Neutron Capture Reactions on Gadolinium Isotopes

Abstract

The neutron capture reaction on 155Gd, 156Gd and 158Gd isotopes has been studied with the DANCE calorimeter at Los Alamos Neutron Science Center. The highly segmented calorimeter provided detailed multiplicity distributions of the capture gamma-rays. With this information the spins of the neutron capture resonances have been determined. The new technique based on the statistical pattern recognition method allowed the determination of almost all spins of 155Gd s-wave resonances. The generalized method was tested for s- and p-wave resonances in 94Mo and 95Mo isotopes. The results were compared with previous resonance data as well as results from other methods. The 155Gd(n,gamma)156Gd cross section has been measured for the incident neutrons energy range from 1 eV to 10 keV. The results are in good agreement with other experiments. Neutron resonances parameters were obtained using the multilevel R-matrix code SAMMY. The fitted radiation and neutron widths were compared with the nuclear data library ENDF/B-VII.0 and with a recent experiment at RPI. With the new spin assignments and resonance parameters, level spacings and neutron strength functions were determined for s-wave resonances in 155Gd. The Monte Carlo code DICEBOX was used to simulate the gamma-decay of the compound nuclei 156Gd, 157Gd and 159Gd. The DANCE detector response was taken into account with a GEANT4 simulation. The simulated and experimental spectra were compared to determine suitable model parameters for the photon strength functions (PSFs) and the level density (LD). The shape of the photon strength function which gave the best agreement with the DANCE spectra contained four low-lying Lorentzian resonances, two for the scissors mode and two for the M1 spin flip mode.