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Öğe Accessing tens-to-hundreds femtoseconds nuclear state lifetimes with low-energy binary heavy-ion reactions(Springer, 2021) Ciemala, M.; Ziliani, S.; Crespi, F. C. L.; Leoni, S.; Fornal, B.; Maj, A.; Bednarczyk, P.A novel Monte Carlo technique has been developed to determine lifetimes of excited states in the tens-to-hundreds femtoseconds range in products of low-energy heavy-ion binary reactions, with complex velocity distributions. The method is based on a detailed study of Doppler-broadened gamma-ray lineshapes. Its relevance is demonstrated in connection with the 18O(7.0MeV/u)+181Ta experiment, performed at GANIL with the AGATA+VAMOS+PARIS setup, to study neutron-rich O, C, N, ... nuclei. Excited states in O-17 and O-19, with known lifetimes, are used to validate the method over the similar to 20-400fs lifetime-sensitivity range. Emphasis is given to the unprecedented position resolution provided by gamma-tracking arrays, which turns out to be essential for reaching the required accuracy in Doppler-shift correction. The technique is anticipated to be an important tool for lifetime investigations in exotic neutron-rich nuclei, produced with intense ISOL-type beams.Öğe Complete set of bound negative-parity states in the neutron-rich nucleus 18N(Amer Physical Soc, 2021) Ziliani, S.; Ciemala, M.; Crespi, F. C. L.; Leoni, S.; Fornal, B.; Suzuki, T.; Otsuka, T.High-resolution. -ray spectroscopy of N-18 is performed with the Advanced GAmma Tracking Array, following deep-inelastic processes induced by an O-18 beam on a Ta-181 target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in N-18 below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the 0(1)(-) and 1(2)(-) excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions and help to simultaneously reproduce the ground and first-excited states in N-16 and N-18, for the first time. Understanding the N-18 structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.Öğe Lifetime analysis of short-lived states in 17N(Soc Italiana Fisica, 2021) Ziliani, S.; Ciemala, M.; Crespi, F. C. L.; Leoni, S.; Fornal, B.; Maj, A.; Bednarczyk, P.A recent extension of the Doppler-shift attenuation method to measure short lifetimes of states populated in low-energy binary reactions is applied to the case of N-17. The reliability of the technique is confirmed by measuring the lifetime of the 5515 keV 3/2(-) state, which is depopulated by two different gamma rays. The method is used to measure the lifetime of the 5170 keV (9/2(+)) state in the same nucleus. Comparisons with large-scale shell-model predictions are given as well.Öğe SPECTROSCOPY OF NEUTRON-RICH NITROGEN ISOTOPES WITH AGATA plus PARIS plus VAMOS(Jagiellonian Univ Press, 2020) Ziliani, S.; Ciemala, M.; Crespi, F. C. L.; Leoni, S.; Fornal, B.; Maj, A.; Bednarczyk, P.Excited states of N-17, N-18 and N-19 were investigated through the measurement of gamma rays, following their population via deep-inelastic reactions induced by an O-18 beam (7 MeV/u) on a thick Ta-181 target. The experimental setup comprised the AGATA+PARIS detection system, coupled to the VAMOS++ magnetic spectrometer. In the N-17 nucleus, the analysis of gamma-ray transitions de-exciting two states around 4-5 MeV clearly pointed to discrepancies with the lifetime values reported in literature. Three new gamma rays were observed in N-18 at the energies of 1662.3 (3) keV, 2073.4 (8) keV and 2300.9 (8) keV, and hints for other two new transitions around 1566 keV and 1720 keV were found. In addition, a new transition with energy of 2489.7 (8) keV was observed in N-19.Öğe Testing ab initio nuclear structure in neutron-rich nuclei: Lifetime measurements of second 2+ state in 16C and 20O(Amer Physical Soc, 2020) Ciemala, M.; Ziliani, S.; Crespi, F. C. L.; Leoni, S.; Fornal, B.; Maj, A.; Bednarczyk, P.To test the predictive power of ab initio nuclear structure theory, the lifetime of the second 2(+) state in neutron-rich O-20, tau(2(2)(+)) = 150(-30)(+80) fs, and an estimate for the lifetime of the second 2(+) state in C-16 have been obtained for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens-to-hundreds of femtoseconds range by analyzing the Doppler-shifted gamma-transition line shapes of products of low-energy transfer and deep-inelastic processes in the reaction O-18 (7.0 MeV/u) + Ta-181. The requested sensitivity could only be reached owing to the excellent performances of the Advanced gamma-Tracking Array AGATA, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The experimental lifetimes agree with predictions of ab initio calculations using two- and three-nucleon interactions, obtained with the valence-space in-medium similarity renormalization group for O-20 and with the no-core shell model for C-16. The present measurement shows the power of electromagnetic observables, determined with high-precision gamma spectroscopy, to assess the quality of first-principles nuclear structure calculations, complementing common benchmarks based on nuclear energies. The proposed experimental approach will be essential for short lifetime measurements in unexplored regions of the nuclear chart, including r-process nuclei, when intense beams, produced by Isotope Separation On-Line (ISOL) techniques, become available.
