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Fast-timing measurements at the focal plane of a separator can suffer from poor timing resolution. This is due to the variations in time-of-flight (ToF) for photons travelling to a given detector, which arise from the changes in the implantation positions of the recoil nuclei emitting the γ rays of interest. In order to minimise these effects on timing measurements, a procedure is presented that improves fast-timing data by performing ToF corrections on an event-by-event basis. This method was used to correct data collected with an array of eight LaBr 3 detectors, which detected γ rays from spatially distributed 138Gd recoil-implants at the focal plane of the Recoil-Ion-Transport-Unit (RITU) spectrometer. The Generalised Centroid Difference (GCD) method was used to extract a lifetime from data in conjunction with a new procedure to calibrate the time walk. The lifetime of the first 2 + state in 138Gd, populated by the decay of the K π = 8 − isomeric state, was measured to be 229(24) ps using the ToF-corrected data, which is consistent within three standard deviations to the literature value. The results together with Monte-Carlo simulations show that the ToF correction procedure reduced the uncertainty in the measured lifetimes by 3 % in the case of the spatially distributed nuclei at the focal plane of RITU. However, ∼ 12 % has been estimated for a similar experiment when using a larger focal plane i.e. the Super-FRS at the FAIR facility.

5 pags., 3 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0, Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δn=0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z=50 (for N>82) and Z=82 (for N>126). The level of forbiddenness of the Δn=1ν1g →π0g transition has been investigated from the β decay of the ground state of Hg into the single-proton-hole nucleus Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0g state in Tl, an upper limit of 3.9×10 % was obtained for the probability of this decay, corresponding to log⁡ft>8.8 within a 95% confidence limit. This is the most stringent test of the Δn=0 selection rule to date., Support from the European Union seventh framework through ENSAR contract no. 262010, the Science and Technology Facilities Council through grants ST/P005314/1, ST/L005743/1 and ST/J000051/1 (UK), the MINECO projects FPA2015-64969-P, FPA2015-65035-P and FPA2017-87568-P (Spain), FWO-Vlaanderen (Belgium), GOA/2015/010 (BOF KU Leuven), the Excellence of Science programme (EOS-FWO), and the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (BriX network P7/12) is acknowledged. ZsP acknowledges support by the ExtreMe Matter Institute EMMI at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. PHR and SMJ ac-knowledge support from the UK Department for Business, Energy and Industrial Strategy via the National Measurement Office.

International audience; Lifetimes of the low-energy levels in Nd136, populated in the reaction Te124(O16,4n), were measured with the ROSPHERE array at the Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN-HH), Bucharest-Magurele. The data were analyzed using the recoil distance Doppler shift method, and, in the cases where lifetimes were τ⩽1 ps, Doppler attenuation effects were taken into account. The deduced electromagnetic transition probabilities are discussed in the framework of the five-dimensional collective Hamiltonian (5DCH) theoretical model implemented with the D1S Gogny force, and detailed systematics of several observables in the even-even Nd isotopic chain are presented that highlight the transitional character of the neutron-deficient Nd isotopes. The 5DCH predictions are in overall good agreement with the present experimental results.

The $^{86}\mathrm{Zr}$ nucleus, which has a low-lying level scheme characteristic of a transitional nucleus, presents, according to the existing electromagnetic transition data, a subunitary ${\mathrm{B}}_{4/2}=B(E2,{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B(E2,{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ ratio, which is anomalously low for a nonmagic nucleus, as it is outside the range of the traditional collective models values. In order to check this anomaly, we performed new measurements of the lifetimes of its low-lying states, with the $\ensuremath{\gamma}$-ray coincident recoil distance Doppler shift method, using the ROSPHERE detector array. New lifetimes were determined for the positive-parity yrast states up to the ${10}^{+}$ one, and for the ${8}_{2}^{+},\phantom{\rule{0.16em}{0ex}}{5}_{1}^{\ensuremath{-}}$, and ${7}_{1}^{\ensuremath{-}}$ states. The newly determined values of the $B(E2)$ values for the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states characterize a nucleus with a moderate quadrupole deformation and are well described by the interacting boson model. The ratio ${B}_{4/2}=1.38(22)$ is no longer anomalous. The low $B(E2)$ value of the ${6}_{1}^{+}$ state indicates a noncollective structure.

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

Here we present new information on the shape evolution of the very neutron-rich ^{92,94}Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of β∼0.25, with the high-K state in ^{94}Se being metastable and K hindered. Following this, ^{94}Se is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ^{94}Se_{60} ground-state band.

Here we present new information on the shape evolution of the very neutron-rich ^{92,94}Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of β∼0.25, with the high-K state in ^{94}Se being metastable and K hindered. Following this, ^{94}Se is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ^{94}Se_{60} ground-state band.

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei Dy164,166 using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an O18 beam and a gold-backed isotopically enriched Dy164 target of thickness 6.3mg/cm2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in Dy164, Dy166, and W178,179 following 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 Dy166 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 Dy164. The half-lives of the yrast 2+ states in Dy164 and Dy166 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 Dy166 is consistent with a quenching of nuclear quadrupole deformation at β≈0.35 as the N=104 mid-shell is approached.

The level structure of La-130 has been investigated using the (121)sb(C-12, 3n) reaction with the ROSPHERE array at IFIN-HH, Bucharest. the level scheme was significantly extended with the observation of 45 new states and 100 new transitions. Several band structures have been identified and a clear connection with the lower-lying states has been established. A lifetime of tau = 3.6(2) ns has been measured for the 346-keV 6(-) state by the in-beam fast timing technique. the lifetimes of 18 high-spin states have been determined by applying the Doppler-shift attenuation method. the deformations derived from the experimental B(E2) transition strengths indicate distinct coexisting shapes at high spins in La-130. the experimental properties of both low- and high-spin states were compared with theoretical calculations performed in the frame of the two-quasiparticles-plus-rotor model. Two new negative-parity decoupled bands, with a deduced quadrupole deformation beta(2) = 0.150(15), were interpreted by coupling the proton in the 1/2[550] and 3/2[541] orbitals with the odd neutron occupying mainly the low-Omega orbitals from the d(3/2) and 5(1/2) states. A quadrupole deformation beta(2) = 0.220(17) was derived for a newly identified positive-parity decoupled band. This enhanced deformation was attributed to the involvement in the band configuration of the Omega = 1/2 (f(7/2), h(9/2)) intruder neutron orbital. the multiparticle configuration pi g(7/2)(h(11/2))(2) circle times nu (h(11/2)) was assigned to a high-spin negative-parity dipole band, based on the comparison of the experimental B(M1) transition strengths with values calculated by applying the geometrical model of Donau and Frauendorf., Ministry of Education and Research, Romania [PN-19-06-01-02], Authors are thankful to the FN Tandem staff of the Horia Hulubei National Institute of Physics and Nuclear Engineering for the good quality delivered beam. This work was supported by the Ministry of Education and Research, Romania, Contract No. PN-19-06-01-02.

Physics letters / B 762, 404 - 408 (2016). doi:10.1016/j.physletb.2016.09.058, Published by North-Holland Publ., Amsterdam

The ROmanian array for SPectroscopy in HEavy ion REactions (ROSPHERE) has been designed as a multi-detector setup dedicated to γ-ray spectroscopy studies at the Bucharest 9 MV Tandem accelerator. Consisting of up to 25 detectors (either Compton suppressed HPGe detectors or fast LaBr3(Ce) scintillator detectors) together with a state of the art plunger device, ROSPHERE is a powerful tool for lifetime measurements using the Recoil Distance Doppler Shift (RDDS) and the in-beam Fast Electronic Scintillation Timing (FEST) methods. The array's geometry, detectors, electronics and data acquisition system are described. Selected results from the first experimental campaigns are also presented.

Using a hybrid Gammasphere array coupled to 25 LaBr3(Ce) detectors, the lifetimes of the first three levels of the yrast band in ¹¹⁴Pd populated via ²⁵²Cf decay, have been measured. The measured lifetimes are τ₂+=103(10)ps, τ₄+=22(13)ps, and τ₆+≤10ps for the 2⁺₁, 4⁺₁, and 6⁺₁ levels, respectively. Palladium-114 was predicted to be the most deformed isotope of its isotopic chain, and spectroscopic studies have suggested it might also be a candidate nucleus for low-spin stable triaxiality. From the lifetimes measured in this work, reduced transition probabilities B(E2;J→J−2) are calculated and compared with interacting boson model, projected shell model, and collective model calculations from the literature. The experimental ratio RB(E₂)=B(E2;4⁺₁→2⁺₁)/B(E2;2⁺₁→0⁺₁)=0.80(42) is measured for the first time in ¹¹⁴Pd and compared with the known values RB(E₂) in the palladium isotopic chain: the systematics suggest that, for N=68, a transition from γ-unstable to a more rigid γ-deformed nuclear shape occurs.

The reaction of a pulsed 18O beam on a self-supporting and gold-backed isotopically-enriched 164Dy target of thickness 6.3 mg/cm2 at separate primary beam energies of 71, 76 and 80 MeV was studied at the accelerator at the ALTO facility of the IPN Orsay. The γ rays produced were detected using the newly-constructed ν-Ball spectrometer which comprised of HPGe and LaBr3(Ce) detectors. This conference paper describes the methodology and effectiveness of multiplicity/sum-energy gating, for channel selection between fusion evaporation events and lower multiplicity/energy events from inelastic nuclear scattering and Coulomb excitation of the target, and from two-neutron transfer reactions to 166Dy.

WOS: 000450997800001, The lifetimes of the excited states belonging to the chiral-candidate doublet bands built on the pi h(11/2) circle times vh(11/12)(-1) configuration in La-130 have been measured by applying the Doppler-shift attenuation method and the in-beam fast timing technique. Excited states were populated in the Sb-121(C-12, 3n) reaction at a bombarding energy of 54 MeV. The 7(+) bandhead of the yrast band was identified as an isomeric state with a lifetime of tau(7(+)) = 0.38(7) ns Similar reduced transition probabilities B(M1) and B(E2) were found for the states with the same spin in the partner bands. The results revealed an even-odd spin dependence in the B(E2) values, whereas the B (M1) values vary nearly monotonically with increasing spin. The experimental properties of the doublet bands in La-130 are compared with theoretical calculations done in the frame of the two-quasiparticles-plus-triaxial-rotor model. The static chirality is not supported by the obtained data; however, chiral vibrations cannot be excluded., Scientific and Technological Research Council of Turkey (TUBTAK) [BIDEB-2219], Authors are thankful to the FN Tandem staff of the Horia Hulubei National Institute of Physics and Nuclear Engineering for the good quality of the delivered beam. S.A. acknowledges financial support from the Scientific and Technological Research Council of Turkey (TUBTAK) BIDEB-2219 Postdoctoral Research program.

This paper describes present capabilities of the new Target Preparation Laboratory constructed at Horia Hulubei National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, Romania. Self-supporting or backed targets are produced in the laboratory, using thermal evaporation, electron-gun evaporation, sputtering or cold rolling techniques. At present, it is possible to prepare layers with a wide range of thicknesses from several tens of µg/cm2 to several hundreds of mg/cm2. As an example, we briefly describe a 40Ca target preparation.

One of the most successful descriptions of the structure of atomic nuclei is the spherical shell model. It, however, becomes impractical when moving away from closed-shell nuclei. Instead, it is the interplay between the macroscopic shape degrees of freedom and the microscopic nature of the underlying single-particle structure in a deformed basis that determines the nuclear structure. Being the heaviest nucleus precisely in the middle of, known, closed proton and neutron shells, $^{170}$Dy has become a central calibration point for tests of collective models of nuclear physics. However, besides one candidate transition from a previous experiment in Legnaro, Italy, no experimental information is available for this nucleus. Using the EURICA setup at RIKEN, which couples the worlds highest intensity in-flight fission facility with a high-efficiency HPGe array, an experiment in November 2014 produced $^{170}$Dy nuclei by in-flight fission of a $^{238}$U beam. The results from this experiment provide a wealth of information on this elusive nucleus, including the evolution of quadrupole collectivity, rigidity and higher order deformations, as well as the long sought for isomeric $K=6^+$ state, predicted to be exceptionally pure at mid-shell. These results provide us with a rich level scheme for discussing both single-particle and collective structures at mid-shell.

A search for shape isomers in the ^{66}Ni nucleus was performed, following old suggestions of various mean-field models and recent ones, based on state-of-the-art Monte Carlo shell model (MCSM), all considering ^{66}Ni as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an ^{18}O beam on a ^{64}Ni target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0^{+} states in ^{66}Ni were populated and their γ decay was observed by γ-coincidence technique. The 0^{+} states lifetimes were assessed with the plunger method, yielding for the 0_{2}^{+}, 0_{3}^{+}, and 0_{4}^{+} decay to the 2_{1}^{+} state the B(E2) values of 4.3, 0.1, and 0.2 Weisskopf units (W.u.), respectively. MCSM calculations correctly predict the existence of all three excited 0^{+} states, pointing to the oblate, spherical, and prolate nature of the consecutive excitations. In addition, they account for the hindrance of the E2 decay from the prolate 0_{4}^{+} to the spherical 2_{1}^{+} state, although overestimating its value. This result makes ^{66}Ni a unique nuclear system, apart from ^{236,238}U, in which a retarded γ transition from a 0^{+} deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.

The $1{i}_{\frac{13}{2}}\ensuremath{\rightarrow}1{h}_{\frac{9}{2}}\phantom{\rule{0.16em}{0ex}}(M2)$ and $3{s}_{\frac{1}{2}}\ensuremath{\rightarrow}2{f}_{\frac{7}{2}}\phantom{\rule{0.16em}{0ex}}(E3)$ reduced proton transition probabilities in $_{83}^{209}\mathrm{Bi}$ have been determined from the direct half-life measurements of the ${\frac{13}{2}}_{1}^{+}$ and ${\frac{1}{2}}_{1}^{+}$ states using the Romanian array for $\ensuremath{\gamma}$-ray SPectroscopy in HEavy ion REactions (RoSPHERE). The ${\frac{13}{2}}_{1}^{+}$ and ${\frac{1}{2}}_{1}^{+}$ states were found to have ${T}_{\frac{1}{2}}=0.120(15)$ ns and ${T}_{\frac{1}{2}}=9.02(24)$ ns respectively. Angular distribution measurements were used to determine an $E3/M2$ mixing ratio of $\ensuremath{\delta}=\ensuremath{-}0.184(13)$ for the 1609 keV $\ensuremath{\gamma}$-ray transition deexciting the ${\frac{13}{2}}_{1}^{+}$ state. This value for $\ensuremath{\delta}$ was combined with the measured half-life to give reduced transition probabilities of $B(E3,{\frac{13}{2}}_{1}^{+}\ensuremath{\rightarrow}{\frac{9}{2}}_{1}^{\ensuremath{-}})=12(2)\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4.pt}{0ex}}{e}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{fm}}^{6}$ and $B(M2,{\frac{13}{2}}_{1}^{+}\ensuremath{\rightarrow}{\frac{9}{2}}_{1}^{\ensuremath{-}})=38(5){\ensuremath{\mu}}_{N}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{fm}}^{2}$. These values are in good agreement with calculations within the finite Fermi system. The extracted value of $B(E3,{\frac{1}{2}}_{1}^{+}\ensuremath{\rightarrow}{\frac{7}{2}}_{1}^{\ensuremath{-}})=6.3(2)\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4.pt}{0ex}}{e}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{fm}}^{6}$ can be explained by a small ($\ensuremath{\sim}6%$) admixture in the wave function of the ${\frac{1}{2}}_{1}^{+}$ state.