R. Ibbotson

Electromagnetic properties of tungsten nuclei

Abstract Quadrupole collectivity in 182,184 W has been studied by Coulomb excitation with 58 Ni and 136 Xe projectiles. The yields of the de-excitation γ-rays, measured in coincidence with the scattered ions, were used to determine a fairly complete set of electromagnetic matrix elements for the low-lying states. A separate experiment measured lifetimes for five and eight states respectively in 182 W and 184 W by the recoil distance method following Coulomb excitation with a 245 MeV 58 Ni projectile. These lifetime measurements are in good agreement with the results for the Coulomb excitation yield measurements. The correlations of the extracted E2 matrix elements are examined using the Cline-Flaum sum-rule technique and discussed in the framework of the various collective models. These correlations are well described with quadrupole shape degrees of freedom. Strong mixing between the β- and γ-vibration degrees of freedom, claimed in the pairing-plus-quadrupole model of Kumar and Baranger, is not supported by the present results.

Quadrupole and octupole collectivity in 148Nd

Abstract The role of quadrupole and octupole collectivity in the shape-transitional nucleus 148 Nd has been studied by Coulomb excitation using beams of 58 Ni and 92 Mo, and a beam of 148 Nd (using a 208 Pb target). The extracted E1, E2 and E3 matrix elements involving states up to 12 + ·in the ground band and 13 − in the negative-parity band are presented, and compared to calculations that assume a vibrational and rotational octupole nature for the negative-parity band. The positive-parity ground-band states are well described in terms of a prolate deformed shape with Q 20 ≈ 400 e fm 2 ( β 2 rms ≈ +0.18). The present results suggest a vibrational octupole nature for the low-spin negative-parity states, with an intrinsic moment Q 30 ≈ 1500 e fm 3 ( β 3 rms ≈ 0.12). The E2 and E3 matrix elements connecting these bands to the β- and γ-vibrational bands (and within these bands) are also presented, and compared to calculations incorporating the coupling between the rotational and vibrational modes. These calculations describe reasonably well the E2 matrix elements involving the gamma band, but do not reproduce the measured E2 matrix elements for the beta band, implying a complicated intrinsic structure for the beta band. The strong enhancement of the measured E3 matrix elements connecting the negative-parity band to the beta band could be indicative of a significant component of the two-phonon octupole vibration in the wavefunction of the so-called beta band.

High-spin states in odd-odd 164Lu

Abstract High-spin states in odd-odd 164 Lu have been studied using 19 F and 23 Na induced fusion-evaporation reactions. Three strongly coupled rotational bands have been established and three decoupled bands were tentatively assigned to 164 Lu. The band crossing frequencies, energy sig-nature splittings, and relative transition probabilities were deduced from experimental data and compared with neighboring even-even and odd- A nuclei. An anomalously large band crossing frequency was observed in the excited band, and a pronounced signature inversion was observed in the yrast band of 164 Lu. These anomalies may be associated with residual neutron-proton interactions.

A kinematic-coincidence detector system for sub-barrier heavy-ion reaction studies

A detector system has been developed to study sub-barrier heavy-ion transfer reactions by means of the coincident detection of both reaction products. Scattered ions are momentum-analyzed by an Enge split-pole magnetic spectrometer, and the recoils are detected in a 3.5 m long time-of-flight arm. Ion trajectories through the spectrometer are determined by an angle measurement at the focal plane, which together with the measured time-of-flight through the spectrometer provides independent measurements of the momentum and velocity of the scattered particles. This raytracing method allows the spectrometer to be operated with a large (10.3 msr) entrance solid angle without significantly degrading the velocity and momentum measurements. Two-body kinematic coincidence is used to measure the mass of the recoil and the reaction Q-value distribution. This detector system has been used to measure the cross sections and Q-value distributions for sub-barrier quasielastic transfer reactions in the system 58Ni + 162Dy. Mass resolution of better than 0.8 u (FWHM) is obtained, resulting in adequate channel separation. The Q-value resolution is approximately 1.4 MeV (FWHM) for ≈ 50 μg/cm2 thick Dy targets. Prospects for the improvement of the performance of such detector systems are discussed.

Electric Transition Strengths in Octupole Nuclei Measured by Coulomb Excitation

There is now a wealth of experimental and theoretical evidence, e.g.1,2, that nuclei with Z ≈ 60, N ≈ 88 and Z ≈ 88, N ≈ 134 are reflection-asymmetric in the intrinsic frame. The usual experimental signature for such pear-shaped nuclei is the observation of low-lying negative parity states which become interleaved with the yrast positive parity states at medium values of angular momentum. These data have been interpreted using geometric models within the mean field approach as arising from the presence of static octupole deformation, i.e. \({\bar \beta _3} \ne 0\) As with quadrupole degrees of freedom, the most reliable measurements of collective octupole strength are the Eλ matrix elements connecting the rotational states in the octupole and ground state bands. While many E1 transition strengths have been determined in ‘octupole’ nudei, they are strongly affected by fluctuating single particle contributions and are not a reliable indicator of octupole strength. While observation of real photon E3 emission is not yet technically possible, the determination of E3 matrix elements from Coulomb excitation (Coulex) is feasible. Results of such measurements of E1, E2 and E3 matrix elements in 148Nd are presented here, and are used to extract deformation parameters which can be compared with theoretical predictions. Recent measurements in other nudei are also briefly discussed.