Unresolved Resonance Region (URR) description with Probability Tables (PT)

The unresolved resonance region is the energy domain in which nuclear resonances cannot be longer experimentally determined (resolved) because the distance between two resonances becomes small compared to the neutron beam experimental energy resolution and because the resonance widths are such that the resonances begin to overlap each other.

In the URR, the cross section is obtained from adjusted average resonance parameters. The ENDF files compile the distribution and average of the partial widths and spacings of the resonances obtained from this adjustment. The partial width distribution is a Wigner distribution and the the spacing distribution is a chi-square law.

In Geant4 the URR can be described either by the smooth cross sections or by using the probability table method to incorporate resonances in a stochastic way with the resonance average parameters (width and mean). By default Geant4 uses the smooth cross sections. If the probability table method is selected the user has the choice to use probability tables processed either from NJOY [MMB+17] or CALENDF [SRCD11]. This allows the user to test the impact of the treatment performed in the two processing code.

The description of the URR with probability tables can have an impact on shielding and critical calculations.

The implementation of the probability tables has been validated against reference neutron transport code MCNP and Tripoli4 [ZTTD25].

Bibliography

MMB+17

missing publisher in had-Macfarlane_2017

SRCD11

J.-C. Sublet, P. Ribon, and M. Coste-Delclaux. CALENDF-2010: User Manual. 2011.

ZTTD25

M. Zmeškal, L. Thulliez, P. Tamagno, and E. Dumonteil. Improvement of geant4 neutron-hp package: unresolved resonance region description with probability tables. Annals of Nuclear Energy, 211:110914, February 2025. URL: http://dx.doi.org/10.1016/j.anucene.2024.110914, doi:10.1016/j.anucene.2024.110914.