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Hall, O. et al, Agramunt, J., Algora, A., Domingo-Pardo, C., Morales, A. I., Rubio, B., et al. (2021). beta-delayed neutron emission of r-process nuclei at the N=82 shell closure. Phys. Lett. B, 816, 136266–7pp.
Abstract: Theoretical models of beta-delayed neutron emission are used as crucial inputs in r-process calculations. Benchmarking the predictions of these models is a challenge due to a lack of currently available experimental data. In this work the beta-delayed neutron emission probabilities of 33 nuclides in the important mass regions south and south-west of Sn-132 are presented, 16 for the first time. The measurements were performed at RIKEN using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The P-1n values presented constrain the predictions of theoretical models in the region, affecting the final abundance distribution of the second r-process peak at A approximate to 130.
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Fujita, Y. et al, Algora, A., Estevez-Aguado, E., Molina, F., & Rubio, B. (2014). Observation of Low- and High-Energy Gamow-Teller Phonon Excitations in Nuclei. Phys. Rev. Lett., 112(11), 112502–5pp.
Abstract: Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A = 42, 46, 50, and 54 “f-shell” nuclei in (He-3, t) charge-exchange reactions. In the Ca-42 -> Sc-42 reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the Fe-54 -> Co-54 reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei.
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IGISOL Collaboration(Zakari-Issoufou, A. A. et al), Algora, A., Tain, J. L., Valencia, E., Agramunt, J., Estevez, E., et al. (2015). Total Absorption Spectroscopy Study of Rb-92 Decay: A Major Contributor to Reactor Antineutrino Spectrum Shape. Phys. Rev. Lett., 115(10), 102503–6pp.
Abstract: The antineutrino spectra measured in recent experiments at reactors are inconsistent with calculations based on the conversion of integral beta spectra recorded at the ILL reactor. Rb-92 makes the dominant contribution to the reactor antineutrino spectrum in the 5-8 MeV range but its decay properties are in question. We have studied Rb-92 decay with total absorption spectroscopy. Previously unobserved beta feeding was seen in the 4.5-5.5 region and the GS to GS feeding was found to be 87.5(25)%. The impact on the reactor antineutrino spectra calculated with the summation method is shown and discussed.
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Matsubara, H. et al, & Rubio, B. (2015). Nonquenched Isoscalar Spin-M1 Excitations in sd-Shell Nuclei. Phys. Rev. Lett., 115(10), 102501–6pp.
Abstract: Differential cross sections of isoscalar and isovector spin-M1 (0(+) -> 1(+)) transitions are measured using high-energy-resolution proton inelastic scattering at E-p = 295 MeV on Mg-24, Si-28, S-32, and Ar-36 at 0 degrees-14 degrees. The squared spin-M1 nuclear transition matrix elements are deduced from the measured differential cross sections by applying empirically determined unit cross sections based on the assumption of isospin symmetry. The ratios of the squared nuclear matrix elements accumulated up to E-x = 16 MeV compared to a shell-model prediction are 1.01(9) for isoscalar and 0.61(6) for isovector spin-M1 transitions, respectively. Thus, no quenching is observed for isoscalar spin-M1 transitions, while the matrix elements for isovector spin-M1 transitions are quenched by an amount comparable with the analogous Gamow-Teller transitions on those target nuclei.
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Grodner, E. et al, Gadea, A., Algora, A., Agramunt, J., Domingo-Pardo, C., Molina, F., et al. (2014). Hindered Gamow-Teller Decay to the Odd-Odd N = Z Ga-62: Absence of Proton-Neutron T=0 Condensate in A=62. Phys. Rev. Lett., 113(9), 092501–5pp.
Abstract: Search for a new kind of superfluidity built on collective proton-neutron pairs with aligned spin is performed studying the Gamow-Teller decay of the T = 1, J(pi) = 0(+) ground state of Ge-62 into excited states of the odd-odd N = Z nucleus Ga-62. The experiment is performed at GSI Helmholtzzentrum fur Shwerionenforshung with the Ge-62 ions selected by the fragment separator and implanted in a stack of Si-strip detectors, surrounded by the RISING Ge array. A half-life of T-1/2 = 2 82.9(14) ms is measured for the Ge-62 ground state. Six excited states of Ga-62, populated below 2.5 MeV through Gamow-Teller transitions, are identified. Individual Gamow-Teller transition strengths agree well with theoretical predictions of the interacting shell model and the quasiparticle random phase approximation. The absence of any sizable low-lying Gamow-Teller strength in the reported beta-decay experiment supports the hypothesis of a negligible role of coherent T = 0 proton-neutron correlations in Ga-62.
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