Title: Angular momentum effects in fission

Abstract

Background: The role of angular momentum in fission has long been discussed but the observable effects are difficult to quantify. Purpose: Here we discuss a variety of effects associated with angular momentum in fission and present quantitative illustrations. Methods: We employ the fission simulation model freya, which is well suited for this purpose because it obeys all conservation laws, including linear and angular momentum conservation at each step of the process. We first discuss the implementation of angular momentum in freya and then assess particular observables, including various correlated observables. We also study potential effects of neutron-induced fission of the low-lying isomeric state of ${}_{235}\mathrm{U}$ relative to the ground state. Results: The fluctuations inherent in the fission process ensure that the spin of the initial compound nucleus has only a small influence on the fragment spins, which are therefore nearly uncorrelated. There is a marked correlation between the spin magnitude of the fission fragments and the photon multiplicity. We also consider the dynamical anisotropy caused by the rotation of an evaporating fragment and study especially the distribution of the projected neutron-neutron opening angles, showing that while it is dominated by the effect of the evaporation recoils, it is possible to extract the signal of the dynamical anisotropy by means of a Fourier decomposition. Finally, we note that the use of an isomeric target, ${}_{235\mathrm{m}}\mathrm{U}$( $n_{\mathrm{th}}$,f), may enhance the symmetric yields and can thus result in higher neutron multiplicities for low total fragment kinetic energies. Conclusions: While the initial angular momentum of the fissioning nucleus tends to have little effect on the observables, those of the produced fragments influence the emitted neutrons and photons in a significant and correlated manner which may be exploited experimentally to elucidate the fission process.

Authors:

Vogt, R.  ^[1];

• Search DOE PAGES for author "Vogt, R."
• Search DOE PAGES for ORCID "0000-0002-0633-1260"

Randrup, J.  ^[2]

• Search DOE PAGES for author "Randrup, J."
• Search DOE PAGES for ORCID "0000-0002-7618-4876"

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+ Show Author Affiliations

1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of California, Davis, CA (United States)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Publication Date:

Tue Jan 19 00:00:00 EST 2021

Research Org.:

Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Nuclear Physics (NP); USDOE Laboratory Directed Research and Development (LDRD) Program

OSTI Identifier:

1782788

Report Number(s):

LLNL-JRNL-815575
Journal ID: ISSN 2469-9985; 1024821; TRN: US2210040

Grant/Contract Number:  

AC52-07NA27344; AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Physical Review. C

Additional Journal Information:

Journal Volume: 103; Journal Issue: 1; Journal ID: ISSN 2469-9985

Publisher:

American Physical Society (APS)

Country of Publication:

United States

Language:

English

Subject:

73 NUCLEAR PHYSICS AND RADIATION PHYSICS; fission; A ≥ 220Spin; Monte Carlo methods; nuclear physics