Application of modified REFIT code for J-PARC/MLF to evaluation of neutron capture cross section on Gd

Mizuyama, Kazuhito; Iwamoto, Nobuyuki  ; Iwamoto, Osamu  ; Hasemi, Hiroyuki*; Kino, Koichi*; Kimura, Atsushi   ; Kiyanagi, Yoshiaki*

Gadolinium has been used as neutron-absorbing material in a thermal reactor since have large thermal neutron capture cross sections. Nevertheless, there is a discrepancy between RPI data and JENDL-4.0 data for Gd. The criticality in the reactor is very sensitive to the capture cross section. The RPI data made the criticality of Gd-loaded thermal systems in ICSBEP overestimated. Recently, the neutron capture cross sections of Gd were measured by the neutron time-of-flight (TOF) method using the Accurate Neutron-Nucleus Reaction measurement Instrument (ANNRI) in the J-PARC/MLF. The pulsed neutron beam from the Japan Spallation Neutron Source (JSNS) was used with a double-bunch structure in this measurement, since the incident proton beam is normally delivered in a double-bunch scheme in the J-PARC. In addition to this, it is necessary to take into account the energy resolution of the pulsed neutron beam at the JSNS for the accurate derivation of resolved resonance parameters. In this study, using the least-squares multilevel R-matrix code REFIT modified to include the double bunch structure and the resolution function for the ANNRI, we fitted the calculated capture cross sections of Gd to the experimental data at the ANNRI. We derived the resonance parameters for some low-lying resonances of the two Gd isotopes.