A.J. Ferguson

Yrast isomers and very high spin states in 148, 149, 151, 152Dy and 147Gd

Abstract We have studied level schemes for the feeding and decay of high-spin isomers in the nuclei 148, 149, 151, 152 Dy and 147 Gd. A variety of techniques involving γ-ray spectroscopy with pulsed heavy-ion beams have been applied including linear polarization, γ-γ correlations and recoil distance measurements. The general aspect of these level schemes is in accordance with the idea that the states near and along the yrast line can be described by a spheroidal shell model in which the nucleus is axially symmetric with respect to the total spin. In these cases the spin is generated not by collective rotation, but by alignment of many single-particle orbits around a common axis.

Knight shifts and absolute magnetic moments of (h9/2) n proton states in trans-bismuth nuclei

The Knight shiftsK(FrT1) andK(RaPb) have been measured in connection with a systematic study on magnetic moments of (h9/2)n proton states. The Knight shifts were obtained directly by a comparison of precession frequencies and, indirectly, from relaxation times involving the Korringa relation. Good agreement is found among the results from the different methods; the average values areK(FrT1)=(0.89±0.09)% andK(RaPb)=(1.05±0.15)%.

A measurement of the radiative width of the 8+ 11.948 MeV level of 20Ne

Abstract With the 4He(16O,γ)20Ne capture reaction, the 8+, 11.948 MeV level in 20Ne was observed as a resonance in the yield of γ-rays corresponding to the 8+→ 6+ transition. A strength, ωγ = 104 ± 35 MeV was obtained for the resonance which gives 7.5 ± 2.5 W.u. for the 8 + → 6 + transition probability establishing the 11.948 MeV level as the 8+ member of the ground state band. The experimental E 2 transition probabilities in the ground state band of 20Ne deviate strongly from those of the pure rotational model, but agree reasonably well with the predictions of the SU3 shell model.

High spin states in 20Ne from 16O(α, α) scattering

Unbound levels in 20Ne have been studied by 16O(α,α) elastic scattering at energies between Eα = 5–11 MeV with an experimental resolution of 3–5 keV. Four resonances have been observed with a width equal to the experimental resolution. From a phase-shift analysis of these resonances the following 20Ne excitation energies, spin-parity assignments and α-decay widths have been derived: 8775 ± 3 keV (Jπ = 6+, Γc.m. = 110 ± 25 eV); 11,925 ± 5 keV (Jπ = 4+, Γc.m. = 440 ± 150 eV); 11,948 ± 5 keV (Jπ = 8+; Γc.m. = 35 ± 10 eV); 13,333 ± 6 keV (Jπ = 7−, Γc.m = 80 ± 30 eV). The Jπ = 6+, 8+ and 7− levels are likely members of the lowest quasi-rotational bands with Kπ = 0+ and Kπ = 2−. The known reduced widths in both bands are small (θ2 < 0.1) indicating that the bands should be adequately described by the spherical shell model. The reduced widths within each band are found to decrease sharply with increasing spin.

Static moments of the first excited states of 32, 34S and 204, 206Pb

Abstract The reorientation effect in Coulomb excitation has been used to measure the following static quadrupole moments: Q 2 + ( 32 S ) = −0.066 ± 0.017 b , Q 2 + ( 34 S ) = 0.026 ± 0.023 b , Q 2 + ( 204 Pb ) = 0.19 ± 0.14 b . Interference effects from higher excited states have been included in the analysis, with the signs of the E2 matrix elements taken from an anharmonic model. The value obtained for Q 2 + ( 32 S ) is in disagreement with two previous measurements. We attribute the discrepancy to the smaller internucleon separation distances involved in the previous experiments, which can cause deviations from Coulomb excitation cross sections. The other quadrupole moments have not been measured previously. The B (E2: 0 + → 2 + ) measured were: 0.0305 ± 0.0016 e 2 · b 2 ( 32 S ), 0.025 ± 0.004 e 2 · b 2 ( 34 S ), and 0.166 ± 0.009 e 2 · b 2 ( 204 Pb ) . From the angular distribution of the de-excitation γ-rays of the Pb nuclei following recoil into vacuum, we have determined the following g- factors : ¦g 2 + ( 204 Pb )¦ (two standard deviations), ¦g 2 + ( 206 Pb )¦ = 0.07 + 0.07 − 0.03 . Our value of g 2 + ( 206 Pb ) is in agreement with a previous measurement.

Nuclear polarizability of 7Li from Coulomb excitation

Abstract Excitation of the first excited state in 7Li in the Coulomb and nuclei fields of 208Pb was studied. Below E( 7 Li) = 24 MeV the polarizability of 7Li was deduced to be α = 0.32 ± 0.07 fm3. At higher energies a large backward cross section for α-particle production was observed reflecting the prominent α-t cluster structure of 7Li.

E1 polarization in coulomb excitation of 7Li

Abstract Coulomb excitation of the 0.478 MeV J π = 1 2 − state of 7Li has been studied by a partiele-γ coincidence technique. From the dependence of the excitation probabilities on bombarding energy and scattering angle a sizeable interference contribution from E1 excitation of continuum states has been determined. General expressions for the size of the E1 polarization effect in Coulomb excitation are given and the observed magnitudes in 6,7Li are compared with schematic model calculations.

Effects of exchange and final-state interactions in the reactions 2H(3He, 3H p)p and 2H(3He, 3He p)n

Abstract Absolute coincidence cross sections for the 2 H( 3 He, 3 He p)n and 2 H( 3 He, 3 H p)p reactions were measured at E He = 35.9 MeV. Spectra dominated by the nucleon-nucleon final-state interaction (FSI) are fitted by a fully antisymmetrized PWBA theory which includes the effects of FSI in all its matrix elements. Previously reported 26.8 MeV data showing both FSI and quasi-elastic scattering (both with and without charge exchange) are also fitted by the theory, which qualitatively describes the shapes of all these spectra and the ratios of the cross sections for the various processes. Predictions of Watson-Migdal theory are fitted to the FSI spectra and differences between the two theories are analyzed.

Alpha-decay widths in 20Ne

Abstract The α-decay of several unbound levels in 20 Ne has been studied by 16 O(α, α) elastic and inelastic scattering. A narrow resonance, Γ c.m. = 13±4 eV, with J π = 5 − was found at E x ( 20 Ne ) = 8.451±0.005 MeV and is associated with the lowest K = 2 − quasirotational band. Several new, narrow resonances were found between E x = 16.0−18.4 MeV. Reduced α-decay widths have been obtained for the lowest K = 0 + , 2 − and 0 − bands . For states described predominantly by the (8, 2) representation of SU(3) we note a reduction of the reduced widths with increasing spin. Reduced widths of positive parity bands are reviewed.

Evidence for vibrational excited states in 32S

Abstract The 0 2 + , 2 2 + and 4 1 + states at 3781, 4283 and 4461 keV respectively in 32 S were studied, with emphasis on the latter two. The spin and parity of 4 + was confirmed for the 4461 keV state. Theγ-ray energies of the 4 1 + → 2 1 + and 2 1 + → O 1 + transitions were determined as 2229.4 ± 1.2 and 2231.7 ± 1.0 keV, respectively. The lifetimes of the 4 1 + and 2 1 + states were measured by the Doppler-shift attenuation method as 0.21 ± 0.06 and 0.35 ± 0.06 ps respectively. The 2 2 + → O 1 + and 2 2 + → 2 1 + transitions have a branching ratio of 5.9 ± 0.3, and the latter is pure E2 within the experimental error. The strengths of the 2 1 + → 0 1 + , 0 2 + → 2 1 + , 2 2 + → 2 1 + and 4 1 + → 2 1 + transitions are respectively 7 ± 1, 15 ± 4, 7.3 ± 0.7 and 12 ± 3 W.u. These values suggest that the 2 1 + level is a one-phonon vibrational state and that the 0 2 + , 2 2 + and 4 1 + levels make up a predominately two-phonon vibrational triplet.