Collectivity at the prolate-oblate transition:the 2₁⁺ lifetime of ¹⁹⁰W

The neutron-rich rare isotope 190W is discussed as a candidate for a prolate-oblate transitional nucleus with maximum γ-softness. The collectivity of this isotope is assessed for the first time by the measurement of the reduced E2 transition probability of its first 2+ state to the ground state. The experiment employed the FAst TIming Array (FATIMA), comprised of 36 LaBr3(Ce) scintillators, which was part of the DESPEC setup at GSI, Darmstadt. The 41+ and 21+ states of 190W were populated subsequently to the decay of its 127(12) μs isomeric Jπ  = 10- state. The mean lifetime of the 21+ state was determined to be τ = 274(28) ps, which corresponds to a B(E2; 21+ → 01+) value of 95(10) W.u. The results motivated a revision of previous calculations within an energy-density functional-based interacting boson model-2 approach, yielding E2 transition properties and spectroscopic quadrupole moments for tungsten isotopes. From comparison to theory, the new data suggest that 190W is at the transition from prolate to oblate structure along the W isotopic chain, which had previously been discussed as a nuclear shape-phase transition

Shell evolution of N = 40 isotones towards 60Ca: First spectroscopy of 62Ti

Excited states in the N=40 isotone 62Ti were populated via the 63V(p,2p)62Ti reaction at ∼200 MeV/nucleon at the Radioactive Isotope Beam Factory and studied using γ-ray spectroscopy. The energies of the 21+→0gs+ and 41+→21+ transitions, observed here for the first time, indicate a deformed 62Ti ground state. These energies are increased compared to the neighboring 64Cr and 66Fe isotones, suggesting a small decrease of quadrupole collectivity. The present measurement is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce our findings. The shell-model calculations for 62Ti show a dominant configuration with four neutrons excited across the N=40 gap. Likewise, they indicate that the N=40 island of inversion extends down to Z=20, disfavoring a possible doubly magic character of the elusive 60Ca

Spectroscopy of 98Cd^{98}\mathrm{Cd} by two-nucleon removal from 100In^{100}\mathrm{In}

International audienceLow-lying states of Cd98 have been populated by the two-nucleon removal reaction (In100,Cd98+γ) and studied using in-beam γ-ray spectroscopy at the Radioactive Isotope Beam Factory at RIKEN. Two new γ transitions were identified and assigned as decays from a previously unknown state. This state is suggested to be based on a π1g9/2−12p1/2−1 configuration with Jπ=5−. The present observation extends the systematics of the excitation energies of the first 5− state in N=50 isotones toward Sn100. The determined energy of the 5− state in Cd98 continues a smooth trend along the N=50 isotones. The systematics are compared with shell-model calculations in different model spaces. Good agreement is achieved when considering a model space consisting of the π(1f5/2, 2p3/2, 2p1/2, 1g9/2) orbitals. The calculations with a smaller model space omitting the orbitals below the Z=38 subshell could not reproduce the experimental energy difference between the ground and first 5− states in N=50 isotones, because proton excitations across Z=38 subshell yield a large amount of correlation energy that lowers the ground states

Shape coexistence in neutron-deficient Hg-188 investigated via lifetime measurements

Shape coexistence in the $Z \approx 82$ region has been established in mercury, lead and polonium isotopes. Even-even mercury isotopes with $100 \leq N \leq 106$ present multiple fingerprints of this phenomenon, which seems to be no longer present for $N \geq 110$. According to a number of theoretical calculations, shape coexistence is predicted in the $^{188}$Hg isotope. The $^{188}$Hg nucleus was populated using two different fusion-evaporation reactions with two targets, $^{158}$Gd and $^{160}$Gd, and a beam of $^{34}$S, provided by the Tandem-ALPI accelerators complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the $\gamma$ rays were detected using the GALILEO $\gamma$-ray array. The lifetimes of the excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Using the two-bands mixing and rotational models, the deformation of the pure configurations was obtained from the experimental results. The extracted transition strengths were compared with those calculated with the state-of-the-art symmetry-conserving configuration-mixing (SCCM) and five-dimentional collective Hamiltonian (5DCH) approaches in order to shed light on the nature of the observed structures in the $^{188}$Hg nucleus. An oblate, a normal- and a super-deformed prolate bands were predicted and their underlying shell structure was also discussed.Comment: v1: 13 pages, 10 figures, comparison between IBM-CM and SCCM calculations; v2: 16 pages, 13 figures, discussion on the mixing amplitudes from the experimental B(E2) values, comparison between SCCM and 5DCH calculation

Spectroscopy of neutron-rich scandium isotopes

Within the SEASTAR III campaign at the Radioactive Isotope Beam Factory, at the RIKEN Nishina Center, neutron-rich isotopes in the vicinity of ⁵³K were produced from the fragmentation of the primary ⁷⁰Zn beam on a ⁹Be target. After nucleon knockout reactions on the secondary liquid hydrogen MINOS target the known γ rays of the neutron-rich ⁵⁵Sc isotope were observed (shown in this proceedings) and γ rays from ⁵⁷,⁵⁹Sc isotopes have been identified for the first time. The evolution of the occupied nucleon orbitals of these nuclei in the ground and excited state is investigated under the prism of the tensor force

Experimental study of ⁴n with ⁸He(p,2p) reaction

The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on ⁸He(p,2p)⁷H{t+⁴ n} reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system

Experimental study of 4n with 8He(p,2p) reaction

The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on 8He(p,2p)7H{t+4 n} reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system

β\beta Decay as a New Probe for the Low-energy E1 Strength

International audienceIn this contribution, it is evaluated whether high Q-value β decays from mothers with low ground-state spin are suitable to probe the structure of 1− levels associated with the pygmy dipole response. A comparison of data from the exemplary 136I →136Xe β decay and the 136Xe(γ,γ′) reaction reveals that some 1− levels are populated in both reactions but with a different pattern. An investigation within the microscopic quasiparticle phonon model shows that the pattern is related to the population of different parts of the wave functions of these 1− levels establishing β decay as a novel probe