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Register a new userDetailed low-spin spectroscopy of 65Ni via neutron capture reaction
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An extended investigation of the low-spin structure of the $^{65}$Ni nucleus was performed at the Institut Laue-Langevin, Grenoble, via the neutron capture reaction $^{64}$Ni(n,$gamma$)$^{65}$Ni, using the FIPPS HPGe array. The level scheme of $^{65}$Ni was significantly expanded, with 2 new levels and 87 newly found transitions. Angular correlation analyses were also performed, allowing us to assign spins and parities for a number of states, and to determine multipolarity mixing ratios for selected $gamma$ transitions. The low-energy part of the experimental level scheme (up to about 1.4 MeV) was compared with Monte Carlo Shell Model calculations, which predict spherical shapes for all states, apart from the 9/2$^+$ and the second excited 1/2$^-$ states of oblate deformation.
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The one neutron knock-out reaction $^1$H($^{20}$C,$^{19}$C$gamma$) was studied at RIKEN using the DALI2 array. A $gamma$ ray transition was observed at 198(10) keV. Based on the comparison between the experimental production cross section and theoretical predictions, the transition was assigned to the decay of the 3/2$_1^+$ state to the ground state.
In this article a method for lifetime measurements in the sub-picosecond regime via the Doppler-shift attenuation method (DSAM) following the inelastic proton scattering reaction is presented. In a pioneering experiment we extracted the lifetimes of 30 excited low-spin states of $^{96}$Ru, taking advantage of the coincident detection of scattered protons and de-exciting $gamma$-rays as well as the large number of particle and $gamma$-ray detectors provided by the SONIC@HORUS setup at the University of Cologne. The large amount of new experimental data shows that this technique is suited for the measurement of lifetimes of excited low-spin states, especially for isotopes with a low isotopic abundance, where $(n,n^{prime}gamma$) or - in case of investigating dipole excitations - ($gamma,gamma^{prime}$) experiments are not feasible due to the lack of sufficient isotopically enriched target material.
Total radiative thermal neutron-capture $gamma$-ray cross sections for the $^{182,183,184,186}$W isotopes were measured using guided neutron beams from the Budapest Research Reactor to induce prompt and delayed $gamma$ rays from elemental and isotopically-enriched tungsten targets. These cross sections were determined from the sum of measured $gamma$-ray cross sections feeding the ground state from low-lying levels below a cutoff energy, E$_{rm crit}$, where the level scheme is completely known, and continuum $gamma$ rays from levels above E$_{rm crit}$, calculated using the Monte Carlo statistical-decay code DICEBOX. The new cross sections determined in this work for the tungsten nuclides are: $sigma_{0}(^{182}{rm W}) = 20.5(14)$ b and $sigma_{11/2^{+}}(^{183}{rm W}^{m}, 5.2 {rm s}) = 0.177(18)$ b; $sigma_{0}(^{183}{rm W}) = 9.37(38)$ b and $sigma_{5^{-}}(^{184}{rm W}^{m}, 8.33 mu{rm s}) = 0.0247(55)$ b; $sigma_{0}(^{184}{rm W}) = 1.43(10)$ b and $sigma_{11/2^{+}}(^{185}{rm W}^{m}, 1.67 {rm min}) = 0.0062(16)$ b; and, $sigma_{0}(^{186}{rm W}) = 33.33(62)$ b and $sigma_{9/2^{+}}(^{187}{rm W}^{m}, 1.38 mu{rm s}) = 0.400(16)$ b. These results are consistent with earlier measurements in the literature. The $^{186}$W cross section was also independently confirmed from an activation measurement, following the decay of $^{187}$W, yielding values for $sigma_{0}(^{186}{rm W})$ that are consistent with our prompt $gamma$-ray measurement. The cross-section measurements were found to be insensitive to choice of level density or photon strength model, and only weakly dependent on E$_{rm crit}$. Total radiative-capture widths calculated with DICEBOX showed much greater model dependence, however, the recommended values could be reproduced with selected model choices. The decay schemes for all tungsten isotopes were improved in these analyses.
We present recent results on the structure of the one-valence-particle $^{41}$Ca and $^{49}$Ca, and one-valence-hole $^{47}$Ca, nuclei. The isotopes of interest were populated via the cold-neutron capture reactions $^{40}$Ca(n,$gamma$), $^{48}$Ca(n,$gamma$) and $^{46}$Ca(n,$gamma$), respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of HPGe detectors. The $gamma$ decay and level schemes of these nuclei were investigated by $gamma$-ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in $^{41}$Ca, $^{47}$Ca, and $^{49}$Ca, respectively. Branching ratios and intensities were extracted for the $gamma$ decay from each state, and $gamma$-ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the Hybrid Configuration Mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the $^{41}$Ca, $^{49}$Ca, and $^{47}$Ca nuclei is characterized by the coexistence of either 2p-1h and 1p-2h excitations, or couplings between single-particle/hole degrees of freedom and collective vibrations (phonons) of the doubly-magic core.
Recent investigations suggest that the neutrino--heated hot bubble between the nascent neutron star and the overlying stellar mantle of a type--II supernova may be the site of the r--process. In the preceding $alpha$--process building up the elements to $A approx 100$, the $^4$He(2n,$gamma$)$^6$He-- and $^6$He($alpha$,n)$^9$Be--reactions bridging the instability gap at $A=5$ and $A=8$ could be of relevance. We suggest a mechanism for $^4$He(2n,$gamma$)$^6$He and calculate the reaction rate within the $alpha$+n+n approach. The value obtained is about a factor 1.6 smaller than the one obtained recently in the simpler direct--capture model, but is at least three order of magnitude enhanced compared to the previously adopted value. Our calculation confirms the result of the direct--capture calculation that under representative conditions in the $alpha$--process the reaction path proceeding through $^6$He is negligible compared to $^4$He($alpha$n,$gamma$)$^9$Be.
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