69 research outputs found
Collectivity at the prolate-oblate transition:the 2<sub>1</sub><sup>+</sup> lifetime of <sup>190</sup>W
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 by two-nucleon removal from
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 region has been established in
mercury, lead and polonium isotopes. Even-even mercury isotopes with present multiple fingerprints of this phenomenon, which seems to be
no longer present for . According to a number of theoretical
calculations, shape coexistence is predicted in the Hg isotope. The
Hg nucleus was populated using two different fusion-evaporation
reactions with two targets, Gd and Gd, and a beam of 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 rays were detected using the GALILEO
-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 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
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
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
Lifetimes of the 2 and the 4 States of Ce and its B 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
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