Search for 22^{22}Na in novae supported by a novel method for measuring femtosecond nuclear lifetimes

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of $^{26}$Al and $^{22}$Na. While gamma rays from the decay of the former radioisotope have been observed throughout the Galaxy, $^{22}$Na remains untraceable. The half-life of $^{22}$Na (2.6 yr) would allow the observation of its 1.275 MeV gamma-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of the nuclear reactions involved in the production and destruction of this nucleus. The $^{22}$Na($p,\gamma$)$^{23}$Mg reaction remains the only source of large uncertainty about the amount of $^{22}$Na ejected. Its rate is dominated by a single resonance on the short-lived state at 7785.0(7) keV in $^{23}$Mg. In the present work, a combined analysis of particle-particle correlations and velocity-difference profiles is proposed to measure femtosecond nuclear lifetimes. The application of this novel method to the study of the $^{23}$Mg states, combining magnetic and highly-segmented tracking gamma-ray spectrometers, places strong limits on the amount of $^{22}$Na produced in novae, explains its non-observation to date in gamma rays (flux < 2.5x$10^{-4}$ ph/(cm$^2$s)), and constrains its detectability with future space-borne observatories.Comment: 18 pages, 3 figures, 1 tabl

Structure Nucléaire aux Extrêmes de Déformation et de Charge

Le principal axe de recherche depuis ma thèse a été l'étude des formes extrêmes du noyau avec un accent particulier sur la désexcitation des noyaux superdéformés. Depuis 2002, je me suis orientée vers l'investigation des propriétés des noyaux très lourds avec l'étude des premiers états excités et états isomériques des noyaux au delà du Fm. Dans les 2 cas, l'outil privilégié est la spectroscopie γ . C'est donc logiquement que je me suis investie dans la recherche et le développement associés à la nouvelle génération de multidétecteurs 4πγ : AGATA

Stablité des Eléments Trans-ferminums à Haut Spin (Mesure de la barrière de fission de 254No)

Les noyaux super lourds offrent la possibilité d étudier la structure nucléaire à trois limites simultanément: en charge Z, spin I et énergie d excitation E . Ces noyaux existent seulement grâce à une barrière de fission créée par les effets de couche. Il est donc important de déterminer cette barrière de fission et sa dépendance en spin Bf(I), qui nous renseigne sur l énergie de couche Eshell(I). Les théories prédisent des valeurs différentes pour la hauteur de la barrière de fission, allant de Bf(I = 0) = 6.8 MeV dans un modèle macro-microscopique à 8.7 MeV pour des calculs de la théorie de la fonctionnelle de la densité utilisant l interaction Gogny ou Skyrme. Une mesure de Bf fournit donc un test des théories.Pour étudier la barrière de fission, la méthode établie est de mesurer, par réaction de transfert, l augmentation de la fission avec l énergie d excitation, caractérisée par le rapport des largeurs de décroissance fission/ total,. Cependant, pour les éléments lourds comme 254No, il n existe pas de cible appropriée pour une réaction de transfert. Il faut s en remettre à un rapport de largeur de décroissance complémentaire: g/ fission et sa dépendance en spin, déduite de la distribution d entrée (I, E ).Des mesures de la multiplicité et l énergie totale des rayons g de254No ont été faites aux énergies de faisceau 219 et 223 MeV pour la réaction 208Pb(48Ca,2n) à ATLAS (Argonne Tandem Linac Accelerator System). Les rayons g du 254No ont été détectés par le multi-détecteur Gammasphere utilisé comme calorimètre et aussi comme détecteur de rayons g de haute résolution. Les coïncidences avec les résidus d évaporation au plan focal du Fragment Mass Analyzer ont permis de séparer les rayons g du 254No de ceux issus de la fission, qui sont > 10^6 fois plus intenses. De ces mesures, la distribution d entrée c est-à-dire la distribution initiale en I et E est reconstruite. Chaque point (I,E ) de la distribution d entrée est un point où la décroissance g l a emporté sur la fission, et ainsi, contient une information sur la barrière de fission.La distribution d entrée mesurée montre une augmentation du spin maximal et de l énergie d excitation entre les énergies de faisceau 219 et 223 MeV. La distribution présente une saturation de E à hauts spins. Cette saturation est attribuée au fait que, lorsque E augmente au-dessus de la barrière, fission domine rapidement. Il en résulte une troncation de la distribution d entrée à haute énergie qui permet la détermination de la hauteur de la barrière de fission.La mesure expérimentale de la distribution d entrée est également comparée avec des distributions d entrée calculées par des simulations de cascades de décroissance qui prennent en compte le processus de formation du noyau, incluant la capture et la survie, en fonction de E et I. Dans ce travail, nous avons utilisé les codes KEWPIE2 et NRV pour simuler les distributions d entrée.Super heavy nuclei provide opportunities to study nuclear structure near three simultaneous limits: in charge Z, spin I and excitation energy E . These nuclei exist only because of a fission barrier, created by shell effects. It is therefore important to determine the fission barrier and its spin dependence Bf(I), which gives information on the shell energy Eshell(I). Theoretical calculations predict different fission barrier heights from Bf(I = 0) = 6.8 MeV for a macro-microscopic model to 8.7 MeV for Density Functional Theory calculations using the Gogny or Skyrme interactions. Hence, a measurement of Bf provides a test for theories.To investigate the fission barrier, an established method is to measure the rise of fission with excitation energy, characterized by the ratio of decay widths fission/ total, using transfer reactions. However, for heavy elements such as 254No, there is no suitable target for a transfer reaction. We therefore rely on the complementary decay widths ratio g/ fission and its spin dependence, deduced from the entry distribution (I, E ).Measurements of the gamma-ray multiplicity and total energy for 254No have been performed with beam energies of 219 and 223 MeV in the reaction 208Pb(48Ca,2n) at ATLAS (Argonne Tandem Linac Accelerator System). The 254No gamma rays were detected using the Gammasphere array as a calorimeter as well as the usual high resolution g-ray detector. Coincidences with evaporation residues at the Fragment Mass Analyzer focal plane separated 254No gamma rays from those from fission fragments, which are > 10^6 more intense. From this measurement, the entry distribution i.e. the initial distribution of I and E is constructed. Each point (I,E ) of the entry distribution is a point where gamma decay wins over fission and, therefore, gives information on the fission barrier.The measured entry distributions show an increase in the maximum spin and excitation energy from 219 to 223 MeV of beam energy. The distributions show a saturation of E for high spins. The saturation is attributed to the fact that, as E increases above the saddle, fission rapidly dominates. The resulting truncation of the entry distribution at high E allows a determination of the fission barrier height.The experimental entry distributions are also compared with entry distributions calculated with decay cascade codes which take into account the full nucleus formation process, including the capture process and the subsequent survival probability as a function of E and I. We used the KEWPIE2 and NRV codes to simulate the entry distribution.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Motional narrowing and ergodic bands in excited superdeformed states of 194Hg

International audienceThe Egamma-Egamma coincidence spectra from the electromagnetic decay of excited superdeformed states in 194Hg reveal surprisingly narrow ridges, parallel to the diagonal. 100-150 excited bands are found to contribute to these ridges, which account for nearly all the unresolved E2 decay strength. Comparison with theory suggests that these excited bands have many components in their wavefunctions, yet they display remarkable rotational coherence. This phenomenon can be explained in terms of the combination of shell effects and motional narrowing

Spectroscopy of 193Bi

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Prompt neutrons accompanying the spontaneous fission of 250^{250}No

International audienceThis paper describes the experiment conducted on the SHELS (Separator for Heavy Element Spectroscopy) separator [1] aimed at studying the properties of spontaneous fission of 250No. A separator detection system consists of a time-of-flight system and SFiNx (Spontaneous Fission, Neutrons and x-rays) detection system [2] is described. The SFiNx detection system consists of an assembly of double-sided Si detectors, around which 116 proportional neutron counters filled with 3He are placed. During experiment, 1366 events of spontaneous fission of 250No were registered. The average number of neutrons per one fission (⁠v¯ = 4.24 ± 0.13) and the distribution of neutron multiplicities were obtained, and the half-lives of the 250No were measured for the ground T1/2 = (3.89 ± 0.17) µs and isomeric states T1/2 = (39.14 ± 3.34) µs

Detailed spectroscopy of 193Bi

An experiment aiming to study shape coexistence in 193Bi has been performed. Due to its transitional character, it has an exceptionally large number of structures identified close to the yrast line. Many new states have been found, significantly extending the previously known level scheme of 193Bi, including several new rotational bands. The π i13/2 band was extended to I π = 45/2+. The I π = 31/2+ member of the π i13/2 band was found to de-excite also to a long-lived isomeric state. This isomeric state is located at 2350 keV and has a spin and parity of 29/2+. The half-life of the isomeric state was measured to be 85(3) μs and it decays via the emission of an 84 keV E2 transition. A level structure feeding this isomeric state was constructed. A low-energy, 49 keV transition has been identified to depopulate the (29/2−) isomeric state, which places it at an energy 2405 keV in the level scheme. This is the first time such a decay has been observed in the neutron-deficient Bi isotopes. A superdeformed band almost identical to that present in the neighboring isotope 191Bi, based on the 1/2[651] Nilsson orbital, has also been identified.peerReviewe

Detailed spectroscopy of 195Bi

An experiment focused on the study of shape coexistence and new high-spin structures in 195 Bi has been performed. The nucleus is in a transitional region of the bismuth isotope chain. A large number of new states have been found, resulting in a significant extension of the previously known level scheme. Several new collective structures have been identified. A strongly coupled rotational band built upon the 13 / 2 + isomeric state was extended up to I π = ( 49 / 2 + ) and an energy of 5706 keV. The I π = 31 / 2 + member of the π i 13 / 2 band was also found to feed a new long-lived isomeric state with an excitation energy of 2616 keV and a spin and parity of I π = 29 / 2 + . The half-life of the 29 / 2 + isomeric state was determined to be 1.49 ( 1 ) μ s . It decays via the emission of 457-keV E 2 and 236-keV E 1 transitions, respectively. A low-energy 46-keV E 2 transition has been identified to depopulate the ( 29 / 2 − ) isomeric state, with a measured half-life of T 1 / 2 = 614 ( 5 ) ns . This transition allows the excitation energy of the isomeric state to be determined as 2381 keV. The feeding patterns of both 29 / 2 + and ( 29 / 2 − ) isomeric states have also been described. This is the first time collective structures have also been observed up to high spins and excitation energies in the neutron-deficient 195 Bi nucleus. Evidence for the manifestation of shape coexistence in 195 Bi is also discussed.peerReviewe

Identification of a dipole band above the Iπ = 31/2- isomeric state in 189Pb

A dipole band of six transitions built upon a firmly established I π = 31/2− isomeric state has been identified in 189Pb using recoil-isomer tagging. This is the lightest odd-mass Pb nucleus in which a dipole band is known. The dipole nature of the new transitions has been confirmed through angular-intensity arguments. The evolution of the excitation energy and the aligned-angular momentum of the states in the new dipole band are compared with those of dipole bands in heavier, odd-mass lead isotopes. This comparison suggests that the new band in 189Pb is based upon a π[s−2 1/2h9/2i13/2]11− ⊗ ν[i −1 13/2+ ]13/2+ configuration. However, the increased aligned-angular momentum in 189Pb may suggest evidence for a reduced repulsive proton/neutron-hole interaction compared to dipole bands in the heavier mass isotopes.peerReviewe