Noah R. Johnson

Heavy-ion inelastic scattering from deformed nuclei

Abstract Theoretical and experimental methods for studying heavy-ion inelastic scattering from deformed nuclei are described. The theoretical methods involve classical-limit approximations, while particle- γ-spectroseopy techniques are employed experimentally. With these approaches, heavy-ion excitation in the Coulomb-nuclear interference region acquires a transparent interpretation, despite the apparent complexity of the multistep excitation processes involved. The examples discussed provide a good illustration of the relationship between classical and quantum physics. The sensitivity of the inelastic scattering to details of the surface ion-ion potential due to radial and angular localization is exploited to provide a method of determining the equipotential contours in a direct manner which bypasses particular model-dependent parametrizations. The method is used to construct ion-ion potentials from inelastic scattering data for the systems 40Ar + 160Gd, 156Gd, 162Dy, 164Dy, and180Hf. The contribution of adiabatic giant resonance polarization to this potential is discussed. The relation between the deformed ion-ion potential and nuclear shapes is illustrated by comparing the experimental potentials to deformed double-folding and deformed proximity-potential calculations.

Reduced transition probabilities for high-spin states in 232Th

Abstract The lifetimes and reduced transition probabilities for spins 6 + − 18 + in the ground-state rotational band of 232 Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using 84 Kr and 136 Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6 + − 18 + . The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.

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.}

Alignment processes and shape variations in 184Pt

Abstract High-spin states in the transitional nucleus 184Pt were populated via the reactions 154Sm(34S, 4n)184Pt and 172Yb(16O, 4n)184Pt. The yrast band was extended up to I = 28 ħ and six new side bands built on both neutron and proton quasiparticle configurations were observed. Shell correction-type calculations indicate variations of the nuclear shape in different bands, especially as a result of band crossings due to the process of angular momentum alignment. Comparison of the band characteristics are made between 184Pt and eight adjacent nuclei. The pattern of band crossings in these nine nuclei is considered from the viewpoint of blocking comparisons and of theoretical calculations. The competition between low-frequency vi 13 2 and πh 9 2 band crossings is discussed.

High-spin states in 183Pt

Abstract High-spin states in 183 Pt have been studied for the first time using the reactions 154 Sm( 34 S, 5n) and 170 Yb( 16 O,3n). Rotational bands built on the Nilsson configurations 1 2 − [521], 7 2 − [514] and 9 2 + [624] were observed up to spin values of 39 2 − 49 2 h . Quasiparticle alignments and band crossing frequencies were investigated in these bands. A large signature splitting was observed in the νi 13 2 - band structure . The experimental results were compared with total routhian surface calculations, in which the shape of the nucleus could be followed as a function of rotational frequency for different quasiparticle configurations.

Nuclear levels of Ge74

Abstract By using gamma-ray scintillation spectrometry, radiations from Ga74 and As74 were studied. By means of coincidence techniques, previous level assignments were confirmed and new states were indicated. A decay scheme consistent with most of the results is proposed with levels at 0.598, 1.20, 1.47, 1.71, 2.18, 2.53, 2.95, 3.05, 3.16, 3.33, 3.41, 3.57 and 3.83 MeV. A measurement of the angular correlation between the 0.598 and 0.60 MeV gamma rays confirmed the predominantly E2 character of the 0.60 MeV (2+→2+) transition.

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.

The K-forbidden beta decay of Tb160

Abstract The β decay of the odd nucleus Tb160(Iπ = 3−) is shown to populate the 2+ and 4+ states of the ground-state band of Dy160 in intensities of 0.34% and 0.12%, respectively. The ratio of the reduced transition probabilities and the absolute transition strength are both compared with theoretical estimates.