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

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

Lifetimes of the 21+^+_1 and the 41+^+_1 States of 148^{148}Ce and its B4/2_{4/2} Ratio from EXILL&FATIMA Experiment

International audienceThe even-even N=90 isotones with Z=60-66, present an interesting phase transition. This phasetransition in nuclei is characterized by a sudden change of the shape of the nucleus [1]. 148Ce lies onthe downboundaries of the phase transition region and therefore further elucidate the underlyingmechanisms that lend the shape phase transition behavior. 235U and 241Pu fission fragments were measured by a mixed spectrometer consisting of highresolution Ge and fast LaBr3(Ce)-scintillator detectors at the high-flux reactor of the ILL. Prompt γ-ray cascades from the nuclei of interest are selected via Ge-Ge-LaBr3-LaBr3 coincidences. The goodenergy resolution of the Ge allows precise gates to be set, selecting the cascade, hence, the nucleusof interest. The excellent timing performance of the LaBr3 detectors in combination with theGeneral Centroid Difference method [2] allows the measurement of lifetimes in the ps range inpreparation for the FATIMA experiment at FAIR. The first results on neutron-rich 148Ce arepresented

ISOMER SPECTROSCOPY AND SUB-NANOSECOND HALF-LIVE DETERMINATION IN W-178 USING THE NuBALL ARRAY

The reaction of a pulsed O-18 beam on a Dy-164 target was studied in the first experiment with the NuBall array at the IPN Orsay, France. Excited state half-lives were measured using the fast timing method with 20 LaBr3 (Ce) detectors. The timing characteristics of the fully digital acquisition system is briefly discussed. A value for the previously unknown half-life of the first excited 4(+) state in W-178 is presented

Shape transitions between and within Zr isotopes

The Zirconium isotopes across the N=56,58 neutron sub­shell closures have been of special interest since years, sparked by the near doubly-magic features of 96Zr and the subsequent rapid onset of collectivity with a deformed ground-state structure already in 100Zr. Recent state-of-the-art shell model approaches did not only correctly describe this shape-phase transition in the Zr isotopic chain, but alsothe coexistence of non-collective structures and pronounced collectivity especially in 96,98Zr. Theisotope 98Zr is located on the transition from spherical to deformed ground state structures. We summarize recent experimental work to obtain the B(E2) excitation strengths of the first 2+ state of98Zr, including a new experiment employing the recoil-distance Doppler-shift method following a two-neutron transfer reaction