Problem on unresolved resonance data in recent nuclear data libraries

Konno, Chikara  ; Nakagawa, Tsuneo; Ochiai, Kentaro; Sato, Satoshi; Yamauchi, Michinori

A leakage neutron spectrum from a niobium sphere of 0.5 m in radius with a 20 MeV neutron in the center, which was calculated with ANISN, MCNP and JENDL-3.3, had a large strange bump around 100 keV, which originated from the unresolved resonance data. We checked cross section data of (n,) reaction and elastic scattering in Nb with and without self-shielding correction by using a multigroup library of JENDL-3.3. The self-shielding correction of the unresolved resonances seems to be too large even around the upper energy of the unresolved resonance region. It is considered that the average reduced neutron widths or the upper energy of the unresolved resonance region in Nb have any problems. It is found out that many nuclei with unresolved resonance data in JENDL-3.3 and ENDF/B-VI have the similar problem. The unresolved resonance data in JENDL-3.3 and ENDF/B-VI should be revised by considering self-shielding correction in the next version.