Inconsistencies in the description of pairing effects in nuclear level densities

The GEF ("GEneral description of Fission observables") model code is documented. It describes the observables for spontaneous fission, neutron-induced fission and, more generally, for fission of a compound nucleus from any other entrance channel, with given excitation energy and angular momentum. The GEF model is applicable for a wide range of isotopes from Z = 80 to Z = 112 and beyond, up to excitation energies of about 100 MeV. The results of the GEF model are compared with fission barriers, fission probabilities, fission-fragment mass-and nuclide distributions, isomeric ratios, total kinetic energies, and prompt-neutron and prompt-gamma yields and energy spectra from neutron-induced and spontaneous fission. Derived properties of delayed neutrons and decay heat are also considered.The GEF model is based on a general approach to nuclear fission that explains a great part of the complex appearance of fission observables on the basis of fundamental laws of physics and general properties of microscopic systems and mathematical objects. The topographic theorem is used to estimate the fission-barrier heights from theoretical macroscopic saddle-point and ground-state masses and experimental ground-state masses. Motivated by the theoretically predicted early localisation of nucleonic wave functions in a necked-in shape, the properties of the relevant fragment shells are extracted. These are used to determine the depths and the widths of the fission valleys corresponding to the different fission channels and to describe the fission-fragment distributions and deformations at scission by a statistical approach. A modified composite nuclear-level-density formula is proposed. It respects some features in the superfluid regime that are in accordance with new experimental findings and with theoretical expectations. These are a constant-temperature behaviour that is consistent with a considerably increased heat capacity and an increased pairing condensation energy that is consistent with the collective enhancement of the level density. The exchange of excitation energy and nucleons between the nascent fragments on the way from saddle to scission is estimated according to statistical mechanics. As a result, excitation energy and unpaired nucleons are predominantly transferred to the heavy fragment in the superfluid regime. This description reproduces some rather peculiar observed features of the prompt-neutron multiplicities and of the even-odd effect in fission-fragment Z distributions. For completeness, some conventional descriptions are used for calculating pre-equilibrium emission, fission probabilities and statistical emission of neutrons and gamma radiation from the excited fragments. Preference is given to simple models that can also be applied to exotic nuclei compared to more sophisticated models that need precise empirical input of nuclear properties, e.g. spectroscopic information.The approach reveals a high degree of regularity and provides a considerable insight into the physics of the fission process. Fission obs...

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“…[63]. This is in conflict with the fact that pairing correlations are only stable, if they increase the nuclear binding.…”

Section: B Nuclear Level Densitiescontrasting

confidence: 44%

“…There exist several descriptions that differ substantially, in particular in their low-energy characteristics. A recent analysis revealed that many of these descriptions are not consistent with our present understanding of nuclear properties [63]. The result can be summarized as follows:…”

Section: B Nuclear Level Densitiesmentioning

confidence: 53%

General Description of Fission Observables: GEF Model Code

Schmidt

Jurado

Amouroux

et al. 2016

Nuclear Data Sheets

444

421

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“…In order to include the effects of pairing correlations on the nuclear level density in a realistic way, the recommended energy shift of the Fermi-gas part of the Gilbert-Cameron composite formula [11] is increased by 2 MeV. Consequently, in our composite formula the transition from the constanttemperature to the Fermi-gas regime occurs at energies of about 8-9 MeV which are higher than the transition energies of the broadly used Gilbert-Cameron formula [12]. For fragment energies larger than the transition energy, the level densities of the fragments follow the Fermi-gas description.…”

Section: -P4 Fission 2013mentioning

confidence: 99%

The even-odd effect in fission-fragment Z yields – a new kind of nuclear clock

Jurado

Schmidt

2013

EPJ Web of Conferences

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Abstract.A model for the even-odd effect in fission based on statistical mechanics is presented. It reveals that the variation of the even-odd effect with the mass of the fissioning nucleus and the increase towards asymmetric splits is due to the important statistical weight of configurations where the light fission fragment populates the ground state of an eveneven nucleus. This implies that excitation energy and unpaired nucleons are predominantly transferred to the heavy fragment. Our results indicate that the time for transfer of excitation energy and nucleons is shorter than the saddle-to-scission time.

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“…We require that the most probable values of the χ k 's at any CN excitation energy E * maximize the entropy S. As regards the choice of the level density (the logarithm of which yields S), it has been suggested [63] that a composite Gilbert-Cameron (CGC) formula, with a larger energy shift and a multiplicative enhancement in the Fermi gas regime, encapsulates qualitatively the influence of shell effects, collective excitations and pairing correlations on the level density. In lieu of specific information on the magnitude of these modifications for 150 Sm, we employ both the back-shifted Fermi gas (BSFG) and the constant temperature (CT) formulas, with 150 Sm parameters (appropriate to E * < 10 MeV) taken from Table II in Ref.…”

mentioning

confidence: 99%