Spherical and deformed isomers in Pb-188

Several isomers in Pb-188 have been identified using pulsed beams, the recoil-shadow technique, and the Er-164(Si-28,4n) Pb-188 reaction. Two of the isomers feed the 10(+) state of the yrast sequence and are suggested to be the 11(-) and 12(+) states from oblate and spherical configurations, respectively. The 12(+) isomer is fed weakly by another isomer with a relatively long lifetime, but it has not been characterized. A fourth isomer with a lifetime of about 1.2 mu s leads via a complicated path to the 8(+) and lower spin yrast states. It is a candidate for the K-pi = 8(-), two-quasineutron state which occurs systematically in N = 106 prolate-deformed nuclei, supporting the assumption that the intruding collective well is prolate

Anomalous isomeric decays in 174Lu as a probe of K-mixing and interactions in deformed nuclei

A Kπ=13+, 280 ns four-quasiparticle isomer in the odd-odd nucleus 174Lu has been identified and characterized. The isomer decays to both Kπ=7+ and Kπ=0+ rotational bands obtained from the parallel and antiparallel coupling of the proton 7/2+[404] and neutron 7/2+[633] orbitals. K mixing caused by particle-rotation coupling explains the anomalously fast transition rates to the 7+ band but those to the 0+ band are caused by a chance degeneracy between the isomer and a collective state, allowing the mixing matrix element for a large K difference to be deduced

Anomalous isomeric decays in Lu-174 as a probe of K mixing and interactions in deformed nuclei

A K-pi=13(+), 280 ns four-quasiparticle isomer in the odd-odd nucleus Lu-174 has been identified and characterized. The isomer decays to both K-pi=7(+) and K-pi=0(+) rotational bands obtained from the parallel and antiparallel coupling of the proton 7/2(+)[404] and neutron 7/2(+)[633] orbitals. K mixing caused by particle-rotation coupling explains the anomalously fast transition rates to the 7(+) band but those to the 0(+) band are caused by a chance degeneracy between the isomer and a collective state, allowing the mixing matrix element for a large K difference to be deduced

Identification of yrast states in 187Pb

g -ray spectroscopy of the high-spin states of the neutron-deficient nucleus 187Pb has been conducted with the 155Gd(36Ar,4n) reaction. A cascade of three transitions was deduced from g -g coincidence data gated by detection of recoiling evaporation residues in a gas-filled recoil separator. In an earlier, separate experiment, two of these g rays were positively identified with 187Pb by recoil-g coincidence measurements with a high-resolution, recoil mass spectrometer. From comparison with similar sequences in heavier odd-A lead isotopes, the cascade in 187Pb is associated with the sequence of three E2 transitions from the yrast 25/2 + level to a low-lying 13/2 + isomer. The variation of excitation energy with mass number of the levels concerned suggests that their structure can be associated with weak coupling of an odd i13/2 neutron to states in the spherical well. However, the possibility that they are influenced by mixing with states in the prolate-deformed well cannot be discounted

Incomplete fusion as a spectroscopic tool

Particle-gamma coincidence studies have been a promising but not well exploited means of using incomplete fusion reactions to gain access to states in heavy nuclei near stability. At beam energies near the Coulomb barrier, fusion of the heavy fragment from break-up of the projectile with emission of non-equilibrated alpha particles or other charged particles can be used to study relatively neutron-rich nuclei which cannot be reached by fusion, evaporation reactions with stable beams. Qualitative features which make the reactions attractive as a spectroscopic tool include a spin input which is higher than that achievable if fusion reactions were carried out with beams equivalent to the massive fragment and a correlation between the angle of emission of the light fragment and the number of evaporated neutrons which assists channel identification

Population of high-spin states in U-234 by an incomplete-fusion reaction

Excited states in U-234 have been populated using the incomplete-fusion reaction Th-232(Be-9,alpha 3n) at 52 MeV. The emitted gamma rays were observed using the CAESAR array, while the ct particles were detected with an array of 14 plastic scintillator detectors of phoswich type. This reaction can populate U-234 at higher spin than the conventional Th-232(alpha,2n) reaction because the ''He-5'' fragment from breakup of the beam can be viewed as initiating a Th-232(He-5,3n) reaction. Similar reactions could provide a Valuable alternative technique for the study of relatively heavy, neutron-rich isotopes. In the present work, states in the ground-state band of U-234 were observed up to J(pi)=18(+) and previous tentative observations of (9(-)) and (11(-)) states in the octupole band were confirmed. A new state at 1366 keV, which is possibly the 8(+) member of the gamma band, has also been identified. [S0556-2813(99)02911-8]

Fragment yields from the fission of 238^{\bf 238}U by fast neutrons

Independent yields of post-neutron emission, secondary fragments from the fission of $^{238}$U induced by fast neutrons have been measured using $\gamma$-$\gamma$ coincidence data. The yields of these products are found to be centred on more neutron-rich isotopes than the yields from fission processes previously used for spectroscopic studies on neutron-rich nuclei. The average angular momentum in the fragments is found to be similar to that in fragments formed in other low-energy fission processes

High-spin isomers in 211Po and related structures in 210Po and 212Po

High-spin states in Po have been populated through the incomplete fusion reactions Pb (Be,α2n) and Pb(Li,p3n). The yrast sequence decaying to the τ = 36.4 s isomer in Po has been established to a spin of (43/2) at an excitation energy of 4873 keV. Two other isomeric levels have been identified, with mean lives of 350(30) ns and 4(1) μs at excitation energies of 2135.7 keV and 4873.3 keV, respectively. Relationships between the structure of high-spin states in Po and those previously identified in Po and Po are discussed

Population of High-Spin States in 234U by an Incomplete-Fusion Reaction

Excited states in 234U have been populated using the incomplete-fusion reaction 232Th(9Be, α3n) at 52 MeV. The emitted γ rays were observed using the CAESAR array, while the α particles were detected with an array of 14 plastic scintillator detectors