The lifetimes of first excited 2 + , 4 + and 6 + states in 98 Zr were measured with the Recoil-Distance Doppler-Shift method in an experiment performed at GANIL. Excited states in 98 Zr were populated using the fission reaction between a 6.2 MeV/u 238 U beam and a 9 Be target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified in mass and atomic number in the VAMOS++ spectrometer. Our result shows very small B(E2; 2 + 1 → 0 + 1 ) value in 98 Zr thereby confirming the very sudden onset of collectivity at N = 60. The experimental results are compared to large-scale Monte Carlo Shell model and beyond mean field calculations. The present results indicate coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.The study of various modes of excitations and the associated evolution of nuclear shapes along spin and isospin axes in atomic nuclei is one of the fundamental quests in nuclear physics. While nuclei with "magic numbers" of protons and/or neutrons have spherical ground states, as one moves away, the polarizing effect of added nucleons leads to deformation. Throughout the nuclear landscape, this onset of deformation is usually a gradual process, however in neutron rich nuclei around mass A ∼ 100 the shape change is rather drastic and abrupt. The ground states of Sr and Zr isotopes with N ranging from the magic number N = 50 up to N < 60 are weakly deformed, however, they undergo a rapid shape transition from nearly spherical to well deformed prolate deformations as N = 60 is approached. The sudden nature of shape transition in Sr and Zr isotopes is evident from the abrupt changes in the two neutron separation energies [1] and mean-square charge radii [2, 3], but also from the excitation energies of 2 + 1 states and B(E2) values [4]. On the other hand, in isotopes with Z ≥ 42 the shape change is rather gradual [1,5] showing also characteristic signatures of triaxiality. This strong dependence of the observed spectroscopic properties, both on the number of protons and neutrons, makes the neutron-rich A ∼ 100 nuclei an excellent mass region for testing various theoretical models.Many experimental and theoretical studies have already been reported on the structure of these nuclei. More specifically for the Zr isotopes, the onset of deformation at N = 60 has been described by a number of theoretical models [6][7][8][9][10][11][12][13][14][15][16][17][18][19], however, none of the models have been able to successfully reproduce the aforementioned rapid change. Very recently, the abrupt shape changes were correctly described by large-scale Monte-Carlo Shell Model (MCSM) calculations [20,21]. In the so-called type-II shell evolution scenario, the (prolate) deformed states in the isotopes with N ≥ 60 are associated with proton excitations to the 0g 9/2 orbital. Driven by the central and tensor components of the effective (proton-neutron) interactions, these excitations result in a lowering and subsequent filling of the neutron 0g ...

We investigated the 238 U(d,p) reaction as a surrogate for the n + 238 U reaction. For this purpose we measured for the first time the gamma-decay and fission probabilities of 239 U* simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupledchannels method and the distorted-wave Born approximation (DWBA) were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission and gamma emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the gamma-decay probability is much higher than the neutroninduced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the 238 U(d,p) reaction with the DWBA to interpret these results.

Prompt fission γ-rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modelling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission, for the first time as a function of the nuclear excitation energy of the fissioning system. Emitted γ-ray spectra were measured, and γ-ray multiplicities and average and total γ energies per fission were determined for the 233 U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the 239 Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra ∼5 MeV of energy is potentially available in the excited fragments for γ-decay. The measured results are compared to model calculations made for prompt γ-ray emission with the fission model code GEF. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction. PACS numbers: 25.85.Ge 24.75.+i 07.85.Nc

International audienceLifetimes of excited states in ${}^{99}$Y , ${}^{101}$Y, ${}^{101}$Nb, ${}^{103}$Nb, and ${}^{105}$Nb were measured in an experiment using the recoil distance Doppler shift method at GANIL (Grand Accélérateur National d’Ions Lourds). The neutron-richnuclei were produced in fission reactions between a ${}^{238}$U beam and a ${}^9$Be target. Prompt γ rays were measuredwith the EXOGAM array and correlated with fission fragments that were identified in mass and atomic numberwith the VAMOS$\ {++}$ spectrometer. The measured lifetimes, together with branching ratios, provide $\it B(M1)$ and$\it B(E2)$ values for the strongly coupled rotational bands built on the [422]5/2$^+$ ground state in the Y and Nb nucleiwith neutron number N $\geq$ 60. The comparison of the experimental results with triaxial particle-rotor calculationsprovides information about the evolution of the nuclear shape in this mass region

Particle-γ coincidence experiments were performed at the Oslo Cyclotron Laboratory with the 181 Ta(d, X) and 181 Ta(3 He, X) reactions to measure the nuclear level densities (NLDs) and γ-ray strength functions (γ SFs) of 180,181,182 Ta using the Oslo method. The back-shifted Fermi-gas, constant temperature plus Fermi gas, and Hartree-Fock-Bogoliubov plus combinatorial models were used for the absolute normalizations of the experimental NLDs at the neutron separation energies. The NLDs and γ SFs are used to calculate the corresponding 181 Ta(n, γ) cross sections and these are compared to results from other techniques. The energy region of the scissors resonance strength is investigated and from the data and comparison to prior work it is concluded that the scissors strength splits into two distinct parts. This splitting may allow for the determination of triaxiality and a γ deformation of 14.9 • ± 1.8 • was determined for 181 Ta.