Les isotopes d'azote au-delà de la limite de stabilité neutronique : $^{23}N$, $^{24}N$ et $^{25}N$

Abstract : To study the most neutron-rich nitrogen isotopes an experiment has been undertaken at the RIBF-RIKEN using the SAMURAI spectrometer and NEBULA neutron array. The nuclei of interest - 23N*, 24N and 25N - were produced via nucleon knockout and fragmentation reactions from high-energy (~250 MeV/nucleon) secondary beams. The technique of invariant-mass spectroscopy, which requires the measurement in complete kinematics of the beam-like reaction products, was employed to characterise these unbound systems. In the case of the 23N, in-flight gamma-ray spectroscopy was performed and it has been possible to confirm that it has no bound excited states. Three reaction channels - the knockout of a proton from 24O, the fragmentation of 27Ne and inelastic scattering – were employed to search for unbound excited states. In all these channels, an l=0 resonance was observed at around 3.5 MeV excitation energy. This resonance is interpreted, through comparison with shell model calculations, as the Jpi=3/2- first excited state of 23N. In all channels, another resonance with an excitation energy of close to 5 MeV was necessary to fully describe the fragment-neutron relative energy spectra. The nucleus 24N was observed here for the first time as resonance-like peak some 1.3 MeV above the one-neutron decay threshold. Four reaction channels were investigated: the knockout of two and three protons from 26F and 27Ne, respectively, and fragmentation of 27F and 28Ne. All the relative energy spectra were consistent with the population of an l=2 resonance. Simple considerations suggest that this is the 2-,1- ground-state doublet predicted by the shell model. The nucleus 25N was also observed here for the first time. Despite the relatively limited statistics, both two-proton and three-proton removal from 27F and 28Ne, exhibited a clear structure some 1.7 MeV above the two-neutron decay threshold which, based on simple considerations, may be identified with the expected 1/2- ground state.
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Quentin Deshayes. Les isotopes d'azote au-delà de la limite de stabilité neutronique : $^{23}N$, $^{24}N$ et $^{25}N$. Physique Nucléaire Expérimentale [nucl-ex]. Normandie Université, Université Caen Normandie, 2017. Français. ⟨NNT : 2017NORMC243⟩. ⟨tel-01696611⟩

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