No abstract

High-spin states in 2~were populated in the 2~ 4n) reaction using e-particles in the energy region 42-51 MeV. Prompt and delayed y-rays as well as conversion electrons were studied in addition to excitation functions, angular distributions and 7-7 coincidences. In this way a stretched cascade of y-rays from a level at 8125.9 keV was found to feed the previously known isomeric 9-level at 2185.7 keV. Spins and parities were established for levels up to and including a 19-level at 6098.0 keV. The levels with J== 17-and 19-at excitation energies of 5664.3 and 6098.0 keV are likely to be due to the simple p~/1 i~z and f5721 ii-~2 configurations. The agreement between calculated and experimental energies for all observed levels in the region J=9-19 is very good in cases where the empirical two-particle interactions used are satisfactorily well known. Above the 19-level there are three weakly populated levels at 7402.1, 7849.2 and 8125.9 keV, which are likely to have J > 20. None of these energies agrees with the calculated value 7695 +20 keV for the 20 + state of the ila~2 configuration which has the highest angular momentum produced by the four valence neutron holes. This apparent anomaly can be understood if the yrast levels with J > 20 have angular momentum contribution from the core. It seems likely that the states at 7402.1, 7849.2 and 8125.9 keV are due to proton core excited states of the type Irh9/2 h11~/zXVp~/2 i1~/2 with J==20 + and J==21 + and =h9/2 h;11/2 x vpT/1 f57~ i~32/2 with J==22 + or 23 +, respectively. The state at 8126keV has the highest energy so far directly observed in a stretched cascade of y-rays from the decay of a heavy nucleus produced in (e, x n) reactions.

Using the 2~ 3n) reaction with e-particles of about 40 MeV, we have proved by applying nowadays conventional y-ray spectroscopy in-beam technique, that there are two isomeric states in 2~ at the excitation energies 5,161.3 and 3,195.5 keV having the half-lives 71 ___ 3 and 217_+ 5 ns, respectively. These isomeric states have spins and parities 33/2 + and 25/2-and are mainly due to the i{3~2and .-2 -1 113,2 Pl, 2 configurations, respectively. This conclusion is supported by the experimental g-factors of these states being -0.159_+ 0.008 and -0.0676_+0.0011, respectively. It is furthermore shown that the E2 effective neutron charge is the same for E2 transitions from the 33/2 + state in 2~and from the 12 + state in 2~as required by the assumption that the 2~ core is responsible for the total E2 strength of the neutron holes, and that these states are due to the ii-332 and i-2 configura-13:2 tions. The calculated B(E3) values of E3 transitions from isomeric states in 2~ and 2~ agree reasonably well with the experimental values as expected from the assumption that the E3-strength should come from particle coupling to the octupole states of the 2~ core. The energies of the six most well established excited states in 2~ with angular momenta in the region 19/2-33/2 were calculated using empirical single-particle energies, empirical two-particle interactions and angular momentum algebra. The average deviation between experimental and calculated energies is -3 keV and the root mean square deviation 6 keV as compared to the uncertainty _+ 5 keV in the nuclear masses used in the calculation. For the orbits concerned the shell model is thus valid with an extremely high precision. The contribution of effective three-particle interaction in these orbits must consequently be less than about 5 keV.

The energies of high-spin states formed by adding a few valence nucleons to 208 Pb can be accurately calculated by using empirical interaction matrix elements. It is shown that the energies of high-spin states with 207 Pb, 206 Pb, and 204 Pb as the core can also be accurately calculated if an additional quadrupole interaction with a strength proportional to the increased polarizability of the core is included. This interaction may contribute to the formation of yrast traps from states with a few valence nucleons with aligned angular momenta.

High spin states have been studied in 133La via the ~=Sn (lSN, 4n),) fusion evaporation reaction. Bands build on low lying h~/2,g7/2 and d5/2 proton states have been identified. At higher spin a h~/2 neutron alignment is observed. The softness with respect to the triaxial deformation makes the nuclear shape sensitive to the quasiparticle configurations and coexistence between states with y ~-+ 30 ~ )' ~ -30 ~ and y <_ -60 ~ was found. The results have been interpreted using total routhian surface (TRS) model calculations.