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Siciliano, M. et al, & Gadea, A. (2020). Shape coexistence in neutron-deficient Hg-188 investigated via lifetime measurements. Phys. Rev. C, 102(1), 014318–16pp.
Abstract: Background: Shape coexistence in the Z approximate to 82 region has been established in mercury, lead, and polonium isotopes. For even-even mercury isotopes with 100 <= N <= 106 multiple fingerprints of this phenomenon are observed, which seems to be no longer present for N >= 110. According to a number of theoretical calculations, shape coexistence is predicted in the Hg-188 isotope. Purpose: The aim of this work was to measure lifetimes of excited states in Hg-188 to infer their collective properties, such as the deformation. Extending the investigation to higher-spin states, which are expected to be less affected by band-mixing effects, can provide additional information on the coexisting structures. Methods: The Hg-188 nucleus was populated using two different fusion-evaporation reactions with two targets, Gd-158 and Gd-160, and a beam of S-34 provided by the Tandem-ALPI accelerator complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the gamma rays were detected using the GALILEO gamma-ray spectrometer. Lifetimes of excited states were determined using the recoil-distance Doppler-shift method, employing the dedicated GALILEO plunger device. Results: Lifetimes of the states up to spin 16 (h) over bar were measured and the corresponding reduced transition probabilities were calculated. Assuming two-band mixing and adopting, as done commonly, the rotational model, the mixing strengths and the deformation parameters of the unperturbed structures were obtained from the experimental results. In order to shed light on the nature of the observed configurations in the Hg-188 nucleus, the extracted transition strengths were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach, limited to axial shapes, and the five-dimensional collective Hamiltonian, including the triaxial degree of freedom. Conclusions: The first lifetime measurement for states with spin >= 6 suggested the presence of an almost spherical structure above the 12(1)(+) isomer and allowed elucidating the structure of the intruder band. The comparison of the extracted B(E2) strengths with the two-band mixing model allowed the determination of the ground-state band deformation. Both beyond-mean-field calculations predict coexistence of a weakly deformed band with a strongly prolate-deformed one, characterized by elongation parameters similar to those obtained experimentally, but the calculated relative position of the bands and their mixing strongly differ.
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Siciliano, M. et al, Gadea, A., & Perez-Vidal, R. M. (2021). Lifetime measurements in the even-even Cd102-108 isotopes. Phys. Rev. C, 104(3), 034320–16pp.
Abstract: Background: The heaviest T-z = 0 doubly-magic nucleus, Sn-100, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction. For instance, the structure of light-Sn nuclei has been shown to be affected by the delicate balance between nuclear-interaction components, such as pairing and quadrupole correlations. From Cd to Te, many common features and phenomena have been observed experimentally along the isotopic chains, leading to theoretical studies devoted to a more general and comprehensive study of the region. In this context, having only two proton holes in the Z = 50 shell, the Cd isotopes are expected to present properties similar to those found in the Sn isotopic chain. Purpose: The aim of this work was to measure lifetimes of excited states in neutron-deficient nuclei in the vicinity of Sn-100. Methods: The neutron-deficient nuclei in the N approximate to Z approximate to 50 region were populated using a multinucleon transfer reaction with a Cd-106 beam and a Mo-92 target. The beamlike products were identified by the VAMOS++ spectrometer, while the gamma rays were detected using the AGATA array. Lifetimes of excited states were determined using the recoil distance Doppler-shift method, employing the Cologne differential plunger. Results: Lifetimes of low-lying states were measured in the even-mass Cd-102-(108) isotopes. In particular, multiple states with excitation energy up to MeV, belonging to various bands, were populated in approximate to 3 Cd-106 via inelastic scattering. The transition strengths corresponding to the measured lifetimes were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach. Conclusions: Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z = 48 and Z = 50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with beta(2) approximate to 0.2. At this deformation Z = 48 becomes a closed-shell configuration, which is favored with respect to the spherical one.
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Soderstrom, P. A. et al, Agramunt, J., Egea, J., Gadea, A., & Huyuk, T. (2019). Neutron detection and gamma-ray suppression using artificial neural networks with the liquid scintillators BC-501A and BC-537. Nucl. Instrum. Methods Phys. Res. A, 916, 238–245.
Abstract: In this work we present a comparison between the two liquid scintillators BC-501A and BC-537 in terms of their performance regarding the pulse-shape discrimination between neutrons and gamma rays. Special emphasis is put on the application of artificial neural networks. The results show a systematically higher gamma-ray rejection ratio for BC-501A compared to BC-537 applying the commonly used charge comparison method. Using the artificial neural network approach the discrimination quality was improved to more than 95% rejection efficiency of gamma rays over the energy range 150 to 1000 keV for both BC-501A and BC-537. However, due to the larger light output of BC-501A compared to BC-537, neutrons could be identified in BC-501A using artificial neural networks down to a recoil proton energy of 800 keV compared to a recoil deuteron energy of 1200 keV for BC-537. We conclude that using artificial neural networks it is possible to obtain the same gamma-ray rejection quality from both BC-501A and BC-537 for neutrons above a low-energy threshold. This threshold is, however, lower for BC-501A, which is important for nuclear structure spectroscopy experiments of rare reaction channels where low-energy interactions dominates.
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AGATA Collaboration(Soderstrom, P. A. et al), & Gadea, A. (2011). Interaction position resolution simulations and in-beam measurements of the AGATA HPGe detectors. Nucl. Instrum. Methods Phys. Res. A, 638(1), 96–109.
Abstract: The interaction position resolution of the segmented HPGe detectors of an AGATA triple cluster detector has been studied through Monte Carlo simulations and in an in-beam experiment. A new method based on measuring the energy resolution of Doppler-corrected gamma-ray spectra at two different target to detector distances is described. This gives the two-dimensional position resolution in the plane perpendicular to the direction of the emitted gamma-ray. The gamma-ray tracking was used to determine the full energy of the gamma-rays and the first interaction point, which is needed for the Doppler correction. Five different heavy-ion induced fusion-evaporation reactions and a reference reaction were selected for the simulations. The results of the simulations show that the method works very well and gives a systematic deviation of <1 mm in the FVVHM of the interaction position resolution for the gamma-ray energy range from 60 keV to 5 MeV. The method was tested with real data from an in-beam measurement using a (30)5i beam at 64 MeV on a thin C-12 target. Pulse-shape analysis of the digitized detector waveforms and gamma-ray tracking was performed to determine the position of the first interaction point, which was used for the Doppler corrections. Results of the dependency of the interaction position resolution on the gamma-ray energy and on the energy, axial location and type of the first interaction point, are presented. The FVVHM of the interaction position resolution varies roughly linearly as a function of gamma-ray energy from 8.5 mm at 250 key to 4 mm at 1.5 MeV, and has an approximately constant value of about 4 mm in the gamma-ray energy range from 1.5 to 4 MeV.
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AGATA Collaboration(Soderstrom, P. A. et al), & Gadea, A. (2012). High-spin structure in K-40. Phys. Rev. C, 86(5), 054320–9pp.
Abstract: High-spin states of K-40 have been populated in the fusion-evaporation reaction C-12(Si-30,np)K-40 and studied by means of gamma-ray spectroscopy techniques using one triple-cluster detector of the Advanced Gamma Tracking Array at the Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro. Several states with excitation energy up to 8 MeV and spin up to 10(-) have been discovered. These states are discussed in terms of J = 3 and T = 0 neutron-proton hole pairs. Shell-model calculations in a large model space have shown good agreement with the experimental data for most of the energy levels. The evolution of the structure of this nucleus is here studied as a function of excitation energy and angular momentum.
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