Nitrogen isotopes beyond the neutron drip line: 23N, 24N et 25N
Description
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 Jπ = 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. (author)
Abstract (French)
Afin d'etudier les limites d'existence de la chaine isotopique des azotes, une experience a ete menee au RIBF-RIKEN en utilisant le spectrometre SAMURAI couple au detecteur de neutrons NEBULA. Les systemes etudies - 23N*, 24N et 25N ont ete produits via des reactions de knockout de quelques nucleons ou de fragmentation a partir de faisceau secondaires de haute energie (∼250 MeV/nucleon). La methode utilisee pour caracteriser ces systemes est celle de la masse invariante qui necessite la cinematique complete des reactions etudiees. Pour interpreter les resultats, une simulation de la totalite du dispositif experimental a ete utilisee. L'etalonnage des detecteurs et les techniques d'analyse ont ete testes en sondant l'etat fondamental connu du 16B. Dans le cas du 23N, une etude de spectroscopie gamma en vol a permis de confirmer qu'il ne possedait pas d'etat excite lie. Nous avons pu le sonder a travers 3 voies de reactions distinctes: Le knockout d'un proton du 24O, la fragmentation a partir du 27Ne et la diffusion inelastique. Dans tous les cas, nous avons observe une resonance l = 0 a environ 3,5 MeV d'energie d'excitation. Cette resonance a ete interpretee, en s'appuyant sur des calculs de modele en couche, comme le premier etat excite du 23N de spin parite Jπ = 3/2-. Dans l'ensemble des voies une seconde resonance possedant une energie d'excitation d'environ 5 MeV etait necessaire pour decrire les spectres en energie-relative fragment-neutron mesures. Le 24N a ete observe pour la premiere fois lors de notre experience comme une resonance autour de 1,3 MeV au-dessus du seuil d'emission neutron. Nous avons pu sonder ce systeme via 4 reactions, le knockout de deux ou trois protons respectivement du 26F et du 27Ne et des reactions de fragmentation a partir du 27F et du 28Ne. L'ensemble de ces spectres peut etre ajuste a l'aide d'une resonance l=2. Des considerations theoriques simples nous suggerent que cette derniere correspond au doublet 2-,1- predit comme l'etat fondamental du 24N par le modele en couche. Le 25N a egalement ete observe pour la premiere fois lors de notre experience. Malgre une statistique relativement limitee, les spectres des reactions de knockout de deux et trois protons du 27F et 28Ne, montrent une structure claire environ 1,7 MeV au-dessus du seuil d'emission de deux neutrons qui peut etre identifiee comme l'etat fondamental 1/2- predit par le modele en couche. (auteur)
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Additional details
Additional titles
- Original title (French)
- Les isotopes d'azote au-dela de la limite de stabilite neutronique: 23N, 24N et 25N
Publishing Information
- Imprint Pagination
- 129 p.
- Report number
- FRNC-TH--13835
INIS
- Country of Publication
- France
- Country of Input or Organization
- France
- INIS RN
- 54020349
- Subject category
- S73: NUCLEAR PHYSICS AND RADIATION PHYSICS;
- Resource subtype / Literary indicator
- Thesis
- Descriptors DEI
- BEAM PRODUCTION; DATA ANALYSIS; ENERGY LEVELS; NUCLEAR STRUCTURE; PARTICLE IDENTIFICATION; RADIOACTIVE ION BEAMS
- Descriptors DEC
- BEAMS; DATA PROCESSING; ION BEAMS; PROCESSING
Optional Information
- Notes
- 49 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses