Search

New levels have been established above the 861-keV, 9/2+ isomeric state in Fe61. The observations can be reproduced satisfactorily by shell model calculations, but only after a significant lowering of the νg9/2 single-particle energy with respect to the value determined empirically from levels outside a Ca48 core. The results are also described well within the scope of the particle-triaxial-rotor model assuming a deformation β2~0.24. The present findings, together with the recently measured magnetic and quadrupole moments of this state, point toward a structure possibly associated with a prolate shape. © 2008 The American Physical Society.

Excited states with seniority ν=3, 5, and 7 have been investigated in odd neutron-rich Sn119,121,123,125 isotopes produced by fusion-fission of 6.9-MeV/ACa48 beams with Pb208 and U238 targets and by fission of a U238 target bombarded with 6.7-MeV/ANi64 beams. Level schemes have been established up to high spin and excitation energies in excess of 6 MeV, based on multifold gamma-ray coincidence relationships measured with the Gammasphere array. In the analysis, the presence of isomers was exploited to identify gamma rays and propose transition placements using prompt and delayed coincidence techniques. Gamma decays of the known 27/2- isomers were expanded by identifying new deexcitation paths feeding 23/2+ long-lived states and 21/2+ levels. Competing branches in the decay of 23/2- states toward two 19/2- levels were delineated as well. In Sn119, a new 23/2+ isomer was identified, while a similar 23/2+ long-lived state, proposed earlier in Sn121, has now been confirmed. In both cases, isomeric half-lives were determined with good precision. In the range of ν=3 excitations, the observed transitions linking the various states enabled one to propose with confidence spin-parity assignments for all the observed states. Above the 27/2- isomers, an elaborate structure of negative-parity levels was established reaching the (39/2-), ν=7 states, with tentative spin-parity assignments based on the observed deexcitation paths as well as on general yrast population arguments. In all the isotopes under investigation, strongly populated sequences of positive-parity (35/2+), (31/2+), and (27/2+) states were established, feeding the 23/2+ isomers via cascades of three transitions. In the Sn121,123 isotopes, these sequences also enabled the delineation of higher-lying levels, up to (43/2+) states. In Sn123, a short half-life was determined for the (35/2+) state. Shell-model calculations were carried out for all the odd Sn isotopes, from Sn129 down to Sn119, and the results were found to reproduce the experimental level energies rather well. Nevertheless, some systematic deviations between calculated and experimental energies, especially for positive-parity states, point to the need to improve some of the two-body interactions used in calculations. The computed wave-function amplitudes provide for a fairly transparent interpretation of the observed level structures. The systematics of level energies over the broad A = 117-129 range of Sn isotopes displays a smooth decrease with mass A, and the observed regularity confirms most of the proposed spin-parity assignments. The systematics of the B(E2) reduced transition probabilities extracted for the 23/2+ and 19/2+ isomers is discussed with an emphasis on the close similarity of the observed A dependence with that of the E2 transition rates established for other ν=2, 3, and 4 isomers in the Sn isotopic chain.

High-spin states with the seniority v ≥ 2 have been investigated in the neutron-rich 118,120,122,124,126Sn isotopes. They were produced in fusion-fission processes following 48Ca + 208Pb, 48Ca + 238U reactions and via fission of target nuclei in the 64Ni + 238U system. By employing techniques of delayed- and cross-coincidences, it was possible to establish level schemes up to an 8 MeV excitation energy. The 13- and 15- states were identified as being isomeric and their half-lives were determined. The reduced transition probabilities extracted for isomeric transitions behave very regularly with the mass number A. The spin-parity values assigned to or suggested for the identified states were supported by shell-model calculations and by systematics.

Excited states above the seniority ν=2 isomers have been investigated in even neutron-rich 118-128Sn isotopes produced by fusion-fission of 6.9 MeV/A Ca48 beams with Pb208 and U238 targets and by fission of 6.7 MeV/A Ni64 beams on a U238 target. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold γ-ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques, including prompt and delayed cross-coincidence methods. As a result, seniority ν=4, 15-, and 13- isomers were observed and their half-lives determined. These long-lived states in turn served as steppingstones to delineate the isomeric decays and to locate higher-lying states with good sensitivity. As the observed isomeric decays feed down to 10+ and 7- isomers, firm spin-parity assignments could be proposed for most of the seniority ν=4 states. Higher-lying, seniority ν=6 levels were assigned tentatively on the basis of the observed deexcitation paths as well as of general yrast population arguments. Shell-model calculations were carried out down to Sn122 in the g7/2, d5/2, d3/2, s1/2, and h11/2 model space of neutron holes with respect to a Sn132 core. Effective two-body interactions were adjusted such that satisfactory agreement with data was achieved for Sn130. The results reproduce the experimental level energies and spin-parity assignments rather well. The intrinsic structure of the states is discussed on the basis of the calculated wave functions which, in many instances, point to complex configurations. In a few cases, the proposed assignments lead to unresolved issues. The smooth, systematic decrease of the level energies with mass A is accompanied by the similarly regular behavior with A of the reduced transition probabilities extracted from the isomeric half-lives. This A dependence is discussed for the E1 and E2 transitions in the decay of the seniority ν=4 isomers and is compared to that determined in earlier work for the E2 transition rates from the ν=2,3 isomers.

Excited states of 76Ge have been populated in above-barrier Coulomb excitation and inelastic scattering of a 530-MeV 76Ge beam on a 238U target and studied using in-beam γ-ray spectroscopy with the Gammasphere array. The γ band was extended considerably and one new band was identified. Comparisons of the γ band with collective- and shell-model calculations suggest that 76Ge may be a rare example of a nucleus exhibiting rigid triaxial deformation in the low-lying states.

The nuclei 65,67Cu were studied in reactions between a 430-MeV 64Ni beam and a thick 238U target with the Gammasphere array. Decay schemes for both nuclei have been extended, with spin and parity assignments of observed states constrained by measured γ-ray angular distributions and correlations. Positive-parity level structures, based on p3/2 protons coupled to negative-parity states in the Ni cores, have been identified above the known 9/2+ states. In 67Cu, a negative-parity dipole band built upon a πf7/2-1 state has been observed, as were two shorter negative-parity sequences. A qualitative description of the level structures has been obtained through comparison with systematics of the odd-A57-71Cu isotopes and with states in the neighboring even-even Ni and Zn cores. Shell-model calculations using JUN45 and jj44b effective interactions were performed for 65,67Cu, with jj44b providing overall better agreement with the data. Both are limited, however, by the restriction that the f7/2 and g9/2 orbitals are not available simultaneously in the basis. Proton f7/2 and neutron g9/2 orbitals are required for the full negative-parity spectrum of states, while g9/2 protons and neutrons are both important for positive-parity levels. The latter states are found to be better described in terms of weak coupling of a proton to the Ni core.

Excited states in 64Ni, 66Ni, and 68Ni were populated in quasielastic and deep-inelastic reactions of a 430-MeV 64Ni beam on a thick 238U target. Level schemes including many nonyrast states were established up to respective excitation energies of 6.8, 8.2, and 7.8 MeV on the basis of γ-ray coincidence events measured with the Gammasphere array. Spin-parity assignments were deduced from an angular-correlation analysis and from observed γ-decay patterns, but information from earlier γ-spectroscopy and nuclear-reaction studies was used as well. The spin assignments for nonyrast states were supported further by their observed population pattern in quasielastic reactions selected through a cross-coincidence technique. Previously established isomeric-state decays in 66Ni and 68Ni were verified and delineated more extensively through a delayed-coincidence analysis. A number of new states located above these long-lived states were identified. Shell-model calculations were carried out in the p3/2f5/2p 1/2g9/2 model space with two effective interactions using a 56Ni core. Satisfactory agreement between experimental and computed level energies was achieved, even though the calculations indicate that all the states are associated with rather complex configurations. This complexity is illustrated through the discussion of the structure of the negative-parity states and of the M1 decays between them. The best agreement between data and calculations was achieved for 68Ni, the nucleus where the calculated states have the simplest structure. In this nucleus, the existence of two low-spin states reported recently was confirmed as well. Results of the present study do not indicate any involvement of collective degrees of freedom and confirm the validity of a shell-model description in terms of neutron excitations combined with a closed Z = 28 proton shell. Further improvements to the calculations are desirable.

The high-spin structure of the neutron-rich 204Tl isotope has been studied up to a 11.2-MeV excitation energy and a I=30 spin range using the deep-inelastic heavy-ion γ-spectroscopy method with reactions of 48Ca on thick 208Pb and 238U targets. The established structure of yrast levels involves four isomeric states up to Iπ=22-, the highest spin state available for the maximally aligned four valence holes. The observations are interpreted and quantitatively confirmed by shell-model calculations. The rates of the identified M2 and E3 isomeric decays are discussed and a striking analogy is found for the yrast level structures and γ decays observed in the 18+ to 22- and 45/2- to 53/2+ spin ranges in 204Tl and 203Hg, respectively. In the highest spin part of the scheme, two prominently populated yrast states are tentatively identified as the 3 - 208Pb core excitation built on the 22- and 20 + maximally aligned four-hole states. Their energies are reproduced well by using energy shifts observed in experiments for the 208Pb core octupole excitation coupled to simpler intrinsic structures.

Excited states in the Z=19, N=29 neutron-rich 48K isotope have been studied using deep-inelastic transfer reactions with a thick target at Gammasphere and with a thin target at the PRISMA-CLARA spectrometer. The lowest excited states were located; they involve a proton hole in the s1/2 or d 3/2 orbital coupled to a p3/2 neutron. A new 7.1(5)-ns, 5+ isomer, the analog of the 7/2- isomer in 47K, was identified. Based on the observed γ-decay pattern of the isomer a revised spin-parity assignment of 1- is proposed for the ground state of 48K.

The decay of a new, 53/2+, isomer at 8281 keV in Hg203 has been studied by γ coincidence spectroscopy. A half-life of 146(30) ns was measured. In addition, another isomeric, 39/2+, level with a half-life of 7.8(1.5) ns was observed. Some elements of the Rydstroem shell-model interaction have been adjusted to reproduce level energies in nuclei with two to four holes in the Pb208 core. With this interaction, the new states in the five-hole nucleus Hg203 are reproduced with an rms error of 105 keV.

Gamma rays assigned to 2872Ni44 have been identified with Gammasphere in deep-inelastic reactions involving a 450-MeV 76Ge beam and a 198Pt target. Using a combination of spectra produced by double gates on the known 454-, 843-, and 1095-keV members of the ground-state cascade, a coincident line at 199 keV has been identified and is tentatively assigned as the 8+→6 + transition. These γ-ray coincidences have been observed only in prompt events, indicating an 8+ half-life below 20 ns and requiring a large B(E2) enhancement compared to that expected from a seniority scheme. This value is consistent with models showing decay to a seniority ν=4, 6+ level that is depressed by the same two-body interaction responsible for the rather low 1095-keV 21+ energy, as compared to the valence-symmetry counterpart 4494Ru50.

Level structures in 60,62Fe34,36 and adjacent nuclei were studied with Gammasphere using the reaction of a 430-MeV 64Ni beam on a 55-mg/cm2 thick 238U target. High-spin level schemes were deduced from singlyand doubly-gated prompt coincidence events. Levels populated in the β decay of 60,62Mn isomers were studied in singly- and doubly-gated delayed coincidences. Spin and parity assignments were deduced from angular correlations of γ rays observed in both nuclei, leading to firm positive-parity yrast assignments up through 8+ in 62Fe. The observed levels for 60Fe up to ~3.5 MeV compare well with pf shell-model calculations available in the literature. Calculations performed in a limited pfg basis exhibit reasonable agreement for the 2+, 4+, and 6+ levels in 62Fe. However, theoretical descriptions that include the newly identified negative-parity levels in 62Fe require a much larger basis, necessarily including g9/2 neutrons, as well as improved interactions leading to negative-parity levels.

Excited states in the N=30 neutron-rich isotope K49 have been studied using multinucleon transfer reactions with thin targets and the PRISMA-CLARA spectrometer combined with thick-target γ-coincidence data from Gammasphere. The d3/2 proton-hole state is located 92 keV above the s1/2 ground state, and the proton-particle f7/2 state is suggested at 2104 keV. Three other levels are established as involving the coupling to 2+ of two neutrons above the N=28 shell. The measured or estimated lifetimes served to reinforce the interpretation of the observed level structure, which is found to be in satisfactory agreement with shell-model calculations.

Level structures in Mn61,62, 36,37 were studied with Gammasphere in the reaction of a 430-MeV Ni64 beam and a thick U238 target. The newly identified levels decrease in excitation energy compared to the analogous structures in the lighter Mn isotopes and behave similarly to states in the corresponding Fe isotones that involve g9/2 neutron excitations. This behavior illustrates the importance of the inclusion of the νg9/2 orbital in any realistic shell-model calculations in this region.

The yrast structure of the neutron-rich nucleus Zr97 has been studied using fission of targetlike products in the reaction of a Ca48 beam on a thick U238 target. The level scheme, known from previous studies up to an excitation energy and spin-parity of approximately 4619 keV and 23/2-, has been extended by about 3 MeV and a few units of angular momentum. Two states fed in the β decay of the 27/2- high-spin isomer in Y97 were identified at 5570 and 5606 keV. The located level structure can be discussed in terms of shell-model configurations.

The yrast structure of the neutron-rich nucleus 97Zr has been studied using fission of target-like products in the reaction of a 48Ca beam on a thick 238U target. The level scheme known from the previous studies up to an energy and spin of approx. 4619 keV and 23/2-, respectively, has been extended by about 3 MeV and a few units of angular momentum. The located structure can be discussed in terms of shell model configurations.

The lifetimes of the first excited states of the N = 30 isotones 50Ca and 51Sc and the Z = 18 isotopes 44−46Ar isotopes have been determined using a novel technique that combines the Recoil Distance Doppler Shift method with the CLARA-PRISMA spectrometers in multinucleon transfer reactions. The results allow determinination of the effective charges above 48Ca and test the strength of the N = 28 magic number when moving away from the stability line. Gadea Raga, Andrés, Gadea.Andres@ific.uv.es ; Algora, Alejandro, Alejandro.Algora@ific.uv.es ; Rubio Barroso, Berta, Berta.Rubio@ific.uv.es

The lifetimes of the first excited states of the N=30 isotones 50Ca and 51Sc and the Z=18 isotopes 44-46Ar isotopes have been determined using a novel technique that combines the Recoil Distance Doppler Shift method with the CLARA-PRISMA spectrometers in multinucleon transfer reactions. The results allow determinination of the effective charges above 48Ca and test the strength of the N=28 magic number when moving away from the stability line.

The level structures of 55V and 55Ti, the two nuclei with a single nucleon outside the semi-magic 5422Ti32 core, have been investigated in order to provide new tests of full pf-shell calculations with the GXPF1A interaction. The addition of a proton does not appear to affect the N = 32 shell gap significantly, although comparisons between calculations and experiment at high spins (Iπ ≥ 21 / 2-) indicate the need for a larger model space for an accurate description of the data in this regime. The energy separation between the ν p1 / 2 and ν f5 / 2 orbitals in neutron-rich Ti isotopes is not large enough to result in an N = 34 shell gap. However, comparisons between the 55Ti data and the calculations argue for the presence of a sizable N = 34 gap in 54Ca.

The structure of the Cr56,58,60 nuclei was investigated at Gammasphere using both deep inelastic and fusion-evaporation reactions. As a result of this work, expanded level schemes are now available for these three neutron-rich, even-even isotopes. Experimental signatures were found for both single-particle and collective excitations whose relative importance varies with neutron number. Together with recent structural information on the odd-A Cr55,57,59 neighbors, the present data highlight the changing role of the g9/2 orbital with neutron number. Comparisons with shell-model calculations using the recently developed GXPF1A interaction and with total Routhian surface calculations are presented.

The level structure of the N=38 isotone Fe64 was studied with the Ni64 + U238 reaction at 430 MeV. Several new levels were identified and compared to shell model calculations. Results show no evidence for deformation in the ground state, but a possible contribution from intruder orbitals at higher energy and spin.

γ rays in the πν-1 nucleus 208Bi have been studied at the Gammasphere using deep-inelastic reactions induced by a 305-MeV 48Ca beam on a thick 208Pb target. Previously unknown yrast γ-ray cascades above the 10- millisecond isomer in 208Bi were identified in cross coincidence with known γ rays from complementary potassium products. Yrast and near-yrast levels up to 5.6 MeV in 208Bi have been located, and they are interpreted in light of earlier charged particle spectroscopy results, and with the help of shell model calculations.

Data from three gamma speetroscopy experiments using deep-inelastic heavy ion reactions provided new information on high-spin states in the neutron-rich JV = 30, 50Ca and 51Sc isotones. Shell model calculations restricted to neutron excitations only are shown to reproduce with good accuracy some of the experimental levels. It is demonstrated that proton excitations not accounted in these calculations are abundantly present in the observed yrast structures. High energy of the 4+ state in 50Ca underlines the validity of the TV = 32 shell closure.