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

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

Fragment yields from the fission of U-238 by fast neutrons

Independent yields of post-neutron emission, secondary fragments from the fission of U-238 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

Normal and anomalous K-hindered decays from four-quasiparticle isomers in Lu-176

Two four-quasiparticle isomers, with K-pi=12(+) and (14(+)) and mean lives of 450(100) ns and 58(5) mu s, have been identified in Lu-176, at excitation energies of 1515 and 1588 keV, respectively. The 12(+) isomer exhibits a large number of K-forbidden decay branches, populating the rotational sequences based on the K-pi=7(-) ground state, two K-pi= 8(+) states, and a K-pi=4(+) state from the nu{7/2(-)[514]}x pi{1/2(-)[541]} configuration. Most branches have decay rates that are consistent with normal K-hindrances except for the branch to the K-pi=4(+) band. It has an anomalously low hindrance factor, which is attributed to two-state mixing due to a near-degeneracy between the 12(+) isomer and the 12(+) member of this band. The implied mixing matrix element has a value of only 5 eV, showing explicitly that very small mixing matrix elements may be responsible for anomalous K-hindered decays

Two-quasiparticle configurations, signature inversion, and the pi i(13/2)circle times nu i(13/2) band in Ir-176

High-spin states have been populated in Ir-176 using the Sm-149(P-31,4n) reaction. Gamma-gamma coincidence techniques have allowed numerous bands up to a maximum spin of 32 to be identified. Configurations have been assigned based on spectroscopic properties such as B(M1)/B(E2) values and aligned angular momenta. Signature inversions are observed in bands based on the pih(9/2)xnui(13/2) and pii(13/2)xnui(13/2) configurations. These inversions are compared with particle-rotor model calculations and are qualitatively reproduced when a residual proton-neutron interaction is included. The shape evolution of the bands has been studied using diabatic potential energy surface calculations. A strong configuration dependence is predicted

Intrinsic states and collective structures in Ta-180

Excited states in Ta-180 have been identified using the Yb-176(B-11,alpha 3n)Ta-180 and Yb-176(Li-7,3n)Ta-180 reactions and associated time-correlated gamma-ray spectroscopy, including particle-gamma coincidences for channel selection. As well as identifying the rotational band based on the 9(-) two-quasiparticle state at 75 keV, at least eight other low-lying two-quasiparticle states and associated rotational bands have been established. Lifetimes in the few nanosecond region were isolated using gamma-gamma-time techniques. Most of the observed two-quasiparticle states and some of the band members can be identified with states known from particle transfer studies. The properties of the observed Omega(n)+/-Omega(p) partners of 1(+) and 8(+) bands from the nu 9/2(+)[624]circle times pi 7/2(+)[404] configuration and the 0(-) and 9(-) pair from the nu 9/2(+)[624]circle times pi 9/2(-)[514] configuration are discussed. High-K structures identified include the band based on the four-quasiparticle 45 mu s, 15(-) isomer, a 32 ns, four-quasiparticle 18((+)) isomer, and a six-quasiparticle 19((-)) intrinsic state and its band. Configuration assignments are aided by analysis of the in-band decay properties, which confirm, for example, a predominantly nu pi(3) configuration for the 15- isomer. The results are compared with multiquasiparticle calculations. A number of yrast high-K six- and eight-quasiparticle states which could be accessible in future studies are predicted