F.K. McGowan

Octupole deformation in 142,143Ba and 144Ce: new band structures in neutron-rich Ba-isotopes

Abstract New, high spin band structures are established for the neutron-rich nuclei 142,143,145,147Ba, and 144Ce, and levels of 144,146Ba extended to higher spins from the study of γ-γ and γ-γ-γ coincidence studies in spontaneous fission. Alternating parity sequences connected by strong electric dipole transitions are identified in 142,143Ba and 144Ce but not in 145,147Ba to confirm theoretical predictions of stable octupole deformation for N = 86.

New Spontaneous Fission Mode for 252Cf: Indication of Hyperdeformed 144,145,146Ba at Scission.

From {gamma}-ray coincidence studies following spontaneous fission of {sup 252}Cf, direct measurements of yields and neutron multiplicities were made for Sr-Nd, Zr-Ce, Mo-Ba, Ru-Xe, and Pd-Te correlated pairs. A strong enhancement of the 7-10 neutron channels for Mo-Ba pairs is observed. A new fission mode associated with the enhanced neutron yields is identified. These data can be interpreted in terms of one or more of {sup 144,145,146}Ba being hyperdeformed at scission. Mean field calculations predict a hyperdeformed third minimum in {sup 252}Cf and an extremely deformed {sup 146}Ba fragment at scission. {copyright} {ital 1996 The American Physical Society.}

Coulomb excitation of vibrational-like states in 166−170Er

Abstract Direct E2 and E3 Coulomb excitation of 2+ and 3− states with 14 MeV 4He ions on isotopically enriched targets of 166Er, 168Er and 170Er has been measured by means of γ-ray spectroscopy. Several 2+ states and 3− octupole vibrational states were identified in each nucleus. The B(Eλ, 0 → J = λ) is obtained for excitation of each state, and information is given on the reduced transition probabilities for the different decay modes of these states. The experimental results are compared with theoretical predictions of nuclear models describing these states.

Transition quadrupole moments of high-spin states in 172W and their implications for the interpretation of band crossings in the light isotopes of W to Pt

Abstract Lifetimes of states in 172W have been measured by the Doppler-shift recoil-distance method using the reaction 124Sn(52Cr, 4n)172W at a bombarding energy of 225 MeV. The data were collected in the γγ-coincidence mode rather than the customary singles mode in order to reduce the complexities of the γ-ray spectra and to avoid some of the problems associated with side feeding to excited states. At low spin (I⩽8 h ) the experimental transition quadrupole moments, Qt, are all near a constant value of 7.0 e·b. For states in the yrast band with I⩾10 h , the experimental Qt values are all near a constant value of 6.8 e.b. This is the trend predicted by cranked Hartree-Fock-Bogoliubov (HFB) calculations and by the calculated systematics of triaxial shape-driving forces which originate from the aligned i i 3 2 neutron orbitals around N = 98. From the extensive total routhian surface (TRS) calculations in this mass region, it now seems clear that the low lying band crossings in the light W to Pt isotopes are due to the alignment of various neutron

Signature of the shape coexistence in 72Kr: discontinuities of the moment of inertia at low spin

Abstract The low-lying level structure of the N=Z nucleus 72Kr was studied by 40 Ca ( 35 Cl, p , 2 n γ) 72 Kr at 95 MeV. Gamma ray coincidences are used to construct for the first time a level scheme for this nucleus based on coincidence gating with the previously identified 2+−0+ transition. The new level scheme indicates strong prolate-oblate mixing of two shape coexisting bands. The results support theoretical predictions of a stable oblate minimum in this region.

Coulomb excitation of states in 238U

Abstract Twenty-two states in 238 U have been observed with 18 MeV 4 He ions on a thick target. Eight 2 + states between 966 and 1782 keV and three 3 − states are populated by direct E2 and E3, respectively. The remaining states are either weakly excited by multiple Coulomb excitation and /or populated by the γ-ray decay of the directly excited states. Spin assignments are based on γ-ray angular distributions. Reduced transition probabilities have been deduced from the γ-ray yields. The B (E2) values for excitation of the 2 + states range from 0.10 to 3.0 W.u. (281 W.u. for the first 2 + state). For the 3 states, the B (E3, 0 → 3 − ) values are 7.1, 7.8, and 24.2 W.u. Several of the 2 + states have decay branches to the one-phonon states with B (E2) values between 27 and 56 W.u. which are an order of magnitude larger than the B (E2) values between the one- and zero-phonon states. This disagrees with our present understanding of collectivity in nuclei if these 2 + states are considered to be collective two-phonon excitations. However, the excitation energies of these 2 + states with respect to the one-phonon states are only 1.3 to 1.6. The B (E1) values for 17 transitions between the positive- and negative-parity states range between 10 −3 and 10 −7 W.u. The B (E1) branching ratios for many of these transitions have large deviations from the Alaga-rule predictions. These deviations can be understood by the strong Coriolis coupling between the states of the one-phonon octupole quadruplet in deformed nuclei. The general features of the experimental results for the B (E3) values are reproduced by the microscopic calculations of Neergard and Vogel when the Coriolis coupling between the states of the octupole quadruplet is included.

High-spin single-particle states in 193Hg

Abstract A decay sequence above the I= 47 2 , 5.4 MeV level in 193Hg has been established through discrete γ-ray spectroscopy. The new energy levels exhibit single-particle character implying the dominant role of individual particle angular momentum alignment at moderate spins in the neutron-deficient Hg isotopes.

Coulomb excitation of states in 229Th

The Coulomb excitation of states in 229Th has been observed using 17.0-MeV 4He2+ ions. For the 5/2+[633] ground-state rotational band, we deduced an intrinsic quadrupole moment, Q20, of 8.816 ± 0.090 eb and an intrinsic hexadecapole moment, Q40, of 3.69 ± 0.72 eb2. This Q40 value for 229Th is 43% larger than that for the even-even neighbor, 230Th, and is the largest thus far observed in the actinide region. Large E3 matrix elements are extracted for states at 512, 562, and 611 keV, which supports the assignment of these states as members of a Kπ = 0− octupole-vibrational band built on the 5/2+[633] ground state. This 0− octupole excitation occurs at much higher energy than the one inferred from earlier decay studies that is built on the 3/2+[631] state. This suggests state-dependent octupole correlations consistent with octupole shape transitional models of 229Th.

Shape coexistence and disappearance of pairing correlations in 82Sr

Abstract Extensive high-spin band structures in 82 Sr have been established using proton- γ - γ coincidence techniques. On the basis of the Woods-Saxon cranking calculations with pairing, four of these bands are interpreted to have prolate, oblate, or triaxial shapes. Pairing correlations are predicted to be very weak at high spins in this nucleus, and calculations with no static pairing successfully reproduce the experimentally observed crossing frequencies and alignments despite the fact that none of the bands displays a rigid-rotor behavior. It is concluded that observation of rigid-rotor behavior is neither necessary nor sufficient for the disappearance of static pairing in nuclei.

Electromagnetic properties of the [303]52, 72 pseudo-spin doublet in 175Re

Abstract High-spin states in 175Re were studied using the 156Gd(23Na, 4n) reaction. Five rotational decay sequences have been identified and assigned to proton configurations. Eight strong interband transitions are observed connecting the members of the [ 303 ] 5 2 , 7 2 pseudo-spin doublet. Both the interband and intraband M1 and E2 transition rates can be understood in the pseudo SU(3) formalism with Coriolis mixing.