Large-scale prediction of the parity distribution in the nuclear level density and application to astrophysical reaction rates

Abstract

A generalized method to calculate the excitation-energy dependent parity ratio in the nuclear level density is presented, using the assumption of Poisson distributed independent quasi particles combined with BCS occupation numbers. It is found that it is crucial to employ a sufficiently large model space to allow excitations both from low-lying shells and to higher shells beyond a single major shell. Parity ratios are only found to equilibrate above at least 5-10 MeV of excitation energy. Furthermore, an overshooting effect close to major shells is found where the parity opposite to the ground state parity may dominate across a range of several MeV before the parity ratio finally equilibrates. The method is suited for large-scale calculations as needed, for example, in astrophysical applications. Parity distributions were computed for all nuclei from the proton dripline to the neutron dripline and from Ne up to Bi. These results were then used to recalculate astrophysical reaction rates in a Hauser-Feshbach statistical model. Although certain transitions can be considerably enhanced or suppressed, the impact on astrophysically relevant reactions remains limited, mainly due to the thermal population of target states in stellar reaction rates.

The Jina Reaclib Database: Its Recent Updates and Impact on Type-I X-Ray Bursts

“…The NON-SMOKER WEB 5.0w code used here still stays close to the original NON-SMOKER code in the treatment of some of the nuclear properties required to calculate the cross sections and reaction rates but the internal modifications give rise to certain differences in the results, expected to yield improved predictions. Concerning the update of nuclear properties, of relevance here are mainly the inclusion of masses from Wapstra et al (2003); Audi et al (2003b) with the additions described elsewhere in this -30 -work, updated experimental information on ground and excited state spins and parities from Nudat 2.4, National Nuclear Data Center (2008), an improved prediction of ground state properties when no experimental information is available, and a parity-dependent level density (Mocelj et al 2007) to be used above the known experimental levels. The different spins and level densities as well as the parity treatment may lead to considerable changes in the predicted rates compared to the previous NON-SMOKER rates (Rauscher & Thielemann 2000).…”

Experimental study of the astrophysicalγ-process reactionXe124(α,γ)BaHalász1, Somorjai2, Gyürky3 et al. 2016Phys. Rev. CSelf Cite160170View full textAdd to dashboardCite

“…(Similar problems due to incomplete sets of experimentally determined excited states were also discussed in [24,57].) In the present work, the incomplete set of excited states was neglected in the SMARAGD calculations and instead the nuclear level density of [58] with the parity distribution of [59] was used above the ground state.…”

Investigation ofα-induced reactions on127I for the astrophysicalγprocess

“…Version 0.8.4s of the SMARAGD code made use of an updated level scheme from [49], also included in the 2010 version of NuDAT [48]. Above the last included discrete state, a refitted version of the level density of [51] is applied and additionally an energy-dependent parity distribution [52].…”