Steven W. Yates

Proton–neutron structure of the N=52 nucleus 92Zr

Abstract Following the successful identification of mixed-symmetric one- and two-phonon states in the N=52 nuclei 94Mo and 96Ru, we have performed a photon scattering experiment on the N=52 isotone 92Zr. Experimental data and shell model calculations show that both, single particle and collective degrees of freedom are present in the low-lying levels of 92Zr. The second excited quadrupole state shows the signatures of the one-phonon mixed-symmetric 2+ state, while calculations and data indicate an almost pure neutron configuration for the 2+1 state, in contradiction with the F-spin symmetric limit. Furthermore, two strong dipole excitations, which are candidates for the two-phonon quadrupole–octupole coupled E1 excitation and for the mixed-symmetric 1+ two-phonon state, were observed.

Analysis of Doppler-shift attenuation measurements performed with accelerator-produced monoenergetic neutrons

Abstract Methods employed for the analysis of Doppler-shift attenuation data, obtained for nuclei excited by the inelastic scattering of accelerator-produced monoenergetic neutrons on large samples are presented. Analytical formulas were derived to describe the influence of two major considerations in the analysis. The reaction mechanism for stopping of recoil nuclei was found to increase the attenuation factor F ( τ ) calculated with the center-of-mass initial velocity, while geometrical effects tend to decrease F ( τ ). It is also shown that the influence of these major factors can be minimized by keeping the neutron energy close to the level threshold and by minimizing the target size. The formulas obtained can be used for any reaction that proceeds via the compound nucleus mechanism and for targets of sufficient size to stop the recoiling nuclei.

The nature of 0+ excitations in 166Er

Abstract Lifetimes of excited 0 + states in 166 Er have been measured with the (n,n′γ) reaction. The first and second excited 0 + states have no significant collective enhancements of their decays, and their properties are suggestive of pair-type excitations rather than those of a β vibration or phonon excitation built on the γ vibration. The third excited 0 + state at 1934 keV has an enhanced decay to the ground state band with B ( E 2; 0 4 + → 2 gsb + ) = 8.8 ± 0.9 W.u., consistent with that expected for a β vibration.

On the nature of “three-phonon” excitations in 112Cd☆

Abstract The lifetimes of proposed 3-phonon states in 112 Cd have been measured using the Doppler shift attenuation method (DSAM) following the (n,n′γ) reaction. The experimental results allow the determination of relevant B ( E 2) values which are compared to different theoretical descriptions. Collective enhancements favouring the interpretation of collectivity in the phonon model are observed for 3 + and 4 + members of the 3-phonon quintuplet. However, for the proposed 2 + member, this simple model fails most likely due to the subtle interplay between intruder, mixed-symmetry, and phonon states, and the decay pattern is strongly perturbed. The best overall agreement is obtained with IBM-2 calculations.

Elemental analysis by gamma-ray detection following inelastic neutron scattering

A method of elemental analysis based on the detection of prompt γ-rays produced by the inelastic scattering of fast monoenergetic neutrons has been investigated. Time-of-flight discrimination techniques and the use of Ge(Li) detectors provide significant advances over previous studies. This method is multielemental, non-destructive, and essentially free of interferences and matrix effects. Sample analysis is rapid and selective excitation of various elements is possible. Many elements have been examined by this technique and minimum detection limits have been established. This method has been applied to the analysis of metal alloys.

Neutron single particle states above the N=152 subshell☆

Abstract Single-particle states above the N=152 subshell have been studied by high-resolution 250Cf(d,p) reaction. All states between the 152 and 164 neutron subshell have been identified. A tentative assignment has been made for the 1 2 −[761] orbital.

Two-neutron transfer reaction mechanisms in 12 C(6 He, 4 He) 14 C using a realistic three-body 6 He model

The reaction mechanisms of the two-neutron transfer reaction 12C(6He,4He) have been studied at Elab=30 MeV at the TRIUMF ISAC-II facility using the Silicon Highly-segmented Array for Reactions and Coulex (SHARC) charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2+2 8.32 MeV state in 14C, using a realistic three-body 6He model and advanced shell-model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30-MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at Elab=18 MeV, however, is not as well described by the same reaction model, suggesting that one needs to include higher-order effects in the reaction mechanism.

Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in 110–114,116Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited Jπ = 1− states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2+ and 3− states. These excitations are interpreted as the 1− member of the quadrupole-octupole coupled quintuplet (2+⊗3−). The fragmented strength observed in the odd isotopes 111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.

On the first excited state of137Ba

The first excited state of137Ba has been excited by the inelastic scattering of accelerator-produced neutrons. The energy of this state at 283.5 keV is not in agreement with the generally accepted value of 279.2 keV, but is in accord with other recent measurements. No evidence for a doublet of states near this energy is found.

Identification of significant E 0 strength in the 22+→21+ transitions of 58,60,62 Ni

Abstract The E0 transition strength in the 2 2 + → 2 1 + transitions of 58,60,62Ni have been determined for the first time following a series of measurements at the Australian National University (ANU) and the University of Kentucky (UK). The CAESAR Compton-suppressed HPGe array and the Super-e solenoid at ANU were used to measure the δ ( E 2 / M 1 ) mixing ratio and internal conversion coefficient of each transition following inelastic proton scattering. Level half-lives, δ ( E 2 / M 1 ) mixing ratios and γ-ray branching ratios were measured at UK following inelastic neutron scattering. The new spectroscopic information was used to determine the E0 strengths. These are the first 2 + → 2 + E0 transition strengths measured in nuclei with spherical ground states and the E0 component is found to be unexpectedly large; in fact, these are amongst the largest E0 transition strengths in medium and heavy nuclei reported to date.