C. Mayer-Böricke

Evidence for h92 proton induced backbending in the Os region: Investigation of the nuclei 181Re, 181–184Os

Abstract High-spin states in 181, 183, 184Os and 181Re have been populated by (α, xn) reactions and studied using in-beam γ-ray spectroscopy methods. In 181Os three bands have been established up to high-spin states, namely a decoupled 1 2 −[521] band up to 33 2 −, a strongly coupled 7 2 −[514] band up to 35 2 − and a mixed positive-parity i 13 2 neutron band up to 41 2 t+. All three bands show backbending. In 181Re the previously known bands were extended to higher spin states. The strongly coupled 9 2 −[514] and 5 2 +[402] bands, which were established up to 29 2 − and 25 2 +, respectively, do backbend while the decoupled h 9 2 proton band, extended up to 41 2 −, does not. The latter feature has been interpreted as a blocking of backbending by the h 9 2 proton. It was concluded from all these results that the backbending effect known to exist in 182Os is due to the rotation alignment of a pair of h 9 2 protons. The backbending behaviour observed for 183, 184Os seems to indicate that also in 184Os the protons produce backbending.

Excitation of high-spin states in 190, 191, 192, 193, 194hg through (α, xn) reactions

Abstract High-spin states in 190, 191, 192, 193, 194Hg have been populated with (α, xn) reactions and studied using in-beam γ-ray spectroscopy methods. Several new features have been observed in these nuclei. In the ground state bands of190, 192, 194Hg, in addition to the low-spin states up to 8+, a sequence of high-spin states on top of the 10+ isomer (up to 18+ in 192, 194Hg) was observed. The energy spacings of the new level sequences in 190,192,194Hg resemble very closely those observed for the low-lying members of the ground state bands up to 6+. The 10+ state has probably a ( πh 11 2 −2 ) two-proton-hole configuration and the level sequence above the 10+ isomer has possibly a structure similar to the low-lying states of the ground state band. In 192, 194Hg the even-spin members of the negative-parity bands with spins and parities 8(−), 10(−), 12(−) …, 18(−) were established. These states are shifted upwards in energy with respect to the odd-spin members 5−, 7−, 9−…, in accordance with recent theoretical calculations. In 191,193Hg a level structure has been observed which compares remarkably well with the band structure of the neighbouring even nuclei. This can be interpreted in the framework of the rotation-alignment model by Coriolis-decoupling of the extra i 13 2 neutron from the core.

Evidence for strong variation of the moment of inertia at higher angular momenta in the ground state rotational bands of 158Er and 166Yb

Abstract High-spin rotational states in 158Er and 166Yb have been studied with the (α, 8n) reaction. In the plot of the moment of interia associated with these states versus the square of the rotational frequency, S-shaped curves were obtained.

High-spin states and isomers in 136, 137, 138Ce

Abstract High-spin states in 136, 137, 138 Ce have been populated by ( α , x n) reactions and studied using in-beam γ-ray spectroscopic methods. New features result from this investigation. Levels up to I = 15, 33 2 and 16 have been identified in 136, 137, 138 Ce, respectively. In 136, 138 Ce a 10 + isomer has been found which has probably a h 11 2 two-neutron-hole configuration. The level scheme for 137 Ce shows a decoupled band structure different from that of the lighter odd-mass Ce nuclei, where perturbed rotational bands, based on the h 11 2 neutron-hole state were reported. The energies of the 15 2 − → 11 2 − , 19 2 − → 15 2 − and 21 2 + → 19 2 − transitions in 137 Ce are similar to the 2 + → 0 + , 4 + → 2 + and 5 − → 4 + transition energies in 138 Ce, and those of 12 + → 10 + , 14 + → 12 + and 15 → 14 + in 136 Ce. The level structure of these nuclei can be understood if a transition from prolate to oblate shape through a triaxial region occurs between 135 Ce and 137 Ce. Furthermore, in 136 Ce a negative-parity band with even- and odd-spin members was established. In 138 Ce the negative-parity states 7 − and 5 − were observed.

In-beam γ-ray study of 189, 191, 193Au

Abstract High-spin states in 189, 191, 193Au have been populated with (α, xn) reactions and studied by means of in-beam γ-ray spectroscopic methods. Levels are established up to the 43 2 − and 43 2 + states in 193Au. Isomeric states of 5 ns ( 31 2 − ), ≧ 100 ns ( 31 2 + ) in 193Au and of ≧ 100 ns in 191Au were found. A band based on the 35 2 − state which was found in 193Au may be a rotation-aligned band. This 35 2 − state was interpreted to have a [(π h 11 2 ) −1 (ν i 13 2 ) −2 ] 35 2 − configuration. A 10 ns isomer of the h 9 2 proton state was found in 191Au. A band built on this state is established up to the 21 2 − state and discussed within the triaxial-rotor-plus-particle model.

Study of Coriolis-decoupled bands in 155, 157, 159Dy and 155, 157, 159Er using (α, xnγ) reactions

Abstract Levels in 155, 157, 159 Dy and in 155, 157, 159 Er have been populated using (α, x nγ) reactions where x = 5, 7 or 9. The resulting γ-rays have been investigated using in-beam y-spectroscopic techniques. Mixed positive-parity bands were predominantly populated and are identified with the Coriolis-deeoupled bands described in the framework of the rotation-alignment model of Stephens and coworkers. In the case of 157 Dy the band was observed up to the 49 2 + state. The absence of the backbending effect in these nuclei can be explained by the blocking of certain 13 2 neutron orbitals near the Fermi surface which are essential for the development of the baekbending mechanism.

Measurement and interpretation of 197Au(d, xnyp) excitation functions in the energy range from 25 to 86 MeV

Excitation functions of the 197Au(d, xn)199−xHg (x = 4, 6, 7, 8 and 9) and the 197Au (d, pxn)198−xAu (x = 0, 2, 4 and 6) reactions were measured using the stacked foil technique. The experimental data were compared with the theoretical predictions of the equilibrium statistical model and the hybrid model. While the (d, pxn) excitation functions and the high energy “tails” of the (d, xn) excitation functions could not be reproduced by the equilibrium statistical model, the hybrid model gave good overall agreement with the experimental data.

Observation of giant resonances in Mg24,25,26 and Al27 by inelastic α scattering

The E2 giant-resonance region in sd-shell nuclei was studied by inelastic scattering of 106-, 120-, 145-, and 172.5-MeV ..cap alpha.. particles. Strong excitation in this region was observed for all investigated nuclei (/sup 24/,/sup 25/,/sup 26/Mg and /sup 27/Al) at the higher incident energies. At lower incident energies, the tail of the giant resonance tends to be obscured by another broad distribution. In the E2 giant-resonance region, there is more fine structure in the Mg isotopes than in /sup 27/Al. (AIP)

Elastic and inelastic deuteron scattering on 12C in the energy range from 60 to 90 MeV

Abstract Elastic and inelastic scattering of deuterons on 12 C has been investigated in the incident energy range between 60 and 90 MeV. Angular distributions between 10° and 80° (lab) have been measured for states up to approximately 15 MeV; at still higher excitation energies some prominent groups of levels show up in the spectra, including a very broad structure peaked at 27 MeV. The elastic and inelastic scattering angular distributions were analyzed in the frame-work of standard optical model and coupled-channel analysis, respectively. The quadrupole deformation parameter was found to be β 2 = −0.48 ± 0.02, independent of incident energy. The angular distributions for the second 0 + and first 3 − state were analysed using a rotation-vibration coupling scheme.

Investigation of high-spin states in 138, 139Nd

Abstract High-spin states in 138, 139Nd have been populated by (α, xn) reactions and investigated using in-beam γ-ray spectroscopic methods. Levels have been established up to I = 19 and 29 2 in 138, 139 Nd , respectively. The level schemes of 138, 139Nd are similar to those of the isotones 136, 137Ce. The low-spin states ( I ≦ 19 2 − ) in 139Nd can be understood in the framework of the triaxial rotorplus-particle model if a transition from prolate to oblate shape occurs between 137Nd and 139Nd. Cascades with level spacings similar to 140Nd are found above a 10+ isomer in 138Nd and the 11 2 − isomer in 139Nd. This may be understood as coupling of one or two h 11 2 neutron holes to the 140Nd core. Possible proton states and a rotation-aligned band are also observed in 138Nd.