Spectroscopie du 19^{19}Ne par diffusion inélastique. Applications à l'astrophysique.

The gamma observation of novae explosions is a one of the objectives of space telescopes like INTEGRAL. According to astrophysical models, the most intense ray of the gamma spectra of these phenomena has an energy of 511 keV. This ray is mainly due to the annihilation of ß+ coming from 18F decay. The astrophysical interpretation of the gamma observations can be done only if the nucleosynthesis of 18F is well understood. Actually, poor knowledge of the rate of the 18F(p,a)15O reaction, linked to the structure of the compound nucleus 19Ne, creates an uncertainty factor of 300 on the abundance of 18F in novae. In order to reduce these uncertainties, an inelastic scaterring experiment 19Ne(p,p')19Ne* was performed to the Louvain la Neuve laboratory. It allowed us to study excited states of 19Ne above alpha (3.55 MeV) and proton (6.42 MeV) thresholds. The study of angular correlation of particules decay (proton or alpha) was used to assign for the first time the spin of many known levels. In addition, a new large ½+ state, which could influence strongly the astrophysical factor, was observed for the first time.L'observation gamma des explosions novae est l'un des objectifs des télescopes spatiaux comme INTEGRAL. Selon les modèles astrophysiques la raie la plus intense du spectre gamma des novae est celle ayant une énergie de 511 keV. Cette raie provient essentiellement de l'annihilation des ß+ venant de la décroissance du noyau de 18F. L'interprétation astrophysique des observations gamma ne pourra se faire que si la nucléosynthèse du 18F est bien comprise. Actuellement, la mauvaise connaissance du taux de la réaction 18F(p,a)15O, lié à la structure du noyau composé 19Ne, engendre une incertitude d'un facteur 300 sur l'abondance du 18F dans les novae. Dans le but de réduire ces incertitudes, une expérience de diffusion inélastique d'un faisceau de noyaux radioactifs 19Ne sur une cible de proton (19Ne(p,p')19Ne*) a été réalisée au laboratoire de Louvain la Neuve. Elle a permis d'étudier notamment plusieurs états excités du 19Ne au dessus des seuils alpha (3.55 MeV) et proton (6.42 MeV). L'étude des distributions angulaires des particules émises (proton ou alpha) par les états excités du 19Ne a permis d'assigner pour la première fois le spin de nombreux niveaux connus. De plus, un nouvel état ½+ large, pouvant influencer fortement le facteur astrophysique, a été observé pour la première fois

Resonant elastic and inelastic scattering. Astrophysical applications. New paradigm beyond drip-lines?

5 pages, 4 figures, Expérience GANIL/SPIRALInternational audienceTwo experimental techniques have been developed at GANIL using resonant elastic and inelastic scattering reactions in inverse kinematics. These techniques were used to study the structure of unstable nuclei. A brief description of the methods is presented through two examples of application in astrophysics. Moreover, new ideas and simple questions are put forward: what happens in the low energy tail of unbound nuclei ground state resonances

First direct observation of two protons in the decay of 45^{45}Fe with a TPC

The decay of the ground-state two-proton emitter 45Fe was studied with a time-projection chamber and the emission of two protons was unambiguously identified. The total decay energy and the half-life measured in this work agree with the results from previous experiments. The present result constitutes the first direct observation of the individual protons in the two-proton decay of a long-lived ground-state emitter. In parallel, we identified for the first time directly two-proton emission from 43Cr, a known beta-delayed two-proton emitter. The technique developped in the present work opens the way to a detailed study of the mechanism of ground-state as well as beta-delayed two-proton radioactivity.Comment: 4 pages, 5 figure

New pathway to bypass the 15O waiting point

We propose the sequential reaction process $^{15}$O($p$,$\gamma)(\beta^{+}$)$^{16}$O as a new pathway to bypass of the $^{15}$O waiting point. This exotic reaction is found to have a surprisingly high cross section, approximately 10$^{10}$ times higher than the $^{15}$O($p$,$\beta^{+}$)$^{16}$O. These cross sections were calculated after precise measurements of energies and widths of the proton-unbound $^{16}$F low lying states, obtained using the H($^{15}$O,p)$^{15}$O reaction. The large $(p,\gamma)(\beta^{+})$ cross section can be understood to arise from the more efficient feeding of the low energy wing of the ground state resonance by the gamma decay. The implications of the new reaction in novae explosions and X-ray bursts are discussed.Comment: submitte

Collapse of the N=28 shell closure in 42^{42}Si

The energies of the excited states in very neutron-rich $^{42}$Si and $^{41,43}$P have been measured using in-beam $\gamma$-ray spectroscopy from the fragmentation of secondary beams of $^{42,44}$S at 39 A.MeV. The low 2$^+$ energy of $^{42}$Si, 770(19) keV, together with the level schemes of $^{41,43}$P provide evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that $^{42}$Si is best described as a well deformed oblate rotor.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. let

Spectroscopy around 36^{36}Ca

Expérience GANILInternational audienceAn experiment was performed to study excited states in neutron-deficient nuclei around Ca. A one-neutron knockout reaction was used to produce $^{36}$Ca ions from a $^{37}$Ca secondary beam, and in-beam $\gamma$-rays were measured. The $2^+$ energy in $^{36}$Ca is compared to the mirror nucleus $^{36}$S to deduce information on the isospin dependence of the nuclear force near the proton drip line. The energy of the first excited $2^+$ state in $^{36}$Ca and the cross section for the 1-neutron knock-out reaction from $^{37}$Ca at $\sim$ 45 · AMeV were obtained. Furthermore, for two other $T_z$ = −2 nuclei, $^{28}$S and $^{32}$Ar, the de-excitation of the first $2^+$ state has been observed

In-beam spectroscopic studies of 44^{44}S nucleus

The structure of the $^{44}$S nucleus has been studied at GANIL through the one proton knock-out reaction from a $^{45}$Cl secondary beam at 42 A$\cdot$MeV. The $\gamma$ rays following the de-excitation of $^{44}$S were detected in flight using the 70 BaF${_2}$ detectors of the Ch\^{a}teau de Cristal array. An exhaustive $\gamma\gamma$-coincidence analysis allowed an unambiguous construction of the level scheme up to an excitation energy of 3301 keV. The existence of the spherical 2$^+_2$ state is confirmed and three new $\gamma$-ray transitions connecting the prolate deformed 2$^+_1$ level were observed. Comparison of the experimental results to shell model calculations further supports a prolate and spherical shape coexistence with a large mixing of states built on the ground state band in $^{44}$S.Comment: 6 pages, 5 figures, accepted for publication in Physical Review

Spectroscopie du 19Ne par diffusion inélastique (Applications à l'astrophysique)

L observation gamma des explosions novae est l un des objectifs des télescopes spatiaux comme INTEGRAL. Selon les modèles astrophysiques la raie la plus intense du spectre gamma des novae est celle ayant une énergie de 511 keV. Cette raie provient essentiellement de l annihilation des b+ venant de la décroissance du noyau de 18F. L interprétation astrophysique des observations gamma ne pourra se faire que si la nucléosynthèse du 18F est bien comprise. Actuellement, la mauvaise connaissance du taux de la réaction 18F(p,alpha)15O, lié à la structure du noyau composé 19Ne, engendre une incertitude d un facteur 300 sur l abondance du 18F dans les novae. Dans le but de réduire ces incertitudes, une expérience de diffusion inélastique d un faisceau de noyaux radioactifs 19Ne sur une cible de proton (19Ne(p,p )19Ne*) a été réalisée au laboratoire de Louvain la Neuve. Elle a permis d étudier notamment plusieurs états excités du 19Ne au dessus des seuils alpha (3.55 MeV) et proton (6.42 MeV). L étude des distributions angulaires des particules émises (proton ou alpha) par les états excités du 19Ne a permis d assigner pour la première fois le spin de nombreux niveaux connus. De plus, un nouvel état 1/2 + large, pouvant influencer fortement le facteur astrophysique, a été observé pour la première fois.The gamma observation of novae explosions is a one of the objectives of space telescopes like INTEGRAL. According to astrophysical models, the most intense ray of the gamma spectra of these phenomena has an energy of 511 keV. This ray is mainly due to the annihilation of b+ coming from 18F decay. The astrophysical interpretation of the gamma observations can be done only if the nucleosynthesis of 18F is well understood. Actually, poor knowledge of the rate of the 18F(p,alpha)15O reaction, linked to the structure of the compound nucleus 19Ne, creates an uncertainty factor of 300 on the abundance of 18F in novae. In order to reduce these uncertainties, an inelastic scaterring experiment 19Ne(p,p )19Ne* was performed to the Louvain la Neuve laboratory. It allowed us to study excited states of 19Ne above alpha (3.55 MeV) and proton (6.42 MeV) thresholds. The study of angular correlation of particules decay (proton or alpha) was used to assign for the first time the spin of many known levels. In addition, a new large 1/2 + state, which could influence strongly the astrophysical factor, was observed for the first time.CAEN-BU Sciences et STAPS (141182103) / SudocSTRASBOURG-Bib.Central Recherche (674822133) / SudocSudocFranceF