic energies can be captured in the NaI detectors. Taking into account the time resolution of the CACTUS NaI detectors of about 10 ns and the average interaction distance of the neutrons in the NaI crystals of 25 cm [6], the time window of 11 ns suppresses 95% of the emitted neutrons with E n ≤ 0,360 MeV and 68% with E n ≤ 1 MeV. Above a few hundred keV the neutron inelastic cross sections of Na and I are one or more orders of magnitude larger than the capture cross sections, but the gamma rays originating from inelastic scattering on Na and I are also removed by the 1.5 MeV gamma-ray energy threshold.…”

“…However, the radiative-capture cross section of 232 Th obtained via the 232 Th(d,p) surrogate reaction in ref. [6] shows very large discrepancies with respect to the neutroninduced radiative-capture cross section at low neutron energies.…”

Development of a measurement system for the determination of (n,γ) cross-sections using multi-nucleon transfer reactions

“…15(a) for 159 Gd] in a similar manner as in Ref. [33]. This extrapolation technique is based on the assumption that the shape of the γ spectrum below E x À S n does not change with E x as shown in Fig.…”

“…This technique was often used in direct measurements of (n, γ) cross-section (see Ref. [1] and references therein), and some surrogate measurements for determination of the (n; γ) cross-section [32,33]. Since this technique requires an accurate determination of the response functions and efficiency of the γ-ray detector over a wide range of γ-ray energies up to about 10 MeV (see e.g., Mizuno et al [34]), we studied these features of the antiCompton LaBr 3 (Ce) spectrometers, as will be described below.…”