Novel Method to Study Neutron Capture ofU235andU

Abstract

The neutron capture cross sections of the main uranium isotopes, (235)U and (238)U, were measured simultaneously for keV energies, for the first time by combining activation technique and atom counting of the reaction products using accelerator mass spectrometry. New data, with a precision of 3%-5%, were obtained from mg-sized natural uranium samples for neutron energies with an equivalent Maxwell-Boltzmann distribution of kT ∼ 25 keV and for a broad energy distribution peaking at 426 keV. The cross-section ratio of (235)U(n,γ)/(238)U(n,γ) can be deduced in accelerator mass spectrometry directly from the atom ratio of the reaction products (236)U/(239)U, independent of any fluence normalization. Our results confirm the values at the lower band of existing data. They serve as important anchor points to resolve present discrepancies in nuclear data libraries as well as for the normalization of cross-section data used in the nuclear astrophysics community for s-process studies.

Evaluation of the neutron induced reactions on235U from 2.25 keV up to 30 MeV

“…The agreement of several experimental datasets with Wallner data [74] at both 25 and 426 keV supports the changes made in evaluated capture data. It should be noted that changes made in capture and fission cross sections in the evaluated file are well within uncertainties of the measured total cross sections.…”

“…However, recent experimental data for 235 U(n,γ ) reaction have been published by Jandel et al [73] showing fluctuations from 2 keV up to 200 keV. Additionally, unique AMS 235 U(n,γ ) measurements using well-defined neutron spectra with average energy equal to 25 and 426 keV have been published by Wallner et al [74] relative to very well known neutron capture cross section on gold and 238 U targets. Neutron capture cross sections were modified to follow Jandel fluctuations as shown in Fig.…”

“…There is some discrepancy in data near 35 keV, lower data was preferred as done in ENDF/B-VII.1 evaluation. Finally, some reduction of capture from 300 to 500 keV following Wallner measurement [74] was made as shown in Fig. 6(b).…”

Neutron capture cross section measurements for 238U in the resonance region at GELINA

“…The data obtained in this work can also be compared with the result of Wallner et al [72]. For a neutron beam with a distribution that is very similar to a Maxwell-Boltzman distribution of kT ∼ 25.3 keV they derived a spectrum averaged 238 U(n, γ) cross section of (391 ± 17) mb [72].…”

“…For a neutron beam with a distribution that is very similar to a Maxwell-Boltzman distribution of kT ∼ 25.3 keV they derived a spectrum averaged 238 U(n, γ) cross section of (391 ± 17) mb [72]. The latter was obtained from a combination of activation measurements and atom counting of the reaction products using accelerator mass spectrometry.…”