Neutron-induced fission cross section ofU234andNp237

Abstract

A high-resolution measurement of the neutron-induced fission cross section of 234 U and 237 Np has been performed at the CERN Neutron Time-of-Flight facility. The cross sections have been determined in a wide energy range from 1 eV to 1 GeV using the evaluated 235 U cross section as reference. In these measurements the energy determination for the 234 U resonances could be improved, whereas previous discrepancies for the 237 Np resonances were confirmed. New cross-section data are provided for high neutron energies that go beyond the limits of prior evaluations, obtaining important differences in the case of 237 Np.

Fission induced by nucleons at intermediate energies

“…It is to be recalled, however, that the measurements of Ref. [48] are relative to 235 U (n, f ) and that other measurements of relative cross sections exist, including those from n TOF [9], which cover the full energy range of interest: they deserve a more detailed description in the following sub-section.…”

“…Since measuring an absolute cross section requires simultaneous determination of fission events and neutron flux, which is a very difficult task, the measurements performed up to the present time are relative to 235 U (n, f ) and absolute cross sections have been obtained by normalizing the experimental ratios to an evaluated 235 U (n, f ) cross section, commonly taken from the ENDF/B-VII.1 library [7] up to E n = 30 MeV and from the JENDL/HE-2007 library [8] from 30 MeV to 1 GeV. (n, f ) cross sections up to 1 GeV have already been published for 234 U and 237 N p [9], as well as for nat P b and 209 Bi [10]. Preliminary data have been obtained for 232 T h and 233,238 U .…”

Publisher's Note: Measurement of resolved resonances of232Th(n,γ) at the n_TOF facility at CERN [Phys. Rev. C85, 064601 (2012)]

“…The second collimator is located near the experimental area at a distance of 175 m and has an outer diameter of 40 cm and a variable inner diameter. For this capture experiment we used an inner diameter of 1.8 cm while for most fission experiments [36][37][38][39] a diameter of 8 cm has been used. The collimation resulted in a nearly symmetric Gaussian-shaped beam profile at the sample position of 185.2 m with a standard deviation of about 0.77 cm at low neutron energies.…”

Pulse processing routines for neutron time-of-flight data

“…At n TOF a wide variety of detectors is used for measuring neutron induced reactions, including neutron capture (n, γ), neutron induced fission (n, f ) and reactions of type (n, p), (n, t) and (n, α). Among these are solid-state detectors (such as the silicon based neutron beam monitor [7] and CVD diamond detectors [8]), scintillation detectors (an array of BaF 2 scintillator crystals [9], C 6 D 6 liquid scintillators [10]) and gaseous detectors (such as MicroMegas-based detectors [11,12], a calibrated fission chamber from the Physikalisch Technische Bundesanstalt [13], a set of Parallel Plate Avalanche Counters [14]). Several other types of detectors were recently introduced and tested at n TOF, such as solid-state HPGe, scintillation NaI, gaseous 3 He detectors, etc.…”