J. DeGraaf

Collective properties of 48Cr at high spin

Abstract High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

Rotational bands in 238U

Abstract A thick foil of 238 U was bombarded with 209 Bi beams at 1130 and 1330 MeV, delivered by the TASCC facility at Chalk River Laboratories. Gamma-ray spectroscopy of states populated in multiple Coulomb excitation was performed with the 8π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. The event trigger required that 3 BGO elements and 2 HPGe detectors fire in coincidence. The experiment achieved a high degree of sensitivity, the weakest rotational band observed had about 0.16% intensity of the ground state rotational band. Several bands were observed to high spin for the first time, including the γ-vibrational band (spin 27 + ) and the octupole bands with K = 0 (spin 31 − ), K = 1 (spin 28 − ) and K = 2 (spin 25 − ). Results for positive and negative parity vibrational bands are compared with cranked RPA calculations. Although this theory can explain some features of the data, many puzzling aspects remain to be explored.

High-spin proton and neutron intruder configurations in 106Cd

Abstract High-spin states in 106Cd have been studied with the reactions 94Zr(16O, 4n)106Cd and 76Ge(34S, 4n)106Cd. A rotational band, based on a two-quasineutron (h 11 2 ) 2 configuration, is observed from its 10+ bandhead to spin 28 Ħ. This structure undergoes a band-crossing at a rotational frequency of approximately 0.45 MeV /Ħ, interpreted as the alignment of a pair of g 7 2 neutrons. Time-correlated spectroscopy has been used to identify states above the previously observed 12+ isomer at 4660 keV. These include a weakly populated, strongly coupled structure which indirectly feeds the isomer, and whose alignment and in-band decay properties are consistent with a deformed ν( h 11 2 ) 2 π( g 9 2 g 7 2 ) , four-quasiparticle configuration. The results are discussed in terms of cranked-shell-model and total-routhian-surface calculations. No evidence is found for a reported 16+ isomer at 7119 keV.

Collective γ-vibrational bands in 165Ho and 167Er

Abstract The nuclear structures of 165Ho and 167Er have been investigated by means of Coulomb excitation. These nuclei excited at moderate spins exhibit γ-vibrational bands with K π = 11 2 − , 3 2 − in 165Ho and K π = 11 2 + in 167Er. The γ-vibrational bands in 165Ho are found to be isospectral; heaving very nearly identical in-band γ-ray energies. Gamma-ray branching ratios are analysed to extract information on collectivity and Coriolis mixing. Experimental results are compared with calculations performed with the Cranked Shell Moedl + RPA + particle-vibration coupling and by invoking the generalized intensity relations (GIR) in the unified model scheme. Although this model explains many features of the data, puzzling aspects such as identical transition energies for the bands in 165Ho remain unexplained. The role of the K quantum number in identical bands is discussed.

Quasicontinuum ridges in {sup 173,174}W

A strong quasicontinuum ridge with a spacing of 58.1 {plus_minus} 0.3 keV was observed in {sup 173}Wand {sup 174}W when populated at high spin. The ridge is a factor of 4 times more intense than that of the sum of discrete bands over the same range of rotational frequencies. The strength of the ridge may be due to the large number of nearly degenerate proton orbitals lying above the {ital Z} = 73 gap occurring at a deformation {beta}{sub 2}{approx_equal}0.3. {copyright} {ital 1996 The American Physical Society.}

Superdeformation below {ital N}=73

A decoupled rotational band with an average dynamical moment of inertia {ital J}{sup (2)} {approximately}55 {h_bar}{sup 2}/MeV has been observed to high spin in {ital N}=71 {sup 129}Ce. The measured quadrupole moment of {ital Q}{sub 0} = 6.3(4) eb is as large as that of the superdeformed band in {sup 131}Ce. The large deformation and decoupled character of the band suggests that the odd neutron occupies either the {ital h}{sub 9/2}/{ital f}{sub 7/2} [541] 1/2{sup {minus}} orbital or the {ital i}{sub 13/2} [660] 1/2{sup +} intruder orbital. This is the first example of a superdeformed band extending to high spin below {ital N}=73, a neutron number that has long been considered as the boundary for superdeformation in the {ital A}{approximately}130 mass region. {copyright} {ital 1996 The American Physical Society.}

Superdeformation below N = 73

A decoupled rotational band with an average dynamical moment of inertia {ital J}{sup (2)} {approximately}55 {h_bar}{sup 2}/MeV has been observed to high spin in {ital N}=71 {sup 129}Ce. The measured quadrupole moment of {ital Q}{sub 0} = 6.3(4) eb is as large as that of the superdeformed band in {sup 131}Ce. The large deformation and decoupled character of the band suggests that the odd neutron occupies either the {ital h}{sub 9/2}/{ital f}{sub 7/2} [541] 1/2{sup {minus}} orbital or the {ital i}{sub 13/2} [660] 1/2{sup +} intruder orbital. This is the first example of a superdeformed band extending to high spin below {ital N}=73, a neutron number that has long been considered as the boundary for superdeformation in the {ital A}{approximately}130 mass region. {copyright} {ital 1996 The American Physical Society.}

Superdeformation belowN=73

A decoupled rotational band with an average dynamical moment of inertia {ital J}{sup (2)} {approximately}55 {h_bar}{sup 2}/MeV has been observed to high spin in {ital N}=71 {sup 129}Ce. The measured quadrupole moment of {ital Q}{sub 0} = 6.3(4) eb is as large as that of the superdeformed band in {sup 131}Ce. The large deformation and decoupled character of the band suggests that the odd neutron occupies either the {ital h}{sub 9/2}/{ital f}{sub 7/2} [541] 1/2{sup {minus}} orbital or the {ital i}{sub 13/2} [660] 1/2{sup +} intruder orbital. This is the first example of a superdeformed band extending to high spin below {ital N}=73, a neutron number that has long been considered as the boundary for superdeformation in the {ital A}{approximately}130 mass region. {copyright} {ital 1996 The American Physical Society.}