The (n, gamma) campaigns at EXILL

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n, gamma) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns

Lifetime measurement of neutron-rich even-even molybdenum isotopes

Background: In the neutron-rich A approximate to 100 mass region, rapid shape changes as a function of nucleon number as well as coexistence of prolate, oblate, and triaxial shapes are predicted by various theoretical models. Lifetime measurements of excited levels in the molybdenum isotopes allow the determination of transitional quadrupole moments, which in turn provides structural information regarding the predicted shape change. Purpose: The present paper reports on the experimental setup, the method that allowed one to measure the lifetimes of excited states in even-even molybdenum isotopes from mass A = 100 up to mass A = 108, and the results that were obtained. Method: The isotopes of interest were populated by secondary knock-out reaction of neutron-rich nuclei separated and identified by the GSI fragment separator at relativistic beam energies and detected by the sensitive PreSPEC-AGATA experimental setup. The latter included the Lund-York-Cologne calorimeter for identification, tracking, and velocity measurement of ejectiles, and AGATA, an array of position sensitive segmented HPGe detectors, used to determine the interaction positions of the gamma ray enabling a precise Doppler correction. The lifetimes were determined with a relativistic version of the Doppler-shift-attenuation method using the systematic shift of the energy after Doppler correction of a gamma-ray transition with a known energy. This relativistic Doppler-shift-attenuation method allowed the determination of mean lifetimes from 2 to 250 ps. Results: Even-even molybdenum isotopes from mass A = 100 to A = 108 were studied. The decays of the low-lying states in the ground-state band were observed. In particular, two mean lifetimes were measured for the first time: tau = 29.7(-9.1)(+11.3) ps for the 4(+) state of Mo-108 and tau = 3.2(-0.7)(+ 0.7) ps for the 6(+) state of Mo-102. Conclusions: The reduced transition strengths B(E2), calculated from lifetimes measured in this experiment, compared to beyond-mean-field calculations, indicate a gradual shape transition in the chain of molybdenum isotopes when going from A = 100 to A = 108 with a maximum reached at N = 64. The transition probabilities decrease for Mo-108 which may be related to its well-pronounced triaxial shape indicated by the calculations

Search for Tetrahedral Symmetry in Nuclei: a Short Overview

International audienceFollowing a series of experiments launched by the TetraNuc collaboration to possibly demonstrate the existence of high-rank symmetries in subatomic physics, the first experi- mental results on the Rare Earth region appear in publications. Meanwhile an important progress has been made on the theory side strongly suggesting that the original criterion of the static tetrahedral symmetry in the form of vanishing quadrupole moments may need to be revised to include explicitly the vibrational motion. The Actinide region seems of particular interest because of the extra stability provided by the octahedral symmetry. In this article a summary of the current experimental efforts on both the Rare-Earth and Actinide regions is given. Finally the ELMA project addressing the experimental search for the symmetries in the Actinides is briefly discussed

Neutron effective single-particle energies above ⁷⁸Ni:a hint from lifetime measurements in the N=51 isotones ⁸⁵Seand ⁸⁷Kr

International audienceBackground: While the N=50 shell-gap evolution towards Ni78 is presently in the focus of nuclear structure research, experimental information on the neutron effective single-particle energy sequence above the Ni78 core remain scarce. Direct nucleon-exchange reactions are indeed difficult with presently available post-accelerated radioactive-ion beams (especially for high orbital-momentum orbitals) in this exotic region. Purpose: In this study we probe the evolution of the ν1g7/2 effective single-particle energy which is a key to understanding the possible evolution of the spin-orbit splitting due to the proton-neutron interaction in the Ni78 region. To achieve this goal, a method based on lifetime measurements is used for the first time. The obtained lifetimes of the 7/21+ states in Kr87 and Se85 are used to investigate the ν1g7/2 evolution. Method: Yrast and near-yrast states in the light N=51 isotones Se85 and Kr87 were populated via multinucleon transfer reactions, using a Se82 beam and a U238 target at the LNL tandem-ALPI facility. The prompt γ rays were detected by the AGATA Demonstrator and particle identification was performed using the PRISMA spectrometer. Lifetime measurements were performed by using the Cologne plunger device for deep inelastic reactions and the Recoil Distance Doppler Shift technique. Results: We obtain τ(7/21+)=0.4−0.4+1.6 ps for Kr87. In the case of Se85 an upper limit of 3(2) ps is obtained for the τ7/21+ value. Conclusion: For Kr87, the measured (7/21+) lifetime is consistent with a core-coupled 2+⊗ν2d5/2 configuration for this state. This result is consistent with that obtained by direct reaction, which validates our method. For Se85, the measured 7/21+ lifetime limit indicates a very small contribution of the ν1g7/2 configuration to the wave function of this state

Compte rendus de la conférence "Journées Jeunes Chercheurs JJC2012"

From 2nd to 8th decembre 2012, the Journées Jeunes Chercheurs 2012 (Young Reseachers Meeting) in high energy physics were held in Munster (Alsace, France). During all this week, PhD students in various domains (particle and nuclear physics, cosmology and astrophysics) have presented their work in a pedagogic way to other membres. The meeting is organized by young permanent staff. Students were coming from French and Belgain laboratories working on fundamental research on the two infinites (from particles to stars and galaxies). Most of them were French but many were coming other european countries (Belgium, Italy, Spain, Ukraine) as well as the rest of the world (Asia, South America ...). Du 2 au 8 décembre 2012, la maison du Kleebach à Munster a accueilli les Journées Jeunes Chercheurs 2012 (JJC). Durant toute cette semaine, les étudiants en thèse dans des domaines variés (physique des particules et nucléaire, cosmologie et astrophysique) ont présenté leurs travaux de façon pédagogique dans une ambiance conviviale, le tout étant encadré par des jeunes chercheurs en poste. Les participants viennent des nombreux laboratoires de France et de Belgique travaillant sur la recherche fondamentale dans les deux infinis (les particules, les étoiles et galaxies). Ils sont originaires pour la plupart de France mais certains viennent d'autres pays Européens (Belgique, Italie, Espagne, Ukraine, etc) ou du reste du monde (Asie, Amérique du sud, etc)

Compte rendus de la conférence "Journées Jeunes Chercheurs JJC2012"

From 2nd to 8th decembre 2012, the Journées Jeunes Chercheurs 2012 (Young Reseachers Meeting) in high energy physics were held in Munster (Alsace, France). During all this week, PhD students in various domains (particle and nuclear physics, cosmology and astrophysics) have presented their work in a pedagogic way to other membres. The meeting is organized by young permanent staff. Students were coming from French and Belgain laboratories working on fundamental research on the two infinites (from particles to stars and galaxies). Most of them were French but many were coming other european countries (Belgium, Italy, Spain, Ukraine) as well as the rest of the world (Asia, South America ...). Du 2 au 8 décembre 2012, la maison du Kleebach à Munster a accueilli les Journées Jeunes Chercheurs 2012 (JJC). Durant toute cette semaine, les étudiants en thèse dans des domaines variés (physique des particules et nucléaire, cosmologie et astrophysique) ont présenté leurs travaux de façon pédagogique dans une ambiance conviviale, le tout étant encadré par des jeunes chercheurs en poste. Les participants viennent des nombreux laboratoires de France et de Belgique travaillant sur la recherche fondamentale dans les deux infinis (les particules, les étoiles et galaxies). Ils sont originaires pour la plupart de France mais certains viennent d'autres pays Européens (Belgique, Italie, Espagne, Ukraine, etc) ou du reste du monde (Asie, Amérique du sud, etc)

The (n,gamma) campaigns at EXILL

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,gamma) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns