Fission-barrier heights of neutron-deficient mercury nuclei

The recently measured probabilities of the β-delayed fission for 178,180Tl are used to deduce the fission-barrier heights of the daughter isotopes 178,180Hg, undergoing low-energy fission. Four alternative β-decay strength functions and four variants of the statistical model of de-excitation of the daughter nucleus are used to determine the fission-barrier height for 180Hg. Depending on the choice of the model, the deduced fission-barrier heights appear to be between 10% and 40% smaller than theoretical estimates. This observation is verified also for fission-barrier heights extracted using the probability of β-delayed fission of 178Tl. The spread in extracted fission-barrier heights results mainly from uncertainties in the magnitude of the pairing gap at the saddle configuration.status: publishe

Alpha decay of 176Au

International audienceThe isotope Au176 has been studied in the complete fusion reaction Ca40+Pr141 → 176Au+5n at the velocity filter SHIP (GSI, Darmstadt). The complex fine-structure α decay of two isomeric states in Au176 feeding several previously unknown excited states in the daughter nucleus Ir172 is presented. An α-decay branching ratio of bα=9.5(11)% was deduced for the high-spin isomer in Ir172

Alpha-decay studies of the francium isotopes 198Fr and 199Fr nuclei

Very neutron deficient francium isotopes have been produced in fusion evaporation reactions using 60Ni ions on 141Pr targets. The gas-filled recoil separator RITU was employed to collect the fusion products and to separate them from the scattered beam. The activities were implanted into a position sensitive silicon detector after passing through a gas-counter system. The isotopes were identified using spatial and time correlations between the implants and the decays. Two α-particle activities, with Eα = 7613(15) keV and T1/2 = (15+12 −5 ) ms and Eα = 7684(15) keV and T1/2 = (16+13 −5 ) ms were identified in the new isotope 198Fr. In addition, the half-life and α-particle energy of 199Fr were measured with improved precision. The measured decay properties deduced for 199Fr and 198Fr suggest that there is an onset of ground-state deformation at N = 112 in the Fr isotopes.peerReviewe

Determination of α-decay branching ratios for 178,179Hg

The neutron-deficient nuclides 182,183Pb were produced at the Accelerator Laboratory of the University of Jyv¨askyl¨a by bombarding a 107Ag target with 78Kr ions. The nuclei of interest were separated using the ritu gas-filled separator and their radioactive decays investigated using the great spectrometer. Correlating the α decays of these Pb isotopes with those of their respective Hg daughters allowed α-decay branching ratios of bα(178Hg) = 89(4)% and bα(179Hg) = 75(4)% to be deduced.status: publishe

Shape coexistence in odd-mass Au isotopes: Determination of the excitation energy of the lowest intruder state in 179Au

Phenomenon of the shape coexistence has been investigated in 179Au. This very neutron-deficient isotope, 18 neutrons away from the stable gold isotope 197Au, was studied by a combination of α-decay and isomer-decay spectroscopy employing the ritu gas-filled separator and the great focal-plane spectrometer at the University of Jyväskylä. A new isomer with t1/2 = 328(2) ns was observed and states associated with π(d3/2)^-1, π(s1/2)^-1, π(f7/2)^+1 and π(h9/2)^+1 structures were revealed. The implications of these results on the systematics of π(h9/2)^+1 and π(f7/2)^+1 intruder structures in the odd-mass gold isotopes are discussed. The minimum of the parabolic trend is at N = 104, i.e. exactly at the mid-shell point and the π(f7/2)^+1 structure is dropping in energy relative to the π(h9/2)^+1 with decreasing mass number.status: publishe

Gamow-Teller decay population of 64Ni levels in the decay of 1+ 64Co

The 64Co β-decay feeding levels in the well-studied 64Ni nucleus were investigated. Whereas the previously known 64Co decay scheme merely contained 2 γ rays, the decay scheme established in this work contains 18, of which 5 are observed in this work and 6 were previously observed in an (n,γ) study but not placed in the 64Ni level scheme. Surprisingly, one additional level—placed at an excitation energy of 3578.7 keV—could be determined. The observed β-decay paths involve allowed νf5/2 → πf7/2 and νp1/2 → πp3/2 transitions. Three strongly fed levels around 4 MeV are interpreted to possess possible proton-intruder characterstatus: publishe