Search Results (48 total)

Gamma rays from neutron-rich nuclei in the vicinity of Z=20, N=28 have been studied at Gammasphere using deep-inelastic reactions induced by a 330-MeV ⁴⁸Ca beam on a thick ²³⁸U target. The yrast γ-ray cascade of ⁵⁶Ti was identified for the first time and the location in energy of the 2⁺, 4⁺, and 6⁺ states was determined. The low-spin ⁵⁶Ti yrast structure does not support the presence of a subshell closure at N=34 as suggested on the basis of other data on nuclei in the region as well as shell model calculations with a recently proposed interaction.

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.

High-spin states in ¹⁵⁴Dy were studied with the Gammasphere spectrometer using the ³⁶S(¹²²Sn,4n) reaction. Band terminating states were identified in the spin range I=(36–48)ħ, and were found to compete with collective rotational cascades up to the highest observed spins. Several “sidebands” feeding the terminating structures were identified as well. A band dominated by M1 transitions was observed to terminate at I^π=42^-. The data are interpreted within the framework of configuration-dependent cranked Nilsson-Strutinsky calculations without pairing.

Analyses of ²⁴⁸Cm fission product γ-ray coincidence data recorded at Gammasphere have yielded additional information about γ-ray cascades in N=82 isotones ¹³⁴Te and ¹³⁵I. New-yrast and near-yrast states in both nuclei have been identified, and they are interpreted as specific shell model excitations.

Gamma rays in the ¹³²Sb and ¹³³Te N=81 isotones near doubly magic ¹³²Sn have been studied at Gammasphere using a ²⁴⁸Cm fission source. Previously unknown yrast cascades in the two nuclei were identified in cross coincidence with known γ rays from complementary Rh and Ru fission fragments. The ¹³²Sb levels are explained as proton-neutron hole states as well as core excited states of 2p-2h character, while the interpretation of the ¹³³Te level scheme is mainly based on results of shell model calculations using empirical proton-proton interaction energies from ¹³⁴Te together with estimated proton-neutron hole interactions.

Prompt and delayed γ-ray cascades in doubly magic ¹³²Sn and its neighbor ¹³¹Sn have been studied at Gammasphere using a ²⁴⁸Cm fission source. Isotopic assignments of unknown γ rays were based on coincidences with known transitions in A  =  112–116 Pd fission partners. The yrast level spectra of both tin nuclei are interpreted using empirical nucleon-nucleon interactions from the ¹³²Sn and ²⁰⁸Pb regions. Results include identification of the (νf_7/2h_11/2^-1)9⁺ aligned state in ¹³²Sn and of extensive (νf_7/2h_11/2^-2), (νf_7/2d_3/2^-1h_11/2^-1) and (νh_11/2^-1×3^-) multiplets in ¹³¹Sn. The previously reported β^- decay of an unusual ¹³¹In high-spin isomer to levels in ¹³¹Sn is also elucidated.

Gamma-ray cascades in the two- and three-valence-particle nuclei ¹³⁴Sb and ¹³⁵Te have been studied with Gammasphere using a ²⁴⁸Cm spontaneous fission source. Isotopic assignments were based in part on coincidences with γ rays from complementary Rh and Ru fission partners. The ¹³⁴Sb and ¹³⁵Te level schemes have been considerably extended, with placement of many new high-energy γ rays; delayed γ-ray coincidences observed across a 0.51-μs yrast isomer in ¹³⁵Te were especially fruitful. The yrast level spectra of both nuclei are interpreted using empirical nucleon-nucleon interactions and compared with the known yrast excitations of their counterparts ²¹⁰Bi and ²¹¹Po.

Prompt and delayed γ-ray coincidence measurements have located (νh_11/2)ⁿ yrast isomers in ¹²⁵Sn and ¹²⁶Sn among the products of heavy ion reactions on ¹²⁴Sn targets. The decay properties of the two isomers are reported, and the results are related to those obtained for other tin isotopes in earlier works.

Excited states in the nucleus ¹³³Sb, populated in spontaneous fission of ²⁴⁸Cm, were studied with EUROGAM2. Medium-spin structure, described as the ν(f_7/2h_11/2^-1) multiplet of the ¹³²Sn core coupled to the odd proton in the g_7/2 orbital, has been identified in this nucleus. Levels corresponding to the octupole excitations of the ¹³²Sn core were also identified. Some uncertainities concerning isomeric decays in ¹³³Sb, observed in previous works, have been resolved.

Spectra of the E2 quasicontinuum γ rays feeding different spin regions of the ¹⁵⁴Dy yrast line have been extracted. These are compared with corresponding theoretical spectra obtained by numerical simulations based on temperature-dependent Hartree-Fock theory, with thermal shape fluctuations. In this manner, different regions of the spin-energy plane can be examined. The results support the predictions of a smeared-out phase transition at high spin above the yrast line.

Yrast excitations in the N=82 isotone ¹³⁷Cs have been identified for the first time in thick-target γ-ray coincidence measurements for the system ²³²Th+¹³⁶Xe using the Gammasphere array. The ¹³⁷Cs nuclei were produced in deep inelastic one-proton-transfer reactions. By-products of the main investigation were two well-developed rotational bands which were identified in heavy reaction partner products complementary to ¹³⁷Cs and are tentatively assigned to the little studied nucleus ²³¹Ac. There was no difficulty in placing the 11 observed ¹³⁷Cs transitions in a yrast level scheme extending to an I^π=(31/2^-) level at 5494 keV. Interpretation was also straightforward, since the experimental level energies are in close agreement with results of shell model calculations performed earlier using empirical single particle energies and interaction matrix elements derived from experimental data for other N=82 isotones.

Prompt γ-ray cascades in neutron-rich nuclei around doubly magic ¹³²Sn have been studied at Eurogam II using a ²⁴⁸Cm fission source. Here we report results for the four-valence-proton N=82 nucleus ¹³⁶Xe and for its N=83 neighbor ¹³⁷Xe. For both nuclei, the yrast level spectra have been considerably extended, and empirical nucleon-nucleon interactions have been used to assign probable shell model configurations. The ¹³⁶Xe level energies are compared with those calculated using different sets of proton-proton interaction matrix elements, both diagonal and nondiagonal, obtained by fitting experimental data for other N=82 isotones.

Recent shell model calculations using a realistic interaction by Andreozzi et al. [Phys. Rev. C 56, R16 (1997)] have given excellent agreement with the experimental level spectra of ¹³⁴Te and ¹³⁵I. In addition, these authors call into question our contention that there is a serious inconsistency in the measured masses of N=82 isotones near ¹³²Sn. In this Comment, we outline the shell model decomposition method and defend our conclusion that the accepted ground state masses of ¹³⁴Te and/or ¹³³Sb must be inaccurate by much more than their assigned errors.

Prompt γ-ray cascades in N=83 fission product nuclei near ¹³²Sn have been studied at Eurogam II using a ²⁴⁸Cm source. Cross coincidences observed between γ rays from complementary light and heavy fission fragments were vital for isotopic assignments. Yrast states in the N=83 isotones ¹³⁴Sb, ¹³⁵Te, and ¹³⁶I are reported. The interpretation of the level schemes is based mainly on results of shell model calculations using empirical proton-proton interaction energies from ¹³⁴Te, and proton-neutron interactions estimated from the well-known ²¹⁰Bi level spectrum.

Prompt γ-ray cascades in neutron-rich nuclei around doubly magic ¹³²Sn have been studied at Eurogam II using a ²⁴⁸Cm fission source. Yrast states to above 5.5 MeV in the two- and three-proton N  =  82 isotones ¹³⁴Te and ¹³⁵I are reported. They are interpreted in terms of valence proton and particle-hole core excitations with the help of shell model calculations employing empirical nucleon-nucleon interactions from both ¹³²Sn and ²⁰⁸Pb regions. A serious inconsistency in the accepted masses of N  =  82 isotones near ¹³²Sn is discovered but not resolved.

Gamma-ray measurements following fusion-evaporation reactions have identified in the N=84 isotones from Ho to Yb the complete yrast lines formed by the configurations πh_{11 / 2}ⁿνf_{7 / 2}² and πh_{11 / 2}ⁿνf_{7 / 2}h_{9 / 2} from the nuclear ground states up to their highest spins near 25ℏ at 5-8 MeV excitation. Their energies are predicted with high accuracy in parameter-free shell model calculations using experimental one- and two-body matrix elements.

The decays of seniority isomers in the N=82 nuclei ¹⁵⁰Er and ¹⁵²Yb and in their respective N=81 isotopes ¹⁴⁹Er and ¹⁵¹Yb were studied following mass separation by the Argonne Fragment Mass Analyzer. Conversion electrons were detected with Si p-i-n diodes operated at room temperature. The low-energy isomeric transitions in ^151,152Yb have been observed for the first time in the electron spectra. Multipolarity assignments were made for many of the decay γ rays of the four nuclei.

A superdeformed band has been found in the ¹⁵⁴Dy (N=88) nucleus. The dynamic moment of inertia is identical to that of the yrast superdeformed band of ¹⁵²Dy and the transition energies are similar to those of an excited superdeformed band in ¹⁵³Dy. It is proposed that the two valence neutrons above the N=86 shell gap occupy the deformation-driving [514]9/2 orbital.

The ⁶⁸Ni nucleus has been identified among the products of deep-inelastic reactions of ⁶⁴Ni projectiles bombarding ¹³⁰Te and ²⁰⁸Pb targets. Three new states, including the high-lying 2⁺ (2033 keV) and the 0.86 ms 5^- isomer, indicate a substantial subshell closure at neutron number N=40. The level structure and the observed very slow E3 transition speed are discussed within the shell model.

The lifetimes of states in two of the ΔI=1 bands in ¹⁹⁶Pb have been measured using the Doppler shift attenuation method, and a new ΔI=1 band has been observed in this nucleus. States in the bands were populated in the reaction ¹⁰⁷Er(³⁰Si,4n) at a beam energy of 142 MeV. Individual level lifetimes were extracted from the data by a analysis of the Doppler broadened γ-ray lineshapes. Under the assumption of pure M1 radiation, average reduced transition strengths B(M1)∼1.5 W.u. were obtained. The B(M1) values in both the ‘‘regular’’ and ‘‘irregular’’ bands exhibit similar dependence on spin. The neutron and proton configurations and nature of the collectivity in these bands is discussed.

Reanalysis of ichproductsofbinaryreactio nsof$³⁴S, ³⁶S, and ³⁷Cl beams on ¹⁶⁰Gd targets. Gates set on known amma$raysin$A∼160 products selected individual react ion channels and identified coincident amma$raysin$A∼36 partner products. Transfers of protons from projectile to target and of neutrons from target to projectile were generally favored, leading to excited neutron-rich light nuclei, including some difficult to reach by other means. Notable results include the observation of amma$raycascadesup^thehigh e stknownyraststatesinfour$N=20 isotones. In two N=19 nuclei, ³³Si and ³⁴P, the two N=22 nuclei, ³⁸S and ³⁹Cl, previously unknown yrast states were identified.

The superdeformed band in ¹⁹⁶Pb has been studied extensively using the reaction ¹⁷⁰Er(³⁰Si,4n) at beam energies of 142, 146, and 151 MeV. New transitions have been added at the top and bottom of the previously known band. Gamma-ray directionsl correlations were measured for most of the transitions in the band verifying the expected stretched E2 character. The collectivity of the band has been measured using the Doppler shift attenuation method yielding an intrinsic quadrupole moment Q₀ of 18.3±3.0 e b, in good agreement with theoretical predictions. The variations of the dynamic moment of inertia scrI⁽²⁾ as a function of the rotational frequency ħω have been studied and compared with cranked shell model calculations. The dependence of scrI⁽²⁾ on mass for superdeformed bands in the Pb isotopes is also investigated.

Product recoils from the reaction ⁹⁶Ru +255 MeV ⁵⁸Ni → ¹⁵⁴Hf^* were analyzed using the Argonne Fragment Mass Analyzer and transported to a collector foil behind the focal plane, where decay of μs isomers could be studied under low background conditions. The recoils were dispersed in A/q as they traversed the position-sensitive focal-plane detector, and recoil–γ-ray coincidences could thus be used to assign masses for specific γ-ray cascades. By use of these methods, yrast isomers with half-lives of 2.6(7) μs and 20(1) μs have been firmly assigned to the exotic N=81 nucleus ¹⁵¹Yb, and the main features of the isomeric decays have been established. The yrast level structure of ¹⁵¹Yb and lighter N=81 isotones are compared.

Evidence for collective oblate behavior in ¹⁹⁶Pb is presented. One irregular and two regular bands of M1 transitions have been observed following the ¹⁷⁰Er(³⁰Si,4n) and ¹⁷⁶Yb(²⁶Mg,6n) reactions. Transitions linking the most intense regular band to the low-lying negative-parity yrast levels are observed, establishing excitation energies, spins, and probable parities of the band members. In contrast, no such transitions have been found for the irregular band and the weaker regular band. The bands are interpreted as corresponding to collective oblate rotation, arising mainly from deformation-aligned high-j, shape-driving quasiproton excitations across the Z=82 shell gap, coupled to rotation-aligned quasineutrons.

Entry distributions leading to normal and superdeformed (SD) states in ¹⁹²Hg have been measured. A model, based on Monte Carlo simulations of γ cascades, successfully reproduces the entry distribution for SD states, as well as all other known observables connected with the population of SD states. Comparison of experimental and model results, together with the measured SD entry distribution, suggest that the SD band lies 3.3–4.3 MeV above the normal yrast line when it decays around spin 10 and that the SD well depth is 3.5–4.5 MeV at spin 40.