Medium and High Spin Structure in the 94^{94}Y Isotope Produced in Fission Induced by Cold Neutrons

International audienceThe shape of a particular nucleus results from an interplay between the collective (macroscopic)and single-particle (microscopic) energies and, therefore, highly depends on both the atomicnumber Z and the neutron number N. The neutron-rich nuclei around Z=40 and N=60 provide oneof the best territories for the exploration of this sensitivity. Indeed, the sudden onset ofdeformation observed for neutron-rich nuclei with Z = 36–40 nuclei near N=60 is considered themost dramatic shape change in the nuclear chart. Nevertheless, the resent study of the yttriumisotopic chain reveal occurrence of shape coexistence in the 98Y isotope [1] and possible rotationalband above the 6- isomeric state in the 96Y nucleus [2]. This suggests that the evolution of thedeformation is a more gradual process. Therefore, we decided to investigate whether deformedstructures are still present in the 94Y nucleus which lies 5 neutrons away from the N=60 boundary.So far, very little spectroscopic information has been gathered on 94Y – what regards higher spinyrast excitations established was only the presence of the 1.35-μs, (5+) isomer at 1202 keVexcitation energy [3]. This isomer could be used as a starting point for the identification ofstructures on top of it.The 94Y isotope has been produced by fission of 235U target induced by cold neutron from thereactor at Institut Laue-Langevin. The level scheme up to excitation energies of about 7 MeV hasbeen established based on multi-fold gamma-ray coincidence relationships measured with theEXILL spectrometer [4] which consists of up to 46 HPGe detectors. During the analysis, over 30 newgamma transitions, which feed the previously known (5+) isomer, have been identified. Angularcorrelation analysis allows to define spin-parity assignment for most of the identified levels. Theidentified transitions don’t show any pattern characteristic for deformed structures. Furthermore,no higher located isomer has been found, similar to the ones known in heavier yttrium isotopes onwhich the rotational bands are built. Those findings are in line with the expectations that in 94Y onlyspherical structures are present at low excitation energy.[1] S. Brant, G. Lhersonneau, and K. Sistemich, Phys. Rev. C 69, 034327 (2004)[2] Ł. W. Iskra et al., to be published[3] J. Genevey et al., Phys. Rev. C 59, 82 (1999)[4] M. Jentschel et al., EXILL technical paper, JINST (subm. 2015

Spectroscopy of neutron-rich 194,96^{194,96}Y isotope produced in fission induced by cold neutrons

International audienceThe onset of the deformation in neutron-rich nuclei around A = 100 mass region has for many years remainedone of the most interesting subjects for nuclear spectroscopy study. For the neutron number N = 60, a sudden onsetof the deformation has been observed at the ground state, which is manifested by the presence of rotational bands(e.g. [1]). On the other hand the occurrence of shape coexistence in nuclei with N = 58 and 59, in this region (e.g.[2]), suggests that the evolution of the deformation is a more gradual process. In the yttrium isotopic chain, arotational band above the 4-, 496-keV isomer has been observed in the N = 59, 98Y nucleus, while there was noevidence of the deformed structure in the Y isotopes with neutron number less than N = 59. Our goal was toinvestigate N = 57, 96Y isotope where only a few states were known from beta decay study of 96Sr [3] as well as thelong 9.6-s (1140-keV) isomer [4]. Additionally, we decided to investigate whether deformed structures are stillpresent in the 94Y nucleus which lies 5 neutrons away from the N=60 boundary. So far, very little spectroscopicinformation has been gathered on 94Y – what regards higher spin yrast excitations established was only the presenceof the 1.35-μs, (5+) isomer at 1202 keV excitation energy [5]. This isomer could be used as a starting point for theidentification of structures on top of it.The yttrium-94 and 96 isotopes have been produced by fission of 235U and 241Pu targets induced by coldneutron from the reactor at Institut Laue-Langevin. The level scheme up to excitation energies in excess of 5 MeVhas been established based on multi-fold gamma-ray coincidence relationships measured with the EXILLspectrometer [6] which consists of up to 46 HPGe detectors. By exploiting delayed- and cross-coincidencetechniques, extensive structure has been delineated. During the analysis, over 50 new gamma transitions which feedpreviously known low-spin states as well as the long 9.6-s, 8+ isomer in 96Y isotope have been identified. Moreover,a new isomeric state at 1655-keV excitation energy has been located with half-life of 201 ns. Angular correlationanalysis allows to define spin-parity assignment for most of the identified levels, in particular (6+) for new isomer.By using the delayed-coincidence method it was possible to identify above the 201-ns state a few weak transitions,which seem to form a rotational band, in analogy to the structure above the 4- isomer in the 98Y isotope. In the caseof 94Y isotope over 11 new gamma transitions, which feed the previously known (5+) isomer, have been identified.Angular correlation analysis supported by shell-model consideration allowed to propose spin-parity assignmentsfor some of the new levels.The existence of the new isomeric state and the possible deformed band built on that isomer in the N = 57, 96Yisotope shed new light on the study of the onset of deformation in neutron-rich nuclei around N = 60. It shows thatthe deformed structures appear just after the subshell closure at N = 56 and evolve smoothly when passing throughN = 57-59 isotopes, to became a ground state structure in the 99Y isotope, i.e., at N = 60. The findings are in linewith the 94Y results where only spherical structures are present at low excitation energy

Detailed low-spin spectroscopy of Ni 65 via neutron capture reaction

An extended investigation of the low-spin structure of the Ni65 nucleus was performed at the Institut Laue-Langevin, Grenoble, France, via the neutron capture reaction Ni64(n,γ)Ni65, using the Fission Product Prompt Gamma-Ray Spectrometer High-Purity Ge array. The level scheme of Ni65 was significantly expanded, with two 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 γ 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.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

High-precision spectroscopy of 65Ni via neutron capture

Detailed studies of the low-spin structures of neutron-rich Ni isotopes may help shedding light on the shape coexistence phenomenon. Of particular interest is the 65Ni nucleus, since it lies between 64Ni and 66Ni, where shape coexistence has been reported earlier. A spectroscopic investigation of 65Ni has been performed at Institut Laue-Langevin via the reaction64Ni(n,γ)65Ni, using the FIPPS HPGe array. Several new gamma transitions have been observed and angular correlation analyses have been performed. A comparison with Monte Carlo shell-model calculations pointed to a dominance of spherical states up to 1.5 MeV excitation energy, together with the appearance of two states of oblate character.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Testing the predictive power of realistic shell model calculations via lifetime measurement of the 11/2+ state in 131Sb

info:eu-repo/semantics/publishe

Decay of the “Stretched” M4 resonance in 13C

info:eu-repo/semantics/publishe

M4 resonnaces in light nuclei studied at CCB

info:eu-repo/semantics/publishe

Multi-quasiparticle sub-nanosecond isomers in 178W

We report on the first measurement of the half-lives of K(pi) = 11- and 12+ four-quasiparticle states in the even-even nucleus W-178. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3 scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array as a whole. The measured half-lives are 476(44)ps and 275(65)ps for the I(pi) = 11- and 12+, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Half-life measurements in 164,166Dy using γ-γ fast-timing spectroscopy with the ν-Ball spectrometer

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei 164,166Dy using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an 18O beam and a gold-backed isotopically enriched 164Dy target of thickness 6.3 mg/cm2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in 164Dy, 166Dy, and 178,179Wfollowing Coulomb excitation, inelastic nuclear scattering, two-neutron transfer, and fusion-evaporation reaction channels respectively. Gamma rays from excited states were measured using the ν-Ball high-purity germanium (HPGe)-LaBr3 hybrid γ -ray spectrometer with the excited state lifetimes extracted using the fast-timing coincidence method using HPGe-gated LaBr3-LaBr3 triple coincident events. The lifetime of the first Iπ = 2+ excited state in 166Dy was used to determine the transition quadrupole deformation of this neutron-rich nucleus for the first time. The experimental methodology was validated by showing consistency with previously determined excited state lifetimes in 164Dy. The half-lives of the yrast 2+ states in 164Dy and 166Dy were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q0 = 7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2+ state in 166Dy is consistent with a quenching of nuclear quadrupole deformation at β ≈ 0.35 as the N = 104 mid-shell is approached.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard