Search

The level structure of Er158 has been studied using the Gammasphere spectrometer via the Cd114(Ca48,4n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level scheme has been considerably extended with more than 200 new transitions and six new rotational structures, including two strongly coupled high-K bands. Configuration assignments for the new structures are based on their observed alignments, B(M1)/B(E2) ratios of reduced transition probabilities, excitation energies, and comparisons with neighboring nuclei and theoretical calculations. With increasing angular momentum, this nucleus exhibits Coriolis-induced alignments of both neutrons and protons before it then undergoes a rotation-induced transition from near-prolate collective rotation to a noncollective oblate configuration. This transition occurs via the mechanism of band termination around spin 45ħ in three rotational structures. Two distinct lifetime branches, consistent with the crossing of a collective "fast"rotational structure by an energetically favored "slow"terminating sequence, are confirmed for the positive-parity states, and similar behavior is established in the negative-parity states. Weak-intensity, high-energy transitions are observed to feed into the terminating states. At the highest spins, three collective bands with high dynamic moments of inertia and large quadrupole moments were identified. These bands are interpreted as triaxial strongly deformed structures and mark a return to collectivity at ultrahigh spin.

The fusion-evaporation reaction Si28+Ca40 at 122 MeV beam energy was used to populate excited states in the N=Z+1 nucleus Ga63. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array, the level scheme of Ga63 was extended by more than a factor of two in terms of number of γ-ray transitions and excited states, excitation energy (Ex>30MeV), and angular momentum (I>30ℏ). Nine regular sequences of states were newly established in the high-spin part of the level scheme. The majority of these rotational band structures could be connected to the previously known part of the level scheme by high-energy γ-ray transitions in the energy range Eγ=4-6MeV. Low-spin states were assessed by shell-model calculations. The high-spin rotational bands were interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

A nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z=114). An upgraded TASISpec decay station was placed behind the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ^{48}Ca+^{242}Pu and ^{48}Ca+^{244}Pu provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ^{286}Fl and ^{288}Fl, respectively. A prompt coincidence between a 9.60(1)-MeV α particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ^{282}Cn. Spectroscopy of ^{288}Fl decay chains fixed Q_{α}=10.06(1)  MeV. In one case, a Q_{α}=9.46(1)-MeV decay from ^{284}Cn into ^{280}Ds was observed, with ^{280}Ds fissioning after only 518  μs. The impact of these findings, aggregated with existing data on decay chains of ^{286,288}Fl, on the size of an anticipated shell gap at proton number Z=114 is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.

A nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z=114). An upgraded TASISpec decay station was placed behind the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ^{48}Ca+^{242}Pu and ^{48}Ca+^{244}Pu provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ^{286}Fl and ^{288}Fl, respectively. A prompt coincidence between a 9.60(1)-MeV α particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ^{282}Cn. Spectroscopy of ^{288}Fl decay chains fixed Q_{α}=10.06(1)  MeV. In one case, a Q_{α}=9.46(1)-MeV decay from ^{284}Cn into ^{280}Ds was observed, with ^{280}Ds fissioning after only 518  μs. The impact of these findings, aggregated with existing data on decay chains of ^{286,288}Fl, on the size of an anticipated shell gap at proton number Z=114 is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.

The fusion-evaporation reaction Si28+Ca40 at 122 MeV beam energy was used to populate high-spin states in the odd-odd N=Z nucleus Ga62. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array the decay scheme of Ga62 was extended beyond 10 MeV excitation energy. The onset of band structures was observed. These high-spin rotational states are interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

The fusion-evaporation reaction S32+Si28 at 125 MeV beam energy was used to populate high-spin states in the semimagic N=28 nuclei Mn53 and Fe54. With a combination of the Gammasphere spectrometer and ancillary devices including the Microball CsI(Tl) array, extensive high-spin level schemes are derived. They exhibit rotational-like collective structures and competing single-particle excitations. The experimental results are compared with predictions from shell-model calculations, for which the inclusion of isopin-symmetry-breaking terms is found to improve the description. An interpretation of the high-spin states is put forward using cranked Nilsson-Strutinsky calculations, indicative of contributions from collective excitations beyond some 8-MeV excitation energy and highlighting the importance of the g9/2 intruder orbital in this energy range.

The fusion-evaporation reaction S32+Si28 at 125 MeV beam energy was used to populate high-spin states in the semimagic N=28 nuclei Mn53 and Fe54. With a combination of the Gammasphere spectrometer and ancillary devices including the Microball CsI(Tl) array, extensive high-spin level schemes are derived. They exhibit rotational-like collective structures and competing single-particle excitations. The experimental results are compared with predictions from shell-model calculations, for which the inclusion of isopin-symmetry-breaking terms is found to improve the description. An interpretation of the high-spin states is put forward using cranked Nilsson-Strutinsky calculations, indicative of contributions from collective excitations beyond some 8-MeV excitation energy and highlighting the importance of the g9/2 intruder orbital in this energy range.

Short-lived α-decaying nuclei "northeast" of Pb208 in the chart of nuclides were studied using the reaction Ca48+Am243 with the decay station TASISpec at TASCA, GSI Darmstadt. Decay energies and times from pile-up events were extracted with a tailor-made pulse-shape analysis routine and specific α-decay chains were identified in a correlation analysis. Decay chains starting with the even-even Ra220 and its odd-A neighbors, Fr219, and Ra221,219, with a focus on the Ra219→Rn215 decay, were studied by means of α-γ spectroscopy. A revised α-decay scheme of Ra219 is proposed, including a new decay branch from a previously not considered isomeric state at 17 keV excitation energy. Conclusions on nuclear structure are drawn from the experimental data, aided by Geant4 simulations and a discussion on theoretical calculations.

Short-lived α-decaying nuclei "northeast" of Pb208 in the chart of nuclides were studied using the reaction Ca48+Am243 with the decay station TASISpec at TASCA, GSI Darmstadt. Decay energies and times from pile-up events were extracted with a tailor-made pulse-shape analysis routine and specific α-decay chains were identified in a correlation analysis. Decay chains starting with the even-even Ra220 and its odd-A neighbors, Fr219, and Ra221,219, with a focus on the Ra219→Rn215 decay, were studied by means of α-γ spectroscopy. A revised α-decay scheme of Ra219 is proposed, including a new decay branch from a previously not considered isomeric state at 17 keV excitation energy. Conclusions on nuclear structure are drawn from the experimental data, aided by Geant4 simulations and a discussion on theoretical calculations.

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus As67 have been studied by using the Ca40(Ar36,2αp)As67 reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states.

High-spin states in Cs124 were populated in the Ni64(Ni64,p3n) reaction and the Gammasphere detector array was used to measure γ-ray coincidences. Both positive- and negative-parity bands, including bands with chiral configurations, have been extended to higher spin, where a shape change has been observed. The configurations of the bands before and after the alignment are discussed within the framework of the cranked Nilsson-Strutinsky model. The calculations suggest that the nucleus undergoes a shape transition from triaxial to prolate around spin I≃22 of the positive-parity states. The alignment gain of 8â.,, observed in the positive-parity bands, is due to partial alignment of several valence nucleons. This indicates the absence of band crossing due to paired nucleons in the bands.

High-spin states in Xe124 have been populated using the Se80(Ca48,4n) reaction at a beam energy of 207 MeV and high-multiplicity, γ-ray coincidence events were measured using the Gammasphere spectrometer. Six high-spin bands with large moments of inertia, similar to those observed in neighboring nuclei, have been observed. The experimental results are compared with calculations within the framework of the cranked Nilsson-Strutinsky model. It is suggested that the configurations of the bands involve excitations of protons across the Z=50 shell gap coupled to neutrons within the N=50-82 shell or excited across the N=82 shell closure.

Products of the fusion-evaporation reaction 48Ca + 243Am were studied with the TASISpec set-up at the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. Amongst the detected thirty correlated α-decay chains associated with the production of element Z=115, two recoil-α-fission and five recoil-α-α-fission events were observed. The latter five chains are similar to four such events reported from experiments performed at the Dubna gas-filled separator, and three such events reported from an experiment at the Berkeley gas-filled separator. The four chains observed at the Dubna gas-filled separator were assigned to start from the 2n-evaporation channel 289115 due to the fact that these recoil-α-α-fission events were observed only at low excitation energies. Contrary to this interpretation, we suggest that some of these recoil-α-α-fission decay chains, as well as some of the recoil-α-α-fission and recoil-α-fission decay chains reported from Berkeley and in this article, start from the 3n-evaporation channel 288115.

A classic region of band termination at high spin occurs in rare-earth nuclei with around ten valence nucleons above the 146Gd closed core. Results are presented here for such non-collective oblate (γ = 60°) terminating states in odd-Z 155Ho, odd-odd 156Ho, and even-even 156Er, where they are compared with neighbouring nuclei. In addition to these particularly favoured states, the occurrence of collective triaxial strongly deformed (TSD) bands, bypassing the terminating states and extending to over 65h, is reviewed.

Produced in the reaction 48Ca+243Am, thirty correlated α-decay chains were observed in an experiment conducted at the GSI Helmholzzentrum für Schwerionenforschung, Darmstadt, Germany. The decay chains are basically consistent with previous findings and are considered to originate from isotopes of element 115 with mass numbers 287, 288, and 289. A set-up aiming specifically for high-resolution charged particle and photon coincidence spectroscopy was placed behind the gas-filled separator TASCA. For the first time, γ rays as well as X-ray candidates were observed in prompt coincidence with the α-decay chains of element 115.

The N ∼ 90 region of the nuclear chart has featured prominently as the spectroscopy of nuclei at extreme spin has progressed. This talk will present recent discoveries from investigations of high spin behavior in the N ∼ 90 Er, Tm and Yb nuclei utilizing the Gammasphere gamma-ray spectrometer. In particular it will include discussion of the beautiful shape evolution and coexistence observed in these nuclei along with the identification of a remarkable new family of band structures. The latter are very weakly populated rotational sequences with high moment of inertia that bypass the classic terminating configurations near spin 40-50h{stroke}, marking a return to collectivity that extends discrete γ-ray spectroscopy to well over 60h{stroke}. Establishing the nature of the yrast states in these nuclei beyond the oblate band-termination states has been a major goal for the past two decades. Cranking calculations suggest that these new structures most likely represent stable triaxial strongly deformed bands that lie in a valley of favored shell energy in deformation and particle-number space.

The level structure of 60140Nd80 has been established up to spin 48 by in-beam γ-ray spectroscopy by use of the Zr96(Ca48, 4n) reaction. High-fold γ-ray coincidences were measured with the EUROBALL spectrometer. Twelve new rotational bands have been discovered at high spins. They are interpreted as being formed in a deep triaxial minimum at 2 0.25 and γ 35°. Possible configurations are assigned to the observed bands on the basis of configuration-dependent cranked Nilsson-Strutinsky calculations. © 2005 The American Physical Society.

A highly-deformed rotational band has been identified in the N = Z nucleus 36Ar. At high spin the band is observed to its presumed termination at Iπ = 16+, while at low spin it has been firmly linked to previously known states in 36Ar. Spins, parities, and absolute excitation energies have thus been determined throughout the band. Lifetime measurements establish a large low-spin quadrupole deformation (β2 = 0.46 ±0.03) and indicate a decreasing collectivity as the band termination is approached. With effectively complete spectroscopic information and a valence space large enough for significant collectivity to develop, yet small enough to be meaningfully approached from the shell model perspective, this rotational band in 36Ar provides many exciting opportunities to test and compare complementary models of collective motion in nuclei.

The highest-spin discrete states (I ∼ 60ℏ and Ex ∼ 30MeV) in normal deformed nuclei have been observed in the rare-earth isotope 156Dy using the GAMMASPHERE spectrometer. The quadrupole moments for a variety of configurations, including the 9/2+[404] (g9/2) proton, 1/2+[660] (i13/2) and 1/2-[541] (f7/2, h9/2) neutron orbitals, were measured in a wide range of ∼ 130 nuclei.