Evidence for Gamow-Teller Decay ofNi78Core from Beta-Delayed Neutron Emission Studies

Abstract

The β-delayed neutron emission of 83;84 Ga isotopes was studied using the neutron time-of-flight technique. The measured neutron energy spectra showed emission from states at excitation energies high above the neutron separation energy and previously not observed in the β decay of midmass nuclei. The large decay strength deduced from the observed intense neutron emission is a signature of Gamow-Teller transformation. This observation was interpreted as evidence for allowed β decay to 78 Ni core-excited states in 83;84 Ge favored by shell effects. We developed shell model calculations in the proton fpg 9=2 and neutron extended fpg 9=2 þ d 5=2 valence space using realistic interactions that were used to understand measured β-decay lifetimes. We conclude that enhanced, concentrated β-decay strength for neutron-unbound states may be common for very neutron-rich nuclei. This leads to intense β-delayed high-energy neutron and strong multineutron emission probabilities that in turn affect astrophysical nucleosynthesis models. DOI: 10.1103/PhysRevLett.117.092502 β-delayed neutron emission from fission fragments was first observed in 1939 following the neutron bombardment of uranium salts [1]. It was recognized that the delayed neutron energies and emission probabilities, P n , are important parameters to model environments that involve neutron-rich isotopes. Two of the main applications are in nuclear reactor physics [2] and r-process nucleosynthesis [3]. Because β-delayed neutron precursors are neutron rich and far from stability, they are always relatively difficult to produce and study. Advances in detector capabilities allowed for pioneering measurements of neutron emission spectra of fission fragments [4,5]. In these experiments, resonancelike behavior was observed in the neutron emission spectrum [4,6].These efforts were halted in the following decade by several factors. First, it became increasingly difficult to produce species with larger neutron excess. Second, the very influential work by Hardy, Johnson, and Hansen on "pandemonium" attributed the features of the neutron spectra to purely statistical effects and warned against overinterpretation of the measurements [7]. Misinterpretations of their work attributed decay observables of all heavy nuclei to gross features of the decay strength and statistical fluctuations of the level density. A more accurate depiction of their work is that neutron emission characteristics cannot be interpreted without considering the effects of high level density. The pandemonium controversy [8] arose partly from the fact that, at the time, there was no capability to compute nuclear properties using a sufficiently complete microscopic model of the nucleus.State-of-the-art models are now capable of computing decay properties of atomic nuclei, such as lifetimes and branching ratios. It has become increasingly clear that the β-decay observables are profoundly influenced by nuclearPublished by the American Physical Society under the terms of the Creative Commons Attribution 3.0 Lic...

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β -decay properties of neutron-rich Ca, Ti, and Cr isotopes

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