C.A. Fields

High spin states in 88, 87, 86Zr

Abstract Levels populated by the 84, 86 Sr(α, 2nγ) 84, 88 Zr and 84 Sr(α, nγ) 87 Zr reactions were studied at bombarding energies of 28, 35 and 16.7 MeV, respectively. In addition, levels in 88 Zr populated by the 89 Y(p, 2nγ) 88 Zr reaction were studied at 22.7 MeV incident proton energy. The studies were performed by techniques of in-beam γ-ray spectroscopy. A number of new high spin states in 88 Zr and 87 Zr were observed, and a level scheme for 86 Zr is obtained for the first time. The levels observed are interpreted within a shell-model context.

A study of the rotational side-bands in 162Dy

Abstract Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.

A systematic investigation of the (α, 2nγ) reaction on medium-heavy nuclei

Abstract Exclusive neutron spectra and angular distributions have been measured for 28–35 MeV (α, 2nγ) reactions on various nuclei in the 80 ≦ A ≦ 210 region. Pre-equilibrium processes dominate the 35 MeV (α, 2nγ) reaction mechanism in much of this region. Analysis of systematic variation in the neutron spectrum parameters shows that the reaction mechanism is strongly correlated with the target neutron excess parameter ( N − Z / A . Analysis of the γ-decay of the entry states shows that well-defined incident angular momentum windows exist for the pre-etjuilibrium (α, 2nγ) reaction. These features are discussed in terms of various models for the reaction mechanism.

A study of the 84Kr(α, 2nγ)86Sr reaction

Abstract Decay properties of levels of 86 Sr populated by the 28 MeV 84 Kr(α, 2n γ ) reaction have been studied by in-beam γ-ray spectroscopy. The observation of several new levels with J ≧ 6 allows a detailed comparison of the 2p-2n structures of 86 Sr with those of neighboring N = 48 nuclei. The results are discussed in terms of the weak-coupling model.

The level structure of 90Mo

Abstract High-spin states of the N = 48 nucleus 90Mo have been studied using the 33 MeV 90Zr(3He, 3nγ) reaction. A previously unknown level structure above the 8+ isomer and several new lower-lying levels have been identified. The results are discussed in terms of shell-model calculations which allow four protons in the 2 p 1 2 and 1 g 9 2 subshells and two neutron holes in the 1 f 5 2 , 2 p 3 2 , 2 p 1 2 , or 1 g 9 2 orbitals.

ROTATIONAL BANDS IN SM-152 OBSERVED FOLLOWING THE (ALPHA, 2N-GAMMA) REACTION

Abstract The level structure of 152Sm has been studied with various spectroscopic techniques using the (α, 2nγ) reaction. Seven rotational bands were identified up to spin values of 9 to 14ħ, viz. the ground-state band, the β- and γ-bands, and four negative-parity bands (NPBs). In the β-band a beginning of backbending or upbending was observed at I ⋍ 14 h . Two of the NPBs are based upon collective states; the lowest can be interpreted as an aligned octupole band, the other one shows two-quasiparticle characteristics at high spin. The properties of the other NPBs are consistent with Kπ = 5− and Kπ = 7− two-quasiparticle configurations of the bandheads. At high spin these bands are strongly mixed. Excitation energies for all bands with a collective bandhead, as well as B(rmE1) and B(E2) branching ratios and (for the β-band) X(E0/E2) ratios, were compared with IBA calculations.

High-spin states above 3.5 MeV in 91Nb

Abstract High-spin states in 91 Nb populated by the 89 Y(α, 2nγ) reaction were studied at 24.0 MeV and 35.7 MeV incident α-particle energy. Gamma-gamma coincidence and γ-ray angular distribution measurements were made. Several high-spin states in 91 Nb were observed at 35.7 MeV bombarding energy which were not observable at 24.0 MeV. The shell model structure of these states is discussed.

Rotational sidebands in 166Er

Abstract Rotational sidebands in 166 Er were observed using the 24 MeV 164 Dy(α, 2nγ) reactions. The ground-state band was observed up to spin 16 + and does not backbend. A strong backbend is, however, observed in a K π = ( O + ) sideband, indicating that the 12 + state of the previously unknown S-band is at 2656 keV. The γ-band shows significant rotational alignment above I = 10 + . Levels of at least two negative-parity bands, one of which is primarily the K π = 2 − octupole vibration, are also observed.

A study of the 28Si(d, p)29Si reaction

Abstract The 28Si(d, p)29Si reaction has been studied at 17.85 MeV with a very clean silicon target allowing the study of sharp states up to an excitation of 14.8 MeV. Spectroscopic factors for bound states were obtained with-good reliability; these compare well to calculated values for strong low-lying states. A weak-coupling model allowed multistep calculations for several states, resulting in improved fits to the stripping cross-section data. Data for states above the neutron threshold in 29Si were compared to DWBA calculations using unbound form factors. The sums of spectroscopic factors show a more complete closure of the d 5 2 shell at 28Si than is predicted by Hartree-Fock methods.

A study of 80,82,84Sr by (α, 2nγ) reactions

Abstract The AKr(α, 2nγ)A+2Sr reactions have been studied by in-beam γ-ray spectroscopy for A = 78, 80, and 82. States with spins up to 10+ in 82,84Sr and 6+ in 80Sr are identified. The Sr nuclei undergo a transition from spherical to deformed ground states as neutrons are removed from the N = 50 shell. The systematics of these nuclei are compared with calculations based upon the shell model and interacting boson model.