Electron spectra in forbidden β decays and the quenching of the weak axial-vector coupling constant gA Kostensalo, Joel; Haaranen, Mikko; Suhonen, Jouni Kostensalo, J., Haaranen, M., & Suhonen, J. (2017). Electron spectra in forbidden β decays and the quenching of the weak axial-vector coupling constant gA. Physical Review C, 95 (4) Evolution of the electron spectra with the effective value of the weak axial-vector coupling constant g A was followed for 26 first-, second-, third-, fourth-and fifth-forbidden β − decays of odd-A nuclei by calculating the involved nuclear matrix elements (NMEs) in the framework of the microscopic quasiparticle-phonon model (MQPM). The next-to-leading-order terms were included in the β-decay shape factor of the electron spectra. The spectrum shapes of third-and fourth-forbidden nonunique decays were found to depend strongly on the value of g A , while first-and second-forbidden decays were mostly unaffected by the tuning of g A . The g A -driven evolution of the normalized β spectra was found to be quite universal, largely insensitive to the small changes in the nuclear mean field and the adopted residual many-body Hamiltonian producing the excitation spectra of the MQPM. This makes the comparison of experimental and theoretical electron spectra, coined "the spectrum-shape method" (SSM), a robust tool for extracting information on the effective values of the weak coupling constants. In this exploratory work two new experimentally interesting decays for the SSM treatment were discovered: the ground-state-to-ground-state decays of 99 Tc and 87 Rb. Comparing the experimental and theoretical spectra of these decays could shed light on the effective values of g A and g V for second-and third-forbidden nonunique decays. The measurable decay transitions of 135 Cs and 137 Cs, in turn, can be used to test the SSM in different many-body formalisms. The present work can also be considered as a (modest) step towards solving the g A problem of the neutrinoless double beta decay.

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.

We describe here microscopic calculations performed on the dominant forbidden transitions in reactor antineutrino spectra above 4 MeV using the nuclear shell model. By taking into account Coulomb corrections in the most complete way, we calculate the shape factor with the highest fidelity and show strong deviations from allowed approximations and previously published results. Despite small differences in the ab initio electron cumulative spectra, large differences on the order of several percents are found in the antineutrino spectra. Based on the behaviour of the numerically calculated shape factors we propose a parametrization of forbidden spectra. Using Monte Carlo techniques we derive an estimated spectral correction and uncertainty due to forbidden transitions. We establish the dominance and importance of forbidden transitions in both the reactor anomaly and spectral shoulder analysis. Based on these results, we conclude that a correct treatment of forbidden transitions is indispensable in both the normalization anomaly and spectral shoulder.

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...

We study the dominant forbidden transitions in the antineutrino spectra of the fission actinides from 4 MeV onward using the nuclear shell model. Through explicit calculation of the shape factor, we show the expected changes in cumulative electron and antineutrino spectra. Relative to the allowed approximation this results in a minor decrease of electron spectra above 4 MeV, whereas an increase of several percent is observed in antineutrino spectra. We show that forbidden transitions dominate the spectral flux for most of the experimentally accessible range. Based on the shell model calculations we attempt a parametrization of forbidden transitions and propose a spectral correction for all first-forbidden transitions. We enforce correspondence with the Institut Laue-Langevin data set using a summation+conversion approach. When compared against modern reactor neutrino experiments, the resultant spectral change is observed to be of comparable magnitude and shape as the reported spectral shoulder.

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Published version Kirsebom, O. S.; Hukkanen, M.; Kankainen, A.; Trzaska, W. H.; Strömberg, D. F.; Martínez-Pinedo, G.; Andersen, K.; Bodewits, E.; Brown, B. A.; Canete, L.; Cederkäll, J.; Enqvist, T.; Eronen, T.; Fynbo, H. O. U.; Geldhof, S.; de Groote, R., Jenkins, D. G.; Jokinen, A.; Joshi, P.; Khanam, A.; Kostensalo, J.; Kuusiniemi, P.; Langanke, K.; Moore, I.; Munch, M.; Nesterenko, D. A.; Ovejas, J. D.; Penttilä, H.; Pohjalainen, I.; Reponen, M.; Rinta-Antila, S.; Riisager, K.; de Roubin, A.; Schotanus, P.; Srivastava, P. C.; Suhonen, J.; Swartz, J. A.; Tengblad, O.; Vilen, M.; Vínals, S.; Äystö, J. Kirsebom, O. S.; Hukkanen, M.; Kankainen, A.; Trzaska, W. H.; Strömberg, D. F.; Martínez-Pinedo, G.; Andersen, K.; Bodewits, E.; Brown, B. A.; Canete, L.; Cederkäll, J.; Enqvist, T.; Eronen, T. et al. (2019). Measurement of the 2+→0+ ground-state transition in the β decay of 20F.We report the first detection of the second-forbidden, nonunique, 2 + → 0 + , ground-state transition in the β decay of 20 F. A low-energy, mass-separated 20 F + beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the β spectrum measured using a magnetic transporter and a plasticscintillator detector. The β-decay branching ratio inferred from the measurement is b β = [0.41 ± 0.08(stat) ± 0.07(sys)] × 10 −5 corresponding to log f t = 10.89(11), making this one of the strongest second-forbidden, nonunique β transitions ever measured. The experimental result is supported by shell-model calculations and has significant implications for the final evolution of stars that develop degenerate oxygen-neon cores. Using the new experimental data, we argue that the astrophysical electron-capture rate on 20 Ne is now known to within better than 25% at the relevant temperatures and densities.

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