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Lalovic, N., Louchart, C., Michelagnoli, C., Perez-Vidal, R. M., Ralet, D., Gerl, J., et al. (2016). Performance of the AGATA gamma-ray spectrometer in the PreSPEC set-up at GSI. Nucl. Instrum. Methods Phys. Res. A, 806, 258–266.
Abstract: In contemporary nuclear physics, the European Advanced GAmma Tracking Array (AGATA) represents a crucial detection system for cutting-edge nuclear structure studies. AGATA consists of highly segmented high-purity germanium crystals and uses the pulse-shape analysis technique to determine both the position and the energy of the y-ray interaction points in the crystals. It is the tracking algorithms that deploy this information and enable insight into the sequence of interactions, providing information on the full or partial absorption of the 7 ray. A series of dedicated performance measurements for an AGATA set-up comprising 21 crystals is described. This set-up was used within the recent PreSPEC-AGATA experimental campaign at the GSI Helmholtzzentrum fur Schwerionenforschung. Using the radioactive sources Co-56, Co-60 and Eu-152, absolute and normalized efficiencies and the peak-to-total of the array were measured. These quantities are discussed using different data analysis procedures. The quality of the pulse-shape analysis and the tracking algorithm are evaluated. The agreement between the experimental data and the Geant4 simulations is also investigated.
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Banik, R. et al, & Perez-Vidal, R. M. (2020). High-spin states above the isomers in neutron-rich iodine nuclei near N=82. Phys. Rev. C, 102(4), 044329–15pp.
Abstract: Excited states of neutron-rich iodine isotopes I130-134 above the high-spin isomers have been identified using prompt-delayed gamma-ray spectroscopy. The iodine isotopes were produced as fission fragments of fusion-fission and transfer induced fission of 9Be(U-238, f) at a beam energy of 6.2 MeV/u. The complete (A, Z) identification was obtained using the large acceptance magnetic spectrometer VAMOS++. The AGATA gamma-ray tracking array was used to detect the prompt gamma rays while the delayed gamma rays (in the time range of 100 ns to 200 μs) from the isomeric states were identified by the EXOGAM segmented clover detectors, placed behind the focal plane of the VAMOS++ spectrometer. The high-spin states above the (8(-)) isomers in I-130,I-132 were populated for the first time, and a new isomer in I-132 was identified. A new gamma-ray transition was also assigned to the level structure of I-134. Prompt transitions above the 19/2- isomer were identified in I-131,I-133, for the first time. The level structures are interpreted in terms of the systematics of odd-Z nuclei above the Z = 50 shell closure and large-scale shell model calculations.
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AGATA Collaboration(Liu, X. et al), Gadea, A., Jurado, M., Domingo-Pardo, C., Huyuk, T., & Perez-Vidal, R. M. (2022). Evidence for spherical-oblate shape coexistence in Tc-87. Phys. Rev. C, 106(3), 034304–6pp.
Abstract: Excited states in the neutron-deficient nucleus Tc-87 have been studied via the fusion-evaporation reaction 54Fe(36Ar, 2n1p) Tc-87 at 115 MeV beam energy. The AGATA gamma-ray spectrometer coupled to the DIAMANT, NEDA, and Neutron Wall detector arrays for light-particle detection was used to measure the prompt coincidence of gamma rays and light particles. Six transitions from the deexcitation of excited states belonging to a new band in Tc-87 were identified by comparing gamma-ray intensities in the spectra gated under different reaction channel selection conditions. The constructed level structure was compared with the shell model and total Routhian surface calculations. The results indicate that the new band structure in 87Tc is built on a spherical configuration, which is different from that assigned to the previously identified oblate yrast rotational band.
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Boso, A. et al, Domingo-Pardo, C., & Perez-Vidal, R. M. (2019). Isospin dependence of electromagnetic transition strengths among an isobaric triplet. Phys. Lett. B, 797, 134835–6pp.
Abstract: Electric quadrupole matrix elements, M-p, for the J(pi) = 2(+) -> 0(+), Delta T = 0, T = 1 transitions across the A = 46 isobaric multiplet Cr-46-V-46-Ti-46 have been measured at GSI with the FRS-LYCCA-AGATA setup. This allows direct insight into the isospin purity of the states of interest by testing the linearity of M-p with respect to T-z. Pairs of nuclei in the T = 1 triplet were studied using identical reaction mechanisms in order to control systematic errors. The M-p values were obtained with two different methodologies: (i) a relativistic Coulomb excitation experiment was performed for Cr-46 and Ti-46; (ii) a “stretched target” technique was adopted here, for the first time, for lifetime measurements in V-46 and Ti-46. A constant value of M-p across the triplet has been observed. Shell-model calculations performed within the fp shell fail to reproduce this unexpected trend, pointing towards the need of a wider valence space. This result is confirmed by the good agreement with experimental data achieved with an interaction which allows excitations from the underlying sd shell. A test of the linearity rule for all published data on complete T = 1 isospin triplets is presented.
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Biswas, S. et al, & Perez-Vidal, R. M. (2020). Prompt-delayed gamma-ray spectroscopy of neutron-rich In-119, In-121 isotopes. Phys. Rev. C, 102(1), 014326–10pp.
Abstract: Background: The Z = 50 shell closure, near N = 82, is unique in the sense that it is the only shell closure with the spin-orbit partner orbitals, pi g(9/2) and pi g(7/2), enclosing the magic gap. The interaction of the proton hole/particle in the above-mentioned orbitals with neutrons in the nu h(11)(/2) orbital is an important prerequisite to the understanding of the nuclear structure near N = 82 and the nu pi interaction. Purpose: To explore the structural similarity between the high-spin isomeric states in In (Z = 49), Sn (Z = 50), and Sb (Z = 51) isotopes from a microscopic point of view. In addition, to understand the role of a proton hole or particle in the spin-orbit partner orbitals, pi g(9/2) and pi g(7/2), respectively, with neutron holes in the nu h(11)(/2) orbital on these aforementioned isomers. Methods: The fusion and transfer induced fission reaction Be-9(U-238, f) with 6.2 MeV/u beam energy, using a unique setup consisting of AGATA, VAMOS ++, and EXOGAM detectors, was used to populate through the fission process and study the neutron-rich In-119,In-121 isotopes. This setup enabled the prompt-delayed gamma-ray spectroscopy of isotopes in the time range of 100 ns-200 μs. Results: In the odd-A In-119,In-121 isotopes, indications of a short half-life 19/2(-) isomeric state, in addition to the previously known 25/2(+) isomeric state, were observed from the present data. Further, new prompt transitions above the 25/2(+) isomer in In-121 were identified along with reevaluation of its half-life. Conclusions: The experimental data were compared with the theoretical results obtained in the framework of large-scale shell-model calculations in a restricted model space. The <pi g(9/2)nu h(11/2); I vertical bar H vertical bar pi g(9/2) nu h(11/2);I > two-body matrix elements of residual interaction were modified to explain the excitation energies and the B(E2) transition probabilities in the neutron-rich In isotopes. The (i) decreasing trend of E(29/2(+))-E(25/2(+)) in odd-In (with dominant configuration pi g(9/)(2)(-1) nu h(11/2)(-2) and maximum aligned spin of 29/2+) and (ii) increasing trend of E(27/2(+)) – E(23/2(+)) in odd-Sb (with dominant configuration pi g(7/)(2)(+1) nu h(11/2)(-2) and maximum aligned spin of 27/2(+)) with increasing neutron number could be understood as a consequence of hole-hole and particle-hole interactions, respectively.
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