R3B Collaboration(Ponnath, L. et al), Benlliure, J., Cortina-Gil, D., & Nacher, E. (2025). Precise measurement of nuclear interaction cross sections towards neutron-skin determination with R3B. Nucl. Phys. A, 1056, 123022–5pp.
Abstract: The (RB)-B-3 (Reactions with Relativistic Radioactive Beams) experiment as a major instrument of the NUSTAR collaboration for the research facility FAIR in Darmstadt is designed for kinematically complete studies of reactions with high-energy radioactive beams. Part of the broad physics program of (RB)-B-3 is to constrain the asymmetry term in the nuclear equation-of-state and hence improve the description of highly asymmetric nuclear matter (e.g., in neutron stars). For a precise determination of the neutron-skin thickness – an observable which is directly correlated with the symmetry energy in theoretical calculations – by measuring absolute fragmentation cross sections, it is essential to quantify the uncertainty and challenge the reaction model under stable conditions. During the successful FAIR Phase-0 campaign of (RB)-B-3, we precisely measured the energy dependence of total interaction cross sections in C-12+C-12 collisions, for a direct comparison with calculations based on the eikonal reaction theory.
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Rubio, B., Gelletly, W., Algora, A., Nacher, E., & Tain, J. L. (2017). Beta decay studies with total absorption spectroscopy and the Lucrecia spectrometer at ISOLDE. J. Phys. G, 44(8), 084004–25pp.
Abstract: Here we present the experimental activities carried out at ISOLDE with the total absorption spectrometer Lucrecia, a large 4 pi scintillator detector designed to absorb a full gamma cascade following beta decay. This spectrometer is designed to measure beta-feeding to excited states without the systematic error called Pandemonium. The set up allows the measurement of decays of very short half life. Experimental results from several campaigns, that focus on the determination of the shapes of beta-decaying nuclei by measuring their beta decay strength distributions as a function of excitation energy in the daughter nucleus, are presented.
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IDS Collaboration(Andel, B. et al), Algora, A., & Nacher, E. (2024). β decay of the ground state and of a low-lying isomer in Bi-216. Phys. Rev. C, 109(6), 064321–18pp.
Abstract: A detailed beta -decay study of the low- and high -spin states in 216 Bi has been performed at the ISOLDE Decay Station at the CERN-ISOLDE facility. In total, 48 new levels and 83 new transitions in the beta -decay daughter 216 Po were identified. Shell -model calculations for excited states in 216 Bi and 216 Po were performed using the H208 and the modified Kuo-Herling particle effective interactions. Based on the experimental observations and the shell -model calculations, the most likely spin and parity assignments for the beta -decaying states in 216 Bi are (3 – ) and (8 – ), respectively.
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IDS Collaboration(Clisu, C. et al), & Nacher, E. (2024). Observation of the J 7/2 low-spin states in 213Fr populated in the electron capture of the 1/2-ground state of 213Ra. Phys. Rev. C, 110(6), 064315–15pp.
Abstract: A detailed level scheme of 213Fr126 following the EC/beta+ decay of the 1/2- 213 Ra parent ground state was built in an experiment performed at the ISOLDE Decay Station, CERN. The fragmented total beta decay strength favours the direct population of several low-spin (J 7/2) excited states. The analysis of the gamma-singles spectrum and gamma-gamma coincidences allowed us to identify many new gamma-ray transitions and excited states in 213 Fr up to about 3.6 MeV excitation energy. The spins and parities of the newly established levels, on top of the (7/2-1 ) state, were mainly assigned based on the systematics of the N = 126 isotones and further compared with shell-model calculations. The level scheme displays a structural pattern, with several groups of states with negative parity, emerging from the well-defined, simple, pi ( h 59 / 2 ), pi ( h 4 9 / 2 f 17 / 2 ) configurations or from their configuration mixing. The strength of the E 2 transitions within the multiplets is compared with shell-model theoretical calculations performed with the KHPE and H 208 effective interactions. A new (3/2-) isomer with a half-life of 26(3) ns has been identified. An upper limit of 35 ps was determined for the half-life of the first excited state, 7/2-. The possibility of a mixed M 1 + E 2 character is discussed for the 7/2-1 -> 9/2-gs decay in 213 Fr, which leads to an l-forbidden nature of the pi f 7 / 2 -> pi h 9 / 2 transition.
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IDS collaboration(Llanos-Exposito, M. et al), Algora, A., & Nacher, E. (2025). Structure of 128Sn selectively populated in the β decay of the 128In ground state. Phys. Rev. C, 111(6), 064310–13pp.
Abstract: High-resolution gamma-ray spectroscopy and fast-timing methods were employed to study the excited structure of 128Sn, populated via the beta-decay chain of 128Cd -> 128In -> 128Sn. The experiment was performed by online mass separation at the ISOLDE facility at CERN, profiting from intense and pure Cd beams obtained by a temperature-controlled quartz transfer line combined with resonant laser ionization. An extended 128Sn level scheme populated in the beta – decay of the low-spin 128In isomer was constructed, adding a total of 81 new gamma-ray transitions and 30 new levels. Lifetimes of excited states were measured using time-delayed beta gamma (t) and gamma gamma (t) coincidences. The lifetime of the (4+) state was measured for the first time, making it possible to deduce the B(E 2; 4+ -> 2+) transition strength. The previously measured (5-) state was reassessed with improved statistics. Additionally, an upper limit for the lifetime of the state at 2378 keV was established. The derived reduced transition probabilities support a tentative spin-parity assignment of (4-) for this level. The experimental level scheme and transition probabilities are compared with available shell-model calculations.
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Brunet, M. et al, & Nacher, E. (2021). Competition between allowed and first-forbidden beta decays of At-208 and expansion of the Po-208 level scheme. Phys. Rev. C, 103(5), 054327–13pp.
Abstract: The structure of Po-208 populated through the EC/beta(+) decay of At-208 is investigated using gamma-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed. rays which have been (re)assigned a position. The level scheme is compared to shell model calculations. Through this analysis approximately half of the beta-decay strength of At-208 is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations. This mass region provides an excellent testing ground for the competition between allowed and first-forbidden beta-decay calculations, important for the detailed understanding of the nucleosynthesis of heavy elements.
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Briz, J. A., Nacher, E., Borge, M. J. G., Algora, A., Rubio, B., Dessagne, P., et al. (2015). Shape study of the N = Z nucleus Kr-72 via beta decay. Phys. Rev. C, 92(5), 054326–10pp.
Abstract: The beta decay of the N = Z nucleus Kr-72 has been studied with the total absorption spectroscopy technique at ISOLDE (CERN). A total B(GT) = 0.79(4)g(A)(2)/4 pi has been found up to an excitation energy of 2.7 MeV. The B(GT) distribution obtained is compared with predictions from state-of-the-art theoretical calculations to learn about the ground state deformation of Kr-72. Although a dominant oblate deformation is suggested by direct comparison with quasiparticle random phase approximation (QRPA) calculations, beyond-mean-field and shell-model calculations favor a large oblate-prolate mixing in the ground state.
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Perez-Cerdan, A. B., Rubio, B., Gelletly, W., Algora, A., Agramunt, J., Burkard, K., et al. (2011). beta decay of (78)Sr. Phys. Rev. C, 84(5), 054311–15pp.
Abstract: The gamma rays and conversion electrons emitted in the beta decay of (78)Sr to levels in (78)Rb have been studied using Ge detectors and a mini-orange spectrometer. A reliable level scheme based on the results of these experiments has been established. The properties of the levels in (78)Rb have been compared with calculations based on deformed Hartree-Fock with Skyrme interactions and pairing correlations in the BCS approximation. This has allowed an interpretation of the nature of the observed sets of levels in the odd-odd nucleus (78)Rb.
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IDS Collaboration(Andel, B. et al), Algora, A., & Nacher, E. (2021). New beta-decaying state in Bi-214. Phys. Rev. C, 104(5), 054301–13pp.
Abstract: A new beta-decaying state in Bi-214 has been identified at the ISOLDE Decay Station at the CERN-ISOLDE facility. A preferred I-pi = (8(-)) assignment was suggested for this state based on the beta-decay feeding pattern to levels in Po-214 and shell-model calculations. The half-life of the I-pi = (8) state was deduced to be T-1/2 = 9.39(10) min. The deexcitation of the levels populated in Po-214 by the beta decay of this state was investigated via gamma-gamma coincidences and a number of new levels and transitions was identified. Shell-model calculations for excited states in Bi-214 and Po-214 were performed using two different effective interactions: the H208 and the modified Kuo-Herling particle interaction. Both calculations agree on the interpretation of the new beta-decaying state as an I-pi = 8 – isomer and allow for tentative assignment of shell-model states to several high-spin states in Po-214.
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IDS Collaboration(Lica, R. et al), Algora, A., & Nacher, E. (2025). Revealing the Nature of yrast States in Neutron-Rich Polonium Isotopes. Phys. Rev. Lett., 134(5), 052502–7pp.
Abstract: Polonium isotopes having two protons above the shell closure at Z = 82 show a wide variety of lowlying, high-spin isomeric states across the whole chain. The structure of neutron-deficient isotopes up to 210Po (N = 126) is well established as they are easily produced through various methods. However, there is not much information available for the neutron-rich counterparts for which only selective techniques can be used for their production. We report on the first fast-timing measurements of yrast states up to the 8+ level in 214,216,218Po isotopes produced in the beta- decay of 214,216,218Bi at ISOLDE, CERN. In particular, our new half-life value of 607(14) ps for the 8+1 state in 214Po is nearly 20 times shorter than the value available in the literature and comparable with the newly measured half-lives of 409(16) and 628(25) ps for the corresponding 8+1 states in 216,218Po, respectively. The measured B(E2; 8+1 -> 6+1 ) transition probability values follow an increasing trend relative to isotope mass, reaching a maximum for 216Po. The increase contradicts the previous claims of isomerism for the 8+ yrast states in neutron-rich 214Po and beyond. Together with the other measured yrast transitions, the B(E2) values provide a crucial test of the different theoretical approaches describing the underlying configurations of the yrast band. The new experimental results are compared to shell-model calculations using the KHPE and H 208 effective interactions and their pairing-modified versions, showing an increase in configuration mixing when moving toward the heavier isotopes.
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