A. Neskakis

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.

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.

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.

Investigation of high-spin states in odd-odd 190, 192, 194Au nuclei

High-spin states in the odd-odd Au nuclei 190,192,194Au have been populated using (α, xn) reactions and studied by means of in-beam γ-ray and e− spectroscopic methods. In each nucleus two negative-parity rotation-aligned bands were observed, which are based on an 11− isomer and a 12− state, respectively. They may have a configuration resulting from the coupling of a h112 proton-hole and an i132 neutron-hole coupled to a predominantly oblate core in agreement with recent theoretical predictions in the framework of the triaxial-rotor-plus-particle model. Furthermore, side structures based on 15(+) states were established up to spins of I = (20) and (22) in 190,192Au, respectively. The 15(+) and 17(+) members are considered to have a (πh112−1vi132−2(vj) configuration (j = P12, P32 or f52). In the nucleus 194Au a positive-parity rotation-aligned band based on a 7+ state was established up to the 13+ state. This band may have predominantly a (vi132−1πd32−1) configuration.

Study of the level structure in 134Ce

Abstract The nucleus 134 Ce has been investigated by means of γ and e − spectroscopic methods. Four side bands in addition to the ground band have been identified. Previous g -factor measurements indicate that the 10 + isomer and the band built on it as well as the band causing backbending have a h 11 2 two-neutron configuration. This fact may be interpreted as a shape coexistence in the framework of the triaxial cranked shell model. The 10 + isomer may be an yrast trap of prolate deformation with an asymmetry parameter of γ = −120° (“Lund convention”) whereas the other band probably has an oblate deformation with γ ≈ −60°. A part of the level scheme of 134 Ce shows no band structure. These levels may be connected to the intrinsic motion of the nucleus. They can be explained in terms of weak coupling.

Weak coupling and high-spin states in 145Eu

Abstract High-spin states feeding the 11 2 − isomer in 145Eu have been studied using in-beam γ-ray spectroscopy. A new 5.5 ns ( 19 2 − ) isomeric state has also been identified. Most of the levels can be interpreted in simple weak coupling terms as either π d 5 2 −1 ⊗ 146 Gd or π g 7 2 −1 ⊗ 146 Gd or π h 11 2 ⊗ 144 Sm . The 5.5 ns isomer probably has a ( π d 5 2 −2 π h 11 2 ) 19 2 − configuration.

In-beam study of the odd-odd nuclei 134La and 136La

Abstract The level schemes of the nuclei 134,136 La have been studied by means of in-beam γ-ray spectroscopic methods following the (α, 3n) and (p, 3n) reactions. New isomers with half-lives of 29 ± 5 μ s and 17 ± 4 ns have been discovered in 134 La and 136 La, respectively. Low-lying levels and isomers in 134,136 La have been interpreted as members of multiplets having (π d 5 2 v d 3 2 ), (π d 5 2 v s −1 1 2 ) and (π d 5 2 v h −1 11 2 ) configurations.