Ph. Benet

Observation of superdeformation in 191Hg.

The first observation of superdeformation in the A {approx equal} 190 mass region is reported. A rotational band of 12 transitions with an average energy spacing of 37 keV, an average moment of inertia of 110 {Dirac h}{sup 2} MeV{sup {minus}1}, and an average quadrupole moment of 18 {plus minus} 3 eb has been observed in {sup 191}Hg. These results are in excellent agreement with a calculation that predicts an ellipsoidal axis ratio of 1.65:1 for the superdeformed shape in this nucleus. Evidence for another discrete superdeformed band and superdeformed structures in the quasi-continuum was also found in the data. 19 refs., 6 figs.

Rotational bands in the mass 100 region

Abstract Rotational bands up to spin of ≈10 h have been studied in even-even and odd- A nuclei in the mass 100 region, by observing prompt γ-rays from the spontaneous fission of 248 Cm. Transitions in the neutro-rich nuclei 103 Zr, 104 Zr, 107 Mo and 108 Mo have been observed for the first time, and evidence for a band in 105 Nb is presented. Rotational bands are also reported in 98–100 Sr, 99,101 Y, 100–102 Zr, 101,103 Nb and 102–106 Mo. In most cases these bands have been extended to much higher spins than previously known. The data on the even-even nuclei provide information on deformation trends in this region. The level structures of the odd- A nuclei indicates which Nilsson orbitals lie close to the Fermi level. Bands based on the 5 2 + [422] and 5 2 − [303] configurations are identified in the odd- A nuclei with Z = 39 and 41. Evidence is presented that certain bands observed in N = 61 and 63 nuclei are built on the 5 2 − [532] configuration, and the role of the neutron h 11 2 orbital in stabilizing the deformation in this region is discussed.

New neutron-rich nuclei 103,104Zr and the A~100 region of deformation.

Partial decay schemes in the neutron-rich nuclei {sup 103}Zr and {sup 104}Zr have been measured for the first time and rotational bands in {sup 100--102}Zr have been extended to spins of up to 10{h bar} by observing prompt {gamma} rays from the spontaneous fisson of {sup 248}Cm. These nuclei are among the most deformed known at low spin and excitation energy. The level structures in the odd-{ital A} nuclei show that the {ital h}{sub 11/2} intruder orbital plays an important role in stabilizing the deformation in this region.

Excited superdeformed bands in 191Hg

Abstract Two weakly populated rotational bands have been observed in 191 Hg with properties (energy spacings, moments of inertia and lifetimes) very similar to those of the previously reported superdeformed band. Based on cranked Woods-Saxon calculations, these structures are interpreted as the first excited bands in the superdeformed minimum of 191 Hg. Comparisons between the data and the calculations highlight the role of specific orbitals at large deformations.

Multiple band structures in 191Hg

Abstract The level structure of the nucleus 191Hg has been considerably extended from previous studies by using the 160Gd(36S, 5n) reaction in conjunction with an array of Compton-suppressed germanium detectors. A series of 13 different level sequences has been established in addition to three superdeformed bands. A majority of the band structures can be understood in cranked shell model calculations assuming an oblate collective nuclear shape. There is some evidence for the onset of triaxiality. Finally, two bands of single-particle character have been found. They are interpreted as being associated with a prolate non-collective shape (γ = −120°).

Superdeformed bands in 189,190Hg

Abstract Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Proton excitations in the superdeformed well of 193Tl

Abstract Two superdeformed bands of 13 transitions each have been found in 193 Tl with the 160 Gd( 37 Cl, 4n) reaction. The dynamic moments of inertia for the two bands are found to rise with rotational frequency, as for all observed superdeformed bands in other nuclei in this region. The two bands can be interpreted as signature partners which exhibit some signature splitting for rotational frequencies above 0.2 MeV. The data are interpreted with cranked Woods-Saxon calculations and illustrate the role of the proton i 3 2 (Ω = 5 2 ) intruder orbital.

Gamma-ray multiplicity distributions in 16O + 152Sm fusion near and below the Coulomb barrier

Abstract We have studied γ-ray multiplicity distributions for the system 16 O + 152 Sm at five beam energies in the range 60–80 MeV, near and below the Coulomb barrier. The data were obtained using a large array of γ-ray detectors and an efficient evaporation-residue detector system utilizing an electrostatic deflector. The data are in good agreement with predictions of angular momentum distributions calculated using a coupled-channels model incorporating the effects of deformation.

Feeding and decay of superdeformed states

Abstract The mechanisms for feeding and decay of superdeformed (SD) bands are examined. Data connected with both processes in 192 Hg are compared with model calculations. The calculations successfully reproduce the data, suggesting that the mechanisms for both processes are understood. Constraints on the energy of the SD band energies and on the well-depths at low and high spins have been obtained. At the point of decay around spin 10, we suggest that the SD band 1s 3.3–4.3 MeV above the normal yrast line and that the well depths at spin 10 and 40 are 0.5–1.3 and 3.5–4.5 MeV, respectively.

Shape-driving effects in 193Tl from the spectroscopy of yrast and near-yrast states

Abstract The nucleus 193Tl has been populated with the 160Gd(37Cl,4n) reaction and its level scheme has been extended up to spin 41 2 h by using an array of 12 Compton-suppressed Ge detectors. Near-oblate band structures arising from a 9 − 2 state and markedly different, single-particle-like, structures above a 13 + 2 state have been established. The two structures are consistent with total routhian surface calculations indicating the existence of two minima in the total routhian surface of the nucleus which survive separately up to the observed spins. Details of the rotational-like part are discussed in terms of the cranked shell model, while the single-particle-like features of the nucleus are compared with non-collective structures in neighboring nuclei, in particular 191Hg.