The emission of prompt neutrons from the spontaneous fission of No-252 and Fm-244

The neutron detector with (3)He -filled counters placed in the focal plane of the VASSILISSA separator is used for measuring the average number and determining the multiplicity distribution of prompt neutrons from the spontaneous fission of heavy short-lived isotopes. The test reaction is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the (252)No . The average neutron number per (252)No spontaneous fission event is as large as . The short-lived heavy isotope (244)Fm , produced in the complete fusion reaction (40)Ar + (206)Pb , is investigated. The average number of neutrons per spontaneous fission of (244)Fm from the experimental data ( is determined for the first time

Fine structure in the alpha decay of 224^{224}U

224U nuclei were populated in fusion-evaporation reactions using a 206Pb target and an intense 22Ne beam. Fusion-evaporation residues were separated by the new separator SHELS at the FLNR, Dubna and implanted into a large-area double-sided silicon strip detector. Position- and time-correlated alpha decays were used to identify evaporation residues. A new α -decay line at 8095(11) keV was observed in this work and assigned as the decay from 224U to the first excited 2+ in the daughter nucleus 220Th. Coincident photons were also observed allowing to unambiguously determine the excitation energy of the first excited 2+ state in 220Th to be 386.5(1) keV and not 373.3(1)keV as previously reported. The half-life of 224U was measured to be 396(17)μs

Influence of octupole vibration on the low-lying structure of 251^{251}Fm and other heavy N=151N=151 isotones

International audienceThe structure of low-lying excited states in Fm251, populated by the α decay of No255, has been investigated by means of combined γ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the 1/2+→5/2+ and 5/2+→9/2− transitions have been measured. The determined M2/E3 mixing ratio and lifetime for the 5/2+ decay to the ground state allowed to determine the corresponding reduced transitions strengths of B(E3)=18(6) W.u. and B(M2)=3.0(6)×10−3 W.u. These results, as well as the results of previous studies in N=151 isotopes, are compared to theoretical calculations beyond the mean-field approach, including the first QRPA calculations using the Gogny D1M parametrization for such heavy odd-N nuclei. The comparison points to the importance of accounting for the octupole vibrations for a proper understanding of the low-lying nuclear structure of some of the heaviest elements

Evidence of high-KK isomerism in 102256No154_{102}^{256}\mathrm{No}_{154}

International audienceIsomeric states in No256 were investigated using internal conversion electron and γ-ray spectroscopy with the GABRIELA detection system at the focal plane of the SHELS recoil separator, at the Flerov Laboratory for Nuclear Research (FLNR, JINR, Dubna). The nuclei of interest were produced using the highly asymmetric fusion-evaporation reaction U238(Ne22,4n)No256. The emission of internal conversion electrons and γ rays occurring between a No256 implantation and a subsequent α-decay event were studied, resulting in the observation of high-K isomerism in this nobelium isotope. The nature of the isomeric states is discussed in terms of possible two- and four-quasiparticle structures

First experimental tests of shels: A new heavy ion separator at the JINR

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New front and back-end electronics for the upgraded GABRIELA detection system

International audienceThe GABRIELA [1] set-up is used at the FLNR to perform detailed nuclear structure studies of transfermium nuclei. Following the modernization of the VASSILISSA separator (SHELS) [2] the GABRIELA detection system has also been upgraded. The characteristics of the upgraded detection system will be presented along with results from some recent electronics tests

Spectroscopy of Transfermium Nuclei Using the GABRIELA Setup

International audienceHeavy elements above Fm (Z = 100) are nuclei with vanishing liquid-drop fission barriers and are therefore entirely stabilized by quantum shell effects. Due to the large density of single-particle levels and strong polarized Coulomb fields, theoretical predictions of magic numbers are extremely model dependent. Furthermore, shell closures for one nucleon species depend strongly on the number of the other species. Reliable experimental data is needed in order to test and constrain theory. As there is a lack of such data in the region, new data is needed along with the confirmation of previous data. A detection system dedicated to the spectroscopy of transfermium nuclei was constructed in 2004 and installed at the focal plane of the VASSILISSA separator at the FLNR, Dubna, by a Franco-Russian collaboration. The results from the 2009 campaign will be presented

Spectroscopy of 253No and its daughters

New high-statistics data have been obtained on the decay properties of 253No and its daughters using the reaction 207Pb(48Ca, 2n)253No. This was made possible thanks to an improved transmission of fusion–evaporation residues through the VASSILISSA recoil separator and an increased efficiency of the GABRIELA detector setup. The decay schemes of 253No and 249Fm have been revisited. The known level scheme of 249Fm has been confirmed, including a new level at 669 keV excitation energy. The observation of L X-rays in coincidence with the α decay of 249Fm gives additional support to the ground-state configuration of 1/2+[631] instead of 5/2+[622] for 245Cf. In both 249Fm and 245Cf, the interpretation of the data has been checked by comparing experimental α-particle and γ -ray spectra with realistic simulations of the decay cascades and of the interaction of particles and photons in the detectors of GABRIELA. The population of a 0.7 ms isomeric state attributed to 253No is confirmed by an α-tagged calorimetric measurement and the corresponding γ and electron decay spectra are presented. Possible evidence for more than one isomer is given and a tentative partial decay scheme is discussed in the light of the available experimental data, systematics and theoretical expectations

Spectroscopy of transfermium nuclei using the GABRIELA set up at the focal plane of the VASSILISSA recoil separator

International audienceAn IN2P3-JINR collaboration has launched a project called GABRIELA at the Flerov Laboratory for Nuclear Reactions (FLNR) within the Joint Institute for Nuclear Research (JINR) in Dubna (Russia). The goal of the project is to perform gamma-ray and internal conversion electron spectroscopy of heavy nuclei produced in fusion-evaporation reactions and transported to the focal plane of the recoil separator VASSILISSA. During five experimental campaigns of GABRIELA, the detection system has gained in sensitivity and new spectroscopic information has been obtained for 249Fm, 251Fm, 253No and 255Lr