M.P. Fewell

High-spin yrast and non-yrast bands in 176Os, 178Os and 180Os

Abstract High-spin yrast and non-yrast states have been identified in 176 Os, 178 Os and 180 Os using ( 16 O , x n) reactions, and γ-ray techniques. Band crossing anomalies are observed in each of the positive-parity yrast bands. The magnitude of these anomalies decreases with decreasing neutron number, an effect attributed to the change in the moment of inertia of the ground state rotational bands. A 23 ns isomer, predominantly K π = 7 − , is identified at 1930 keV in 180 Os. The configuration of this isomer is discussed on the basis of the properties of its rotational band. Negative parity, odd and even spin, sideband sequences are observed in each isotope. Their relationship to rotation-aligned octupole and 2-quasiparticle bands is discussed from their excitation energies, band spacings, and decay properties. Detailed calculations for Coriolis mixed bands are carried out for the likely 2-quasiproton and 2-quasineutron configurations. An anomaly observed at spin 17 in the odd-spin negative-parity sequence in 180 Os is attributed to a band crossing with a fourquasiparticle configuration.

Electric quadrupole moments of the first excited states of 194Pt, 196Pt and 198Pt

Abstract Coulomb excitation of 194, 196, 198 Pt by 4 He, 12 C and 16 O projectiles has been used to determine the static electric quadrupole moments Q (2 1 + ) of the first excited states of 194,196,198 Pt, together with values of B (E2; 0 1 + → 2 1 + ). It is clearly established that Q (2 1 + ) is positive for each nucleus, having values of 0.48 (14) e · b and 0.66 (12) e · b for 194 Pt and 196 Pt, respectively, and 0.42 (12) e · b or 0.54 (12) e · b for 198 Pt, depending on whether the interference term P 4 (2 2 + ) is positive or negative. Results obtained for B (E2; 0 1 + → 2 1 + ) are 1.661 (11) e 2 · b 2 , 1.382 (6) e 2 · b 2 and 1.090 (7) e 2 · b 2 for 194, 196, 198 Pt respectively. The results are compared with the predictions of various nuclear models.

The static quadrupole moment of the first excited state of 198Hg

Abstract The first direct evidence of oblate nuclear deformation in the even-A mercury isotopes has been obtained by measurement of the static quadrupole moment of the first excited state of 198Hg using the reorientation effect in Coulomb excitation.

Energy calibration of the A.N.U. 14UD Pelletron accelerator

Abstract The energy calibration of the 14UD Pelletron accelerator at the Australian National University has been determined, with an accuracy better than 0.1%, up to 64 MeV equivalent proton energy using a simple alpha-particle back-scattering technique. From the observed width of the 14.2 MeV resonance in the 12 C(p, α 0 ) 9 B reaction, the energy spread ΔE / E of the analysed beam was measured as (1.7±0.3)×10 −4 at terminal potential of 7.1 MV.

Electric moments of the first excited state of 18O

Abstract The static quadrupole moment Q 2 + and the B (E2; 0 + → 2 + ) value of the first excited state of 18 O at e x = 1.982 MeV have been determined using the reorientation effect in Coulomb excitation. Surface-barrier detectors at laboratory angles of 90° and 174° were used to detect 18 O ions elastically and inelastically scattered from 208 Pb. At both angles, we determined experimentally the maximum bombarding energies at which nuclear interference effects were negligible. It is found that Q + = −0.023 ± 0.021 e · b (−0.052 ± 0.021 e · b) for destructive (constructive) interference from higher states. This result is in good agreement with theoretical expectation. For the transition moment we find B (E2; 0 + → 2 + ) = 0.00390 ± 0.00018 e 2 · b 2 (0.00371 ± 0.00018 e 2 · b 2 ) for destructive (constructive) interference.

The quadrupole moment of 196Pt - A crucial test of nuclear models

Abstract The static electric quadrupole moment Θ2+ and the B(E2; 0+ → 2+) of the first excited state of 196Pt have been determined using Coulomb excitation. The results are Θ2+ = 0.79±0.12 eb and B(E2; 0+ → 2+) = 1.382±0.006 e2b2. The Bohr-Mottel-son model and the O(6) limit of the interacting boson model fail to reproduce these values when constrained to fit other known. properties of 196Pt.

Systematics of coulomb-nuclear interference: Implications for reliable measurement of the reorientation effect

Abstract Data are presented which show an inverse correlation between Coulomb excitation probability and the nuclear separation corresponding to onset of Coulomb-nuclear interference. They demonstrate that the safe energy for reorientation effect measurements should always be determined experimentally.

An experimental verification of the validity of the reorientation-effect technique for the determination of nuclear quadrupole moments

Abstract The static electric quadrupole moment of the ground state of 7 Li has been determined using the reorientation effect in Coulomb excitation. The result obtained (−4.0 ± 1.1 e fm 2 ) is in excellent agreement with values reported from standard techniques and thus provides a strong verification of the validity of the reorientation-effect technique for the determination of nuclear quadrupole moments.

Octupole vibrational states in the even-mass barium isotopes

Abstract A low-lying state of 132, 134, 136, 138Ba, strongly excited in the scattering of 20 MeV α-particles at 175°, is identified as the first 3− octupole vibrational state in each case. Angular distribution measurements for α-particle scattering to this state and the well-known first 2+ state in 132, 134, 136, 138Ba are in agreement with harmonic-vibration coupled-channel predictions. Deformation parameters are obtained, and the deduced isoscalar transition rates are compared with measured electromagnetic transition rates. The first 3− states of 132, 134, 136, 138Ba are found to be at excitation energies of 2.070, 2.251, 2.529 and 2.879 MeV, respectively. A state in 130Ba at an excitation energy of 1.948 MeV is tentatively identified as the first octupole-vibrational state in this nucleus.

Static quadrupole moments of the first excited states of 200Hg and 202Hg

Abstract The static quadrupole moments Q2+ and B(E2; 0+ → 2+) values of the 2+ first excited states of 200Hg and 202Hg have been determined using the reorientation effect in Coulomb excitation. An annular silicon surface-barrier detector was used to detect backscattered 4He, 12C and 16O projectiles. It is found that for 200Hg, Q2+ = +1.07 ± 0.19 e · b(+0.98 ± 0.19 e · b) for destructive (constructive) interference from the 2+′ state, and B(E2; 0+ → 2+) = 0.853 ± 0.007 e2 · b2. For 202Hg, we find Q2+ = +1.01 ± 0.13 e · b (+0.87 ± 0.13 e · b) and B(E2; 0+ → 2+) = 0.605 ± 0.005 e2 · b2. The Q2+ value obtained for 200Hg is in agreement with previous work, but that for 202Hg is not. The results obtained are compared with the predictions of various nuclear models, and the mass dependence of Q2+ in the region 182 ≦ A ≦ 206 is examined.