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The Zirconium (Z = 40) isotopic chain has attracted interest for more than four decades. The abrupt lowering of the energy of the first 2(+) state and the increase in the transition strength B(E2; 2(1)(+) -> 0(1)(+) ) going from Zr-98 to Zr-100 has been the first example of "quantum phase transition" in nuclear shapes, which has few equivalents in the nuclear chart. Although a multitude of experiments have been performed to measure nuclear properties related to nuclear shapes and collectivity in the region, none of the measured lifetimes were obtained using the Recoil Distance Doppler Shift method in the gamma gamma-coincidence mode where a gate on the direct feeding transition of the state of interest allows a strict control of systematical errors. This work reports the results of lifetime measurements for the first yrast excited states in Zr98-104 carried out to extract reduced transition probabilities. The new lifetime values in gamma gamma-coincidence and gamma-single mode are compared with the results of former experiments. Recent predictions of the Interacting Boson Model with Configuration Mixing, the Symmetry Conserving Configuration Mixing model based on the Hartree-Fock- Bogoliubov approach and the Monte Carlo Shell Model are presented and compared with the experimental data.

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Lifetimes of yrast states in the rare-earth midshell isotopes W-176,W-178,W-180 have been measured with fast-timing methods using the Cologne iron-free Orange spectrometer and the Cologne HORUS spectrometer and with the recoil distance Doppler shift method using the Cologne coincidence plunger setup. Different fusion evaporation reactions have been used to populated excited states in the investigated nuclei. Lifetimes of the 2(1)(+) , 4(1)(+) , 6(1)(+) , and 8(1)(+) yrast states were measured in all three tungsten isotopes and in addition the lifetime of the 10(1)(+) state was measured in W-176. Quadrupole deformation parameters, reduced transition probabilities, and B-4/2 ratios are extracted and discussed in the context of interacting boson model 1 calculations. The results with newly determined signatures largely confirm the investigated tungsten isotopes to be deformed prolate rotors.

Direct lifetime measurements via gamma -gamma coincidences using a fast timing detector array consisting of LaBr3(Ce) scintillators has been applied to determine the lifetime of low-lying states in the semimagic (N = 50) nucleus Ru-94. The experiment was carried out as the first in a series of FAIR-0 experiments with the DESPEC experimental setup at the Facility for Antiproton and Ion Research (FAIR). Excited states in Ru-94 were populated primarily via the beta-delayed proton emission of Pd-95 nuclei, produced in the projectile fragmentation of an 850 MeV/nucleon Xe-124 beam impinging on a 4 g/cm(2) Be-9 target. While the deduced E2 strength for the 2(+) -> 0(+) transition in the yrast cascade follows the expected behavior for conserved seniority symmetry, the intermediate 4(+) -> 2(+) transition exhibits a drastic enhancement of transition strength in comparison with pure-seniority model predictions as well as standard shell model predictions in the f pg proton hole space with respect to doubly magic Sn-100. The anomalous behavior is ascribed to a subtle interference between the wave function of the lowest seniority v = 2, I-pi = 4(+) state and that of a close-lying v = 4 state that exhibits partial dynamic symmetry. In addition, the observed strongly prohibitive 6(+) -> 4(+) transition can be attributed to the same mechanism but with a destructive interference. It is noted that such effects may provide stringent tests of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations.

This paper reports results of the first experiment of the DESPEC Phase-0 campaign at GSI, which focused on the study of neutron-deficient nuclei approaching 100Sn. These data provide the first extended commissioning experiment for the DESPEC collaboration within NuSTAR. We present results on electromagnetic transition rates associated with the decays from excited states populated following the formation of I pi = 8+ proton 'seniority -isomer' states in the N = 50 isotones 94Ru and 96Pd. Direct half-life measurements via gamma-gamma coincidences using the FATIMA detector array consisting of 36 LaBr3(Ce) scintillators have determined the reduced matrix elements associated with decays between low-lying states in these semi-magic nuclei. The extracted half-lives for yrast I pi = 6+ and 4+ states in 96Pd and the 6+ state in 94Ru are consistent with the published, highest-precision values for these nuclei.

Lifetimes have been measured for the low-lying yrast levels of Te-130,Te-132 using gamma - gamma fast timing methods. The excited states were populated in beta(-) and IT decay of fission fragments, mass-separated by the Lohengrin separator at Institut Laue-Langevin. Four LaBr3(Ce) detectors, placed at the focal plane of the spectrometer, were used for the gamma -gamma fast timing measurement. Lifetimes of mu s isomers were measured using coincidence of an ionization chamber with two Clover HPGe detectors. Themeasured lifetimes and absolute transition probabilities are discussed in the light of systematics with the neighboring nuclei. Large basis shell model calculations have been performed to interpret the level structure and transition probabilities in these even mass Te isotopes.

Lifetimes of low-energy states in the semimagic nucleus 211At were measured employing the recoil-distance Doppler shift and the Doppler-shift attenuation methods. The deduced transition probabilities are compared to two shell-model calculations, one using the modified Kuo-Herling interaction in a multi-j model space and the other using a semiempirical interaction for protons confined to the single-j 0h9/2 orbital. The Kuo-Herling calculations overestimates some of the ground-state transition probabilities, possibly due to contributions not included in the calculated ground-state wave function of 211At. A strong underestimation of the E2 strengths involving the 7/2-1 state is also observed. Therefore, a modification of a single two-body matrix element of the Kuo-Herling interaction is introduced which improves the agreement with the experimental data significantly. The calculations in the single-j approximation agree very well with the measured transition probabilities, indicating that seniority can be regarded as a good quantum number in 211At.

The beta decay of the neutron-rich In-134 and In-135 was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number Z = 50 above the N = 82 shell. The beta-delayed gamma-ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three beta-decay branches of In-134 were established, two of which were observed for the first time. Population of neutron-unbound states decaying via. rays was identified in the two daughter nuclei of In-134, Sn-134 and Sn-133, at excitation energies exceeding the neutron separation energy by 1 MeV. The beta-delayed one- and two-neutron emission branching ratios of In-134 were determined and compared with theoretical calculations. The beta-delayed one-neutron decay was observed to be dominant beta-decay branch of In-134 even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of Sn-134. Transitions following the beta decay of In-135 are reported for the first time, including. rays tentatively attributed to Sn-135. In total, six new levels were identified in Sn-134 on the basis of the beta.. coincidences observed in the In-134 and In-135 beta decays. A transition that might be a candidate for deexciting the missing neutron single-particle 13/2(+) state in Sn-133 was observed in both beta decays and its assignment is discussed. Experimental level schemes of Sn-134 and Sn-135 are compared with shell-model predictions. Using the fast timing technique, half-lives of the 2(+), 4(+), and 6(+) levels in Sn-134 were determined. From the lifetime of the 4(+) state measured for the first time, an unexpectedly large B(E2; 4(+)-> 2(+)) transition strength was deduced, which is not reproduced by the shell-model calculations.

The structure of Ge-80 has been investigated at the ISOLDE facility at CERN. A previous study reported for the first time a low-lying 0(2)(+) intruder state at 639 keV, based on the coincidence with a previously unobserved 1764-keV gamma ray, and suggested it as evidence for shape coexistence in Ge-80. We used the beta decay from the 3(-) 22.4-keV state in Ga-80 to enhance the population of low-spin states in Ge-80, including any excited 0(+) level, and gamma gamma coincidences to investigate it. We observed a 1764-keV gamma ray in coincidence with strong transitions in Ge-80, thus not feeding the proposed 639-keV 0(2)(+). No connecting transitions from previously known levels to the 639-keV and 2403-keV 2(3)(+) states could be established either. Shell-model calculations for Ge isotopes and N = 48 isotones were performed. They succeed to explain most of the experimental levels, but fail to reproduce the presence of a 0(2)(+) state below approximate to 1200 keV in Ge-80. Our experimental findings and shell-model calculations are difficult to reconcile with a very low-lying 0(2)(+) state in Ge-80.

This paper reports preliminary results of the DESPEC campaign at GSI, focused on the study of neutron-deficient nuclei approaching Sn-100. The results presented show the isomeric decays of excited states with I-pi = 14(+) and 8(+) in Pd-96 and Pd-94, respectively. The detailed characterisation of the DESPEC set-up and analysis methodologies, proven in this experimental run, are crucial for the future campaigns.

The 5/2(1)(-), 9/2(1)(-), and 11 /2(1)(-) states in Po-209 were populated in the beta decay of At-209 and their lifetimes measured using the electronic gamma-gamma fast timing technique. The lifetime of the 9/2(1)(-) state is measured for first time. The lifetime of the 5/2(1)(-) is measured to be shorter than the value adopted in the literature while the lifetime of the 11/2(1)(-) state agrees well with the previous measurement. In order to get deeper insight into the structure of the states, a shell-model calculation was carried out adopting a microscopic effective interaction derived from the realistic CD-Bonn potential. The comparison between theoretical and experimental data for the low-lying negative-parity states of Po-209 supports the reliability of the predicted wave functions, which are found to be dominated by the coupling of a neutron hole to the yrast states of Po-210. However, it also points to the important role played by minor wave-function components in describing the reduced electromagnetic strengths, suggesting the need of additional configuration mixing for achieving a better quantitative agreement.

The level lifetimes of the 2(1)(+) and 4(1)(+) states in Pt-182 have been re-measured employing the gamma-gamma fast-timing technique using fast LaBr3(Ce) scintillators. Excited states in the nucleus of interest were populated by the fusion-evaporation reaction Yb-170(O-16, 4n)Pt-182 at a beam energy of 87 MeV provided by the FN Tandem accelerator of the University of Cologne. The lifetime of the 2(1)(+) state was re-measured with high accuracy to be tau = 563(12) ps and resolves inconsistencies from previous measurements. Experimental results are compared to theoretical calculations in the framework of the sd-IBM with and without configuration mixing.

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(Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally 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. The measured half-lives are 476(44)ps and 275(65)ps for the I-pi = 11(-) and 12(+) states, 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. (C) 2019 The Author(s). Published by Elsevier B.V.

Excited states of the neutron-rich nucleus Ce-148 have been populated by neutron-induced fission of a U-235 target. Its electromagnetic decays were studied by means of gamma-ray coincidence spectroscopy with fast-timing capabilities. Lifetimes of the 2(1)(+) and 4(1)(+) states of Ce-148 were obtained and their E2 decay rates deduced. The B-4/2 = B(E2; 4(1)(+) -> 2(1)(+))/B(E2; 2(1)(+) -> 0(1)(+)) ratio indicates that Ce-148 is a transitional nucleus while the N = 88/90 shape phase transition evolves into a gradual change of nuclear deformation for proton numbers Z < 60.

We report on the development of custom Front-End Electronics (FEE) dedicated to large area SiPM arrays for gamma-ray spectroscopy applications in high counting rate conditions. Two types of commercially available 8 x 8 arrays of 6 x 6 mm(2) Silicon Photomultipliers (SiPM) and a quasi-circular array of 30 SiPM cells were coupled to dedicated Front End Electronics (FEE) boards and tested in combination with LaBr3(Ce) crystals. Energy resolution measurements were taken with standard calibration radioactive sources (Co-60, Eu-152 and Co-56) while the timing response was studied in comparison with industry standard photomultiplier tubes. The energy resolution measured in this setup was down to 3.09(9)% at 661 keV, while the best timing resolution measured was 222.3(22) ps. Finally, the design of a SiPM based-ray detector, dedicated to fast-timing applications, is presented.

We report on systematic gamma-gamma fast-timing measurements by using four cylindrical 1.5 '' x 1.5 '' LaBr3(Ce) scintillator detectors which were installed in compact geometry around the focal plane of the Lohengrin fission-fragment separator at the Institut Laue-Langevin in Grenoble, France. Unconventional gamma-ray sources as Os-185 and W-187 were produced by thermal-neutron activation to provide nearly prompt low-energy gamma and K-X rays with average gamma- and X-ray multiplicity equal to two. Due to practically no contribution of Compton background, highly precise results of time-walk measurements down to 40 keV are presented. Timing deviations related to different phenomena have been investigated, such as the geometry of an extended gamma-ray source and the detector arrangement, long-term timing shifts and the timing contributions of the Compton background and the inter-detector Compton-scattering. The geometrical timing deviations are shown to be minimized using a multi-element detector array with a centrally symmetric arrangement relative to the center of the focal plane. Time-correction formula are proposed as analytical corrections for long-term time shifts, the time contributions of the Compton background and the position-dependent change of the time walk for cases where the calibration source cannot be placed at the center of the focal plane.

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei Dy-164,Dy-166 using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an O-18 beam and a gold-backed isotopically enriched Dy-164 target of thickness 6.3 mg/cm(2) at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in Dy-164, Dy-166, and W-178,W-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 nu-Ball high-purity germanium (HPGe)-LaBr3 hybrid gamma-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 I-pi = 2(+) excited state in Dy-166 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 Dy-164. The half-lives of the yrast 2(+) states in Dy-164 and Dy-166 were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q(0) = 7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2(+) state in Dy-166 is consistent with a quenching of nuclear quadrupole deformation at beta approximate to 0.35 as the N = 104 mid-shell is approached.

The beta-decay of Zn-81 to the neutron magic N = 50 nucleus Ga-81, with only three valence protons with respect to Ni-78, was investigated. The study was performed at the ISOLDE facility at CERN by means of gamma spectroscopy. The 81Zn half-life was determined to be T-1/2 = 290(4) ms while the beta-delayed neutron emission probability was measured as P-n = 23(4)%. The analysis of the beta-gated gamma-ray singles and gamma-gamma coincidences from the decay of Zn-81 provides 47 new levels and 70 new transitions in Ga-81. The beta(-)n decay of Zn-81 was observed and a new decay scheme into the odd-odd Ga-80 nucleus was established. The half-lives of the first and second excited states of Ga-81 were measured via the fast-timing method using LaBr3(Ce) detectors. The level scheme and transition rates are compared to large-scale shell-model calculations. The low-lying structure of (81)Gais interpreted in terms of the coupling of the three valence protons outside the doubly magic Ni-78 core.

Lifetimes of low-lying yrast states in Er-162 are measured using the electronic gamma-gamma fast-timing technique. Excited states were populated in a Sm-154(C-12, (4)n)Er-162 fusion-evaporation reaction and gamma rays were detected in a combined setup of high purity germanium and LaBr3(Ce) scintillation detectors. The lifetimes of the 4(1)(+) and 6(1)(+) states are determined for the first time and the lifetimes of the 2(1)(+) and 7- states are remeasured with higher precision. Reduced transition probabilities are extracted and well reproduced by theoretical calculations in the framework of the confined beta soft model and the interacting boson model.

Lifetimes of low-spin excited states in Zr-98 were measured using the recoil-distance Doppler-shift technique , and the Doppler-shift attenuation method. The nucleus of interest was populated in a( 96)zr(O-18, O-16) Zr-98 two- neutron transfer reaction at the Cologne FN Tandem accelerator. Lifetimes of six low-spin excited states, of which four are unknown, were measured. The deduced B(E2) values were compared with Monte Carlo shell model and interacting boson model with configuration mixing calculations. Both approaches reproduce well most of the data but leave challenging questions regarding the structure of some low-lying states.

Lifetimes of low- and high-spin states in the odd-A nucleus Hf-177 were measured during the EXILL&FATIMA campaign using a spectrometer equipped with eight HPGe-Clover detectors and 16 fast-timing LaBr3(Ce) detectors. For the determination of lifetimes in the pico- to nanosecond regime, the well-established generalized centroid difference method was used. Lifetimes of the 9/2(-), 9/2(+), 11/2(-), 11/2(+) states were remeasured, the lifetimes of the 13/2(-), 17/2(-), 13/2(+), 15/2(+), and 17/2(+) states were determined for the first time and an upper limit for the 19/2(+) state has been established. From these lifetimes absolute reduced transition probabilities were extracted and compared to particle-rotor-model calculations and quasiparticle-phonon-model calculations showing the importance of including multipole-multipole interactions in the description of odd-A nuclei in the rare earth region.