Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data

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

Characterisation of the Neutron Wall and of Neutron Interactions in Germanium-Detector Systems

A Monte Carlo simulation of the Neutron Wall detector system has been performed using Geant4, in order to define optimum conditions for the detection and identification of multiple neutrons. Emphasis was put on studying the scattering of neutrons between different detectors, which is the main source of the apparent increase of the number of detected neutrons. The simulation has been compared with experimental data. The quality of neutron gated γ-ray spectra was improved for both two- and three-neutron evaporation channels. The influence of small amounts of γ rays mis-interpreted as neutrons was investigated. It was found that such γ rays dramatically reduce the quality of neutron gated γ-ray spectra. The interaction properties of fast neutrons in a closed-end coaxial and a planar high-purity germanium detector (HPGe) were studied. Digitised waveforms of HPGe preamplifier signals were recorded for time-of-flight separated neutrons and γ rays, emitted by a 252Cf source. The experimental waveforms from the detectors were compared to simulated pulse shapes. In the analysis, special emphasis was given to the detection of elastically scattered neutrons, which may be an important effect to take into account in future spectrometers based on γ-ray tracking. No differences between neutron and γ-ray induced pulse shapes were found in this work. A Monte Carlo simulation of the interactions of fast neutrons in the future 4π γ-ray spectrometer AGATA was also performed, in order to study the influence of neutrons on γ-ray tracking. It was shown that although there is a large probability of detecting neutrons in AGATA, the neutrons decrease the photo-peak efficiency of AGATA by only about 1% for each neutron emitted in coincidence with γ rays. The peak-to-background ratios in γ-ray spectra are, however, reduced to a much larger extent. The possibility of using AGATA as a neutron detector system was also investigated

Nuclear shapes and collectivity investigated using lifetime measurements and γ-ray spectroscopy

Les moments électromagnétiques sont des grandeurs clés pour mieux comprendre la structure des excitations dans les noyaux atomiques. Une observable liée est le temps de vie des états excités dans les noyaux. Dans cette HDR des mesures de temps de vie faites pour élaborer des questions liées avec les modifications de positionrelative des orbitales et la coexistence de forme dans les noyaux sont discutées et mises dans le contexte plus grand des développements de la structure nucléaire des 15 dernières années.Pour mesurer des temps de vie autour de quelques pico secondes jusqu’aux centaines des pico secondes la technique Recoil DistanceDoppler Shift est très bien adaptée. Le développement du dispositif expérimental "the Orsay Univeral Plunger System" (OUPS) estdiscuté en détail et les différentes campagnes de mesures faites avec OUPS sont exposées dans la texte.Une grande partie de cet HDR traite du multi-détecteur AGATA (Advanced GAmma Tracking Array). Un développement qui a commencé à la fin des années 1990 et auquel j'ai participé depuis ma thèse. Pendant la période après ma thèse j'ai, d'abord comme post-doc au CEA et après comme CR au CNRS, travaillé avec presque tous les aspects d'AGATA.Mes travaux portent sur : des simulations Monte Carlo pour estimer la performance d'AGATA dans différentes conditions expérimentales, des calculs des formes des impulsions et du développement de codes pour faire du traitement des signaux pour extraire l'information des positions des interactions des rayons gamma, des méthodes innovantes pour déterminer les formes des impulsions des détecteurs déjà installés dans AGATA.Au final, je présente mes perspectives sur le champ scientifique de la structure du noyau et les développements souhaitables pour AGATA et OUPS

Nuclear shapes and collectivity investigated using lifetime measurements and γ-ray spectroscopy

Les moments électromagnétiques sont des grandeurs clés pour mieux comprendre la structure des excitations dans les noyaux atomiques. Une observable liée est le temps de vie des états excités dans les noyaux. Dans cette HDR des mesures de temps de vie faites pour élaborer des questions liées avec les modifications de positionrelative des orbitales et la coexistence de forme dans les noyaux sont discutées et mises dans le contexte plus grand des développements de la structure nucléaire des 15 dernières années.Pour mesurer des temps de vie autour de quelques pico secondes jusqu’aux centaines des pico secondes la technique Recoil DistanceDoppler Shift est très bien adaptée. Le développement du dispositif expérimental "the Orsay Univeral Plunger System" (OUPS) estdiscuté en détail et les différentes campagnes de mesures faites avec OUPS sont exposées dans la texte.Une grande partie de cet HDR traite du multi-détecteur AGATA (Advanced GAmma Tracking Array). Un développement qui a commencé à la fin des années 1990 et auquel j'ai participé depuis ma thèse. Pendant la période après ma thèse j'ai, d'abord comme post-doc au CEA et après comme CR au CNRS, travaillé avec presque tous les aspects d'AGATA.Mes travaux portent sur : des simulations Monte Carlo pour estimer la performance d'AGATA dans différentes conditions expérimentales, des calculs des formes des impulsions et du développement de codes pour faire du traitement des signaux pour extraire l'information des positions des interactions des rayons gamma, des méthodes innovantes pour déterminer les formes des impulsions des détecteurs déjà installés dans AGATA.Au final, je présente mes perspectives sur le champ scientifique de la structure du noyau et les développements souhaitables pour AGATA et OUPS

B(E2) anomalies in the yrast band of 170Os

Background: The neutron-deficient osmium isotopic chain provides a great laboratory for the study of shape evolution, with the transition from the soft triaxial rotor in 168Os to the well-deformed prolate rotor in 180Os, while shape coexistence appears around N = 96 in 172Os. Therefore, the study of the Os isotopic chain should provide a better understanding of shape changes in nuclei and a detailed scrutiny of nuclear structure calculations. In this paper, the lifetimes of the low-lying yrast states of 170Os have been measured for the first time to investigate the shape evolution with neutron number. Purpose: Lifetimes of excited states in the ground-state band of 170Os are measured to investigate the shape evolution with neutron number in osmium isotopes and compare with state-of-the-art calculations. Methods: The states of interest were populated via the fusion-evaporation reaction 142Nd(32S, 4n) at a bombarding energy of 170 MeV at the ALTO facility from IPN (Orsay, France). Lifetimes of the 2+ 1 and 4+ 1 states in 170Os were measured with the recoil-distance Doppler-shift method using the Orsay universal plunger system. Results: Lifetimes of the two first excited states in 170Os were measured for the first time. A very small B(E2; 4+ 1 → 2+ 1 )/B(E2; 2+ 1 → 0+ 1 ) = 0.38(11) was found, which is very uncharacteristic for collective nuclei. These results were compared to state-of-the-art beyond-mean-field calculations. Conclusions: Although theoretical results give satisfactory results for the energy of the first few excited states in 170Os and the B(E2; 2+ 1 → 0+ 1 ) they fail to reproduce the very small B(E2; 4+ 1 → 2+ 1 ), which remains a puzzle

Lifetime Measurments of Excited States in Neutron-Rich Fission Fragments

Lifetimes of short-lived excited states in a wide range of neutron-rich fission fragments were measured using the recoil distance Doppler shift (RDDS) technique, which was applied to fusion-fission reactions in inverse kinematics for the first time. The fission fragments were identified event-by-event in mass, charge, and atomic number using the VAMOS magnetic spectrometer at GANIL. Gamma rays originating from the fission fragments were measured with the EXOGAM array of Ge Clover detectors around the target position. Using a degrader, the change in the Doppler shift of the gamma ray allows the application of the RDDS method. Details of the experimental technique will be discussed and the status for the ongoing analysis for odd-mass yttrium isotopes will be presented