Forbidden nonunique β decays and effective values of weak coupling constantsHaaranen, Mikko; Srivastava, P. C.; Suhonen, Jouni Haaranen, M., Srivastava, P. C., & Suhonen, J. (2016 Forbidden nonunique β decays feature shape functions that are complicated combinations of different nuclear matrix elements and phase-space factors. Furthermore, they depend in a very nontrivial way on the values of the weak coupling constants, g V for the vector part and g A for the axial-vector part. In this work we include also the usually omitted second-order terms in the shape functions to see their effect on the computed decay half-lives and electron spectra (β spectra). As examples we study the fourth-forbidden nonunique ground-state-to-ground-state β − decay branches of 113 Cd and 115 In using the microscopic quasiparticle-phonon model and the nuclear shell model. A striking new feature that is reported in this paper is that the calculated shape of the β spectrum is quite sensitive to the values of g V and g A and hence comparison of the calculated with the measured spectrum shape opens a way to determine the values of these coupling constants. This article is designed to show the power of this comparison, coined spectrum-shape method (SSM), by studying the two exemplary β transitions within two different nuclear-structure frameworks. While the SSM seems to confine the g V values close to the canonical value g V = 1.0, the values of g A extracted from the half-life data and by the SSM emerge contradictory in the present calculations. This calls for improved nuclear-structure calculations and more measured data to systematically employ SSM for determination of the effective value of g A in the future.

The gallium anomaly, i.e. the missing electron-neutrino flux from 37 Ar and 51 Cr electron-capture decays as measured by the GALLEX and SAGE solar-neutrino detectors, has been among us already for about two decades. We present here a new estimate of the significance of this anomaly based on cross-section calculations using nuclear shell-model wave functions obtained by exploiting recently developed two-nucleon interactions. The gallium anomaly of the GALLEX and SAGE experiments is found to be smaller than that obtained in previous evaluations, decreasing the significance from 3.0σ to 2.3σ. This result is compatible with the recent indication in favor of short-baselineν e disappearance due to small active-sterile neutrino mixing obtained from the combined analysis of the data of the NEOS and DANSS reactor experiments.

A significant fraction of stars between 7-11 solar masses are thought to become supernovae, but the explosion mechanism is unclear. The answer depends critically on the rate of electron capture on 20 Ne in the degenerate oxygen-neon stellar core. However, due to the unknown strength of the transition between the ground states of 20 Ne and 20 F, it has not previously been possible to fully constrain the rate. By measuring the transition, we have established that its strength is exceptionally large and enhances the capture rate by several orders of magnitude. This has a decisive impact on the evolution of the core, increasing the likelihood that the star is (partially) disrupted by a thermonuclear explosion rather than collapsing to form a neutron star. Importantly, our measurement resolves the last remaining nuclear physics uncertainty in the final evolution of degenerate oxygen-neon stellar cores, allowing future studies to address the critical role of convection, which at present is poorly understood. Stars of 7-11 solar masses (M ) are prevalent in the Galaxy, their birth and death rate comparable to that of all heavier stars combined [1]. Yet, the ultimate fate of such "intermediate-mass stars" remains uncertain. According to current models [2-4], a significant fraction explode, but the mechanism is a matter of ongoing debate [5][6][7][8]. The answer-gravitational collapse or thermonuclear explosion-depends critically on the rate of electron capture on 20 Ne in the stellar core. However, due to the unknown strength of the transition between the ground states of 20 Ne and 20 F, it has not previously been possible to constrain this rate in the relevant temperature-density regime [9]. Here, we report the first measurement of this transition, provide the first accurate determination of the capture rate and explore the astrophysical implications.Intermediate-mass stars that undergo central carbon burning become super-AGB stars [1] with a degenerate oxygen-neon (ONe) core consisting mainly of 16 O and 20 Ne and smaller amounts of 23 Na and 24,25 Mg. We are interested in the scenario where the ONe core is able to increase its mass gradually and approach the Chandrasekhar limit, M Ch ∼ 1.37 M . This can occur if nuclear burning continues long enough outside the core or if the core, having lost its outer layers, becoming a white dwarf (WD), is able to accrete material from a binary companion star. As the core approaches M Ch , it contracts and warms up, but only gradually as the heating from compression is balanced by cooling via the emission of thermal neutrinos. The density, on the other hand, rises rapidly eventually triggering a number of electroncapture processes that greatly influence the temperature evolution of the core. First, the core is cooled by cycles of electron capture followed by β decay on the odd-mass nuclei 25 Mg and 23 Na [10]. At higher densities, the core is cooled by another such cycle on 25 Na, and heated by double electron captures on the even-mass nuclei 24 Mg and 20 Ne, which produce substant...

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Abstract. In the present work we calculate the allowed β − -decay half-lives of nuclei with Z = 20 − 30 and N ≤ 50 systematically under the framework of the nuclear shell model. A recent study shows that some nuclei in this region belong to the island of inversion. We perform calculation for f p shell nuclei using KB3G effective interaction. In the case of Ni, Cu, and Zn, we used JUN45 effective interaction. Theoretical results of Q values, half-lives, excitation energies, logf t values, and branching fractions are discussed and compared with the experimental data. In the Ni region, we also compared our calculated results with recent experimental data [Z. Y. Xu et al., Phys. Rev. Lett. 113, 032505, 2014]. Present results agree with the experimental data of half-lives in comparison to QRPA. Nuclear β − -decay half-lives for f p and f pg shell nuclei 2 IntroductionThe neutron density becomes very diffused and the single-particle spectrum shows the similarity of the harmonic-oscillator as we approach towards the neutron-drip line [1,2]. We can see this effect at N = 40 shell gap. Recently intruder configurations are found in the neutron rich nuclei around this shell gap [3,4,5,6,7]. Thus the nuclear structure study of these nuclei in this region is very important [8,9,10].Sorlin et al. Using RIBF facility at RIKEN, the half-lives of twenty neutron-rich nuclei with Z = 27 − 30 are reported in ref. [14]. In that work, sizable magicity was reported for both the proton number Z = 28 and the neutron number N = 50 in 78 Ni. A sudden shortening of half-lives of the nickel isotopes beyond N = 50 were observed in this work, although this effect is not found in the Cu-Ge-Ga chains. The half-lives results from LISE2000 spectrometer at GANIL for 71 Co and 73 Co are reported in ref.[15]. The half-lives of 77,78 Ni were measured for the first time by Hosmer et al. [16], at NSCL, MSU. Recently the beta decay half-lives of 38 very neutron-rich Kr to Tc isotopes are measured at RIKEN [17].Despite the progress in the experimental side, we need theoretical estimates for half-lives of neutron rich nuclei, especially those belonging to the island of inversion. These calculations are based on allowed GT-transitions. Many theoretical calculations from the quasiparticle random phase approximation (QRPA) based on the HartreeFock Bogoliubov theory [18] or other global models [19,20,21] are available in the literature. These calculations underestimate the correlation among nucleons which predict GT-strength at low-energies. Recently, shell model calculations for the β − -decays of Z = 9 − 13 nuclei are reported by Li and Ren [22]. The motivation of the present work is to study β − -decay properties of Z = 20 − 30 nuclei using the nuclear shell model. This paper is organized as follows. In section 2, we present the formulas for the calculation of β − decay half-lives. Shell model spaces, effective interactions and the quenching factors adopted in our calculations are reported in section 3. In section 4, we present theoretical results along w...