V. P. Janzen

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.

Channel selection for high spin γ-ray spectroscopy studies via total energy measurements in fusion-evaporation reactions

Abstract A channel selection method for high spin γ-ray spectroscopy studies based on the measurement of the total energy of all radiations (both charged particle and γ-ray) emitted in heavy-ion fusion reactions is presented. The method is applicable to all reactions in which charged-particle evaporation from the compound system dominates, and is particularly effective in isolating the weakly populated low particle multiplicity channels that leave the final nucleus with the greatest spin and excitation energy. The method is illustrated using data taken with the 8π γ-ray spectrometer and the miniball 4π charged-particle detector array at the Chalk River Tandem Accelerator Superconducting Cyclotron (TASCC) facility. Channel-to-total ratios are improved over those obtained with charged-particle detection alone by factors as large as 46 without significant loss of statistics for the selected channel.

Magnetic rotation in 106Sn and 108Sn

Abstract The nuclei 106Sn and 108Sn have been populated using the 54Fe(58Ni,α2p) and 54Fe(58Ni,4p) reactions, respectively, at a beam energy of 243 MeV and the gamma rays have been detected using the Gammasphere array. Two “rotation-like” structures consisting of magnetic dipole transitions have been observed in each of the nuclei. The bands can be interpreted, using the Tilted Axis Cranking model, as examples of magnetic rotation. The calculations show excellent agreement with the data for 108Sn. However, for the lighter isotope, 106Sn, the model is unable to reproduce the experimental B(M1)/B(E2) ratios. Reasons for this discrepancy are discussed.

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.

A smoothly terminating rotational band in 64Zn

A rotational band has been discovered in the nucleus 64Zn; it extends from spin (12ℏ) to spin (24ℏ), and exhibits strong dipole transitions between its signature partners. In our interpretation, 64Zn takes on triaxial deformation (e∼0.3γ∼30°) at intermediate spin when an f7/2 proton is promoted across the spherical shell-gap at Z=28 to the g9/2 orbital. The situation is parallel to that occurring in the smooth terminating bands in the Sn and Sb isotopes at the Z=50 shell-gap. The band in 64Zn is the first example of a collective band approaching termination in the mass A∼60 region.

Magnetic dipole bands in 198Bi

High spin states in 198{Bi} were populated using a 184W(19F, 5n) reaction at 107 MeV. Three 198Bi γ-ray cascades resulting from a series of magnetic dipole transitions were observed. Measurements of the B(M1)/B(E2) ratio, angular correlation ratios and second moment of inertia are presented.

Investigation of the use of an α + Xn reaction channel to enhance the population of superdeformed states in 193Hg and 195Hg

A study was made to determine whether the population of superdeformed states in 193 Hg and 195 Hg can be enhanced by using reactions in which alpha particles are emitted. The search utilized a 184 W( 18 O ,α Xn) reaction at 115 and 120 MeV for the 193 Hg study and a 186 W( 18 O ,α Xn) reaction at 105 and 110 MeV for the 195 Hg study. Two known superdeformed states of 193 Hg were observed. The intensities of the superdeformed states in 193 Hg populated by a reaction involving the emission of an alpha particle were found to be reduced by at least a factor of four relative to the intensities of these states produced in reactions involving only the emission of neutrons and γ -rays. No rotational bands built on superdeformed states in 195 Hg with transition intensities > 0.8% of the total 195 Hg yield were found. Evidence is shown that the energy of the alpha particle that is associated with superdeformed states may be lower than that of alpha particles associated with normally deformed states.

Orbital configurations & population characteristics of the superdeformed Gd isotopes

We have used the 8π spectrometer at the Chalk River TASCC facility to study superdeformed rotational bands in 146,147,148,149Gd. Various options are discussed for the occupation of single‐particle orbitals at 2:1 deformation. We address the related problem of spin assignments for superdeformed states, and present recent measurements of the relative population of superdeformed bands in 147Gd as a function of excitation energy.