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Jin, S. Y. et al, & Algora, A. (2021). Spectroscopy of Cd-98 by two-nucleon removal from In-100. Phys. Rev. C, 104(2), 024302–6pp.
Abstract: Low-lying states of Cd-98 have been populated by the two-nucleon removal reaction (In-100, Cd-98+gamma) and studied using in-beam gamma-ray spectroscopy at the Radioactive Isotope Beam Factory at RIKEN. Two new gamma transitions were identified and assigned as decays from a previously unknown state. This state is suggested to be based on a pi 1g(/9/2)(-1)2p(1/2)(-2) configuration with J(pi) = 5(-). The present observation extends the systematics of the excitation energies of the first 5(-) state in N = 50 isotones toward Sn-100. The determined energy of the 5(- )state in Cd-98 continues a smooth trend along the N = 50 isotones. The systematics are compared with shell-model calculations in different model spaces. Good agreement is achieved when considering a model space consisting of the pi(1f(5/2), 2p(3/2), 2p(1/2), 1g(9/2)) orbitals. The calculations with a smaller model space omitting the orbitals below the Z = 38 subshell could not reproduce the experimental energy difference between the ground and first 5(-) states in N = 50 isotones, because proton excitations across Z = 38 subshell yield a large amount of correlation energy that lowers the ground states.
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Mengoni, D. et al, Gadea, A., Algora, A., & Rubio, B. (2010). Lifetime measurements of excited states in neutron-rich Ar-44,Ar-46 populated via a multinucleon transfer reaction. Phys. Rev. C, 82(2), 024308–7pp.
Abstract: Lifetimes of low-lying excited states of the neutron-rich Ar-44,Ar-46 nuclei, populated via multinucleon transfer reactions, are measured by means of the differential recoil distance Doppler shift method. The extracted electromagnetic transition probabilities are compared with previous intermediate-energy Coulomb-excitation measurements and with large-scale shell-model calculations. The increase in the deduced B( E2; 2(+) -> 0(+)) transition probability from Ar-44 to the closed-shell nucleus Ar-46 contradicts the earlier results of Coulomb-excitation experiments. Shell-model calculations using different effective interactions agree with the new measured values.
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Sarriguren, P., Algora, A., & Kiss, G. (2018). beta-decay properties of neutron-rich Ca, Ti, and Cr isotopes. Phys. Rev. C, 98(2), 024311–10pp.
Abstract: beta-decay properties of neutron-rich Ca, Ti, and Cr isotopes are studied within a deformed proton-neutron quasiparticle random-phase approximation. The underlying mean field is described self-consistently from deformed Skyrme Hartree-Fock calculations with pairing correlations. Residual spin-isospin interactions in the particle-hole and particle-particle channels are also included in the formalism. The energy distributions of the Gamow-Teller strength, the beta-decay feedings, the beta-decay half-lives, and the beta-delayed neutron emission probabilities are discussed and compared with other theoretical results, as well as with the available experimental information. The evolution of these nuclear beta-decay properties is investigated in isotopic chains in a search for structural changes. A reliable estimate of the beta-decay properties in this mass region is valuable information for evaluating decay rates in astrophysical scenarios.
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Guadilla, V., Algora, A., Tain, J. L., Agramunt, J., Jordan, D., Monserrate, M., et al. (2019). Total absorption gamma-ray spectroscopy of niobium isomers. Phys. Rev. C, 100(2), 024311–15pp.
Abstract: The beta-intensity distributions of the decays of Nb-100gs,Nb-100m and Nb-102gs,Nb-102m have been determined using the total absorption gamma-ray spectroscopy technique. The JYFLTRAP double Penning trap system was employed in a campaign of challenging measurements performed with the decay total absorption gamma-ray spectrometer at the Ion Guide Isotope Separator On-Line facility in Jyvaskyla. Different strategies were applied to disentangle the isomeric states involved, lying very close in energy. The low-spin component of each niobium case was populated through the decay of the zirconium parent, which was treated as a contaminant. We have applied a method to extract this contamination, and additionally we have obtained beta-intensity distributions for these zirconium decays. The beta-strength distributions evaluated with these results were compared with calculations in a quasiparticle random-phase approximation, suggesting a prolate configuration for the ground states of Zr-100,Zr-102. The footprint of the Pandemonium effect was found when comparing our results for the analyses of the niobium isotopes with previous decay data. The beta-intensities of the decay of Nb-102m, for which there were no previous data, were obtained. A careful evaluation of the uncertainties was carried out, and the consistency of our results was validated taking advantage of the segmentation of our spectrometer. The final results were used as input in reactor summation calculations. A large impact on antineutrino spectrum calculations was already reported, and here we detail the significant impact on decay heat calculations.
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IDS Collaboration(Heideman, J. et al), Algora, A., & Morales, A. I. (2023). Evidence of nonstatistical neutron emission following beta decay near doubly magic Sn-132. Phys. Rev. C, 108(2), 024311–9pp.
Abstract: Models of the beta-delayed neutron emission (beta n) assume that neutrons are emitted statistically via an intermediate compound nucleus post beta decay. Evidence to the contrary was found in an In-134 beta-decay experiment carried out at ISOLDE CERN. Neutron emission probabilities from the unbound states in Sn-134 to known low-lying, single-particle states in Sn-133 were measured. The neutron energies were determined using the time-of-flight technique, and the subsequent decay of excited states in Sn-133 was studied using gamma-ray detectors. Individual beta n probabilities were determined by correlating the relative intensities and energies of neutrons and gamma rays. The experimental data disagree with the predictions of representative statistical models which are based upon the compound nucleus postulate. Our results suggest that violation of the compound nucleus assumption may occur in beta-delayed neutron emission. This impacts the neutron-emission probabilities and other properties of nuclei participating in the r-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.
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Ramalho, M., Suhonen, J., Kostensalo, J., Alcala, G. A., Algora, A., Fallot, M., et al. (2022). Analysis of the total beta-electron spectrum of( 92)Rb: Implications for the reactor flux anomalies. Phys. Rev. C, 106(2), 024315–7pp.
Abstract: We present here a microscopic nuclear-structure calculation of a beta-electron spectrum including all the beta-decay branches of a high Q-value reactor fission product contributing significantly to the reactor antineutrino energy spectrum. We perform large-scale nuclear shell-model calculations of the total electron spectrum for the beta(-) decay of Rb-92 to states in Sr-92 using a computer cluster. We exploit the beta-branching data of a recent total absorption gamma-ray spectroscopy (TAGS) measurement to determine the effective values of the weak axial-vector coupling, g(A), and the weak axial charge, g(A)(gamma(5)). By using the TAGS data we avoid the bias stemming from the pandemonium effect which is a systematic error biasing the usual beta-decay measurements. We take fully into account all the involved allowed and forbidden beta transitions, in particular the first-forbidden nonunique ones which have earlier been shown to be relevant in the context of the reactor-antineutrino flux anomaly and the unexplained spectral shoulder, the “bump,” the former one having been interpreted as one of the strongest evidence for the existence of sterile neutrinos. Here we are able to present quantitative evidence for the relevance of forbidden nonunique beta(-) decays in a total beta spectrum of a fission product, in this case( 92)Rb, which is one of the major contributors to the total reactor antineutrino spectral shape. We demonstrate that taking the forbidden spectral shapes fully into consideration leads for Rb-92 to a 2.6%-4.6% reduction in the expected inverse beta-decay rate at the reactor antineutrino telescopes. We also confirm by our calculation of a total beta-electron spectrum that the forbidden transitions can contribute to the formation of the spectral bump in the reactor-antineutrino flux profile.
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Valencia, E. et al, Tain, J. L., Algora, A., Agramunt, J., Estevez, E., Jordan, M. D., et al. (2017). Total absorption gamma-ray spectroscopy of the beta-delayed neutron emitters Br-87, Br-88, and Rb-94. Phys. Rev. C, 95(2), 024320–18pp.
Abstract: We investigate the decay of Br-87,Br-88 and Rb-94 using total absorption gamma-ray spectroscopy. These important fission products are beta-delayed neutron emitters. Our data show considerable beta gamma intensity, so far unobserved in high-resolution gamma-ray spectroscopy, from states at high excitation energy. We also find significant differences with the beta intensity that can be deduced from existing measurements of the beta spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for U-235 fission at cooling times in the range 1-100 s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for. emission from neutron unbound states populated in the daughter nucleus. The. branching is compared to Hauser-Feshbach calculations, which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for Rb-94, although much smaller, hints of the need to increase the radiative width gamma by one order of magnitude. This increase in gamma would lead to a similar increase in the calculated (n, gamma) cross section for this very neutron-rich nucleus with a potential impact on r process abundance calculations.
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Deo, A. Y., Podolyak, Z., Walker, P. M., Algora, A., Rubio, B., Agramunt, J., et al. (2010). Structures of Po-201 and Rn-205 from EC/beta(+)-decay studies. Phys. Rev. C, 81(2), 024322–8pp.
Abstract: Several low-lying excited states in Rn-205(86)119 and Po-201(84)117 were identified for the first time following EC/beta(+) decay of Fr-205 and At-201, respectively, using gamma-ray and conversion electron spectroscopy at the CERN isotope separator on-line (ISOLDE) facility. The EC/beta(+) branch from Fr-205 was measured to be 1.5(2)%. The excited states of the daughter nuclei are understood in terms of the odd nucleon coupling to the neighboring even-even core. The neutron single-particle energies of the p(3/2) orbital relative to the f(5/2) ground state in Rn-205, and the f(5/2) orbital relative to the p(3/2) ground state in Po-201, were determined to be 31.4(2) and 5.7(3) keV, respectively. We tentatively identify a 13/2(+) isomeric level at 657.1(5) keV in Rn-205. The systematic behavior of the 13/2(+) and 3/2(-) levels is also discussed.
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Dombos, A. C. et al, & Algora, A. (2021). Total absorption spectroscopy of the beta decay of Zr-101,102 and Tc-109. Phys. Rev. C, 103(2), 025810–20pp.
Abstract: The beta decay of Zr-101,Zr-102 and Tc-109 was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The beta-decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For Zr-101 and Zr-102, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For Tc-109, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide beta-decay properties in r-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical beta-decay distributions, rather than just half-lives and beta-delayed neutron emission probabilities, as close to the r-process path as possible.
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Caballero, L., Rubio, B., Kleinheinz, P., Yates, S. W., Algora, A., Dewald, A., et al. (2010). Two-phonon octupole excitation in Gd-146. Phys. Rev. C, 81(3), 031301–4pp.
Abstract: Based on experimental evidence from the Sm-144(alpha,2n) reaction, the 3484.7- keV 6(+) state in Gd-146 is identified as the highest-spin member of the 3(-) circle times 3(-) two-phonon octupole quartet. A previously unknown gamma line of 1905.8 keV and E3 character feeding the 3(-) octupole state has been observed. These results represent the first observation of a 6(+) -> 3(-) -> 0(+) cascade of two E3 transitions in an even-even nucleus and provide strong support for the interpretation of the 6(+) state as a two-phonon octupole excitation.
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