P. A. Butler

Heavy-ion inelastic scattering from deformed nuclei

Abstract Theoretical and experimental methods for studying heavy-ion inelastic scattering from deformed nuclei are described. The theoretical methods involve classical-limit approximations, while particle- γ-spectroseopy techniques are employed experimentally. With these approaches, heavy-ion excitation in the Coulomb-nuclear interference region acquires a transparent interpretation, despite the apparent complexity of the multistep excitation processes involved. The examples discussed provide a good illustration of the relationship between classical and quantum physics. The sensitivity of the inelastic scattering to details of the surface ion-ion potential due to radial and angular localization is exploited to provide a method of determining the equipotential contours in a direct manner which bypasses particular model-dependent parametrizations. The method is used to construct ion-ion potentials from inelastic scattering data for the systems 40Ar + 160Gd, 156Gd, 162Dy, 164Dy, and180Hf. The contribution of adiabatic giant resonance polarization to this potential is discussed. The relation between the deformed ion-ion potential and nuclear shapes is illustrated by comparing the experimental potentials to deformed double-folding and deformed proximity-potential calculations.

The GREAT spectrometer

Abstract The GREAT spectrometer is designed to measure the decay properties of reaction products transported to the focal plane of a recoil separator. GREAT comprises a system of silicon, germanium and gas detectors optimised for detecting the arrival of the reaction products and correlating with any subsequent radioactive decay involving the emission of protons, α particles, β particles, γ rays, X-rays or conversion electrons. GREAT can either be employed as a sensitive stand-alone device for decay measurements at the focal plane, or used to provide a selective tag for prompt conversion electrons or γ rays measured with arrays of detectors deployed at the target position. A new concept of triggerless data acquisition (total data readout) has also been developed as part of the GREAT project, which circumvents the problems and limitations of common dead time in conventional data acquisition systems.

Nuclear isomers in superheavy elements as stepping stones towards the island of stability

A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the ‘island of stability’; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons—the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.

The GREAT triggerless total data readout method

Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment.

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Intrinsic dipole moments in reflection-asymmetric nuclei

Abstract Low-energy collective E1 moments in nuclei from the Ra-Th and Ba-Sm region are analysed within the shell-correction method based on the reflectio-asymmetric Woods-Saxon model. Extensive systematics of experimental intrinsic dipole moments are presented. Shell effects on the isovector E1 moment explain local quenchings in E1 strength observed experimentally. It is demonstrated that the inclusion of the neutron-skin contribution to the macroscopic dipole moment is crucial for the understanding of experimental trends.

Calibration of the new composite “clover” detector as a Compton polarimeter for the EUROGAM array

Abstract The application of a composite Ge detector to the measurement of the linear polarisation of γ-rays has been investigated. The polarisation sensitivity of this device has been determined over the energy range 197–1368 keV. Comparison to a previous design of a similar detector has been made.

Construction and use of a three Ge(Li) Compton polarimeter

Abstract A three Ge(Li) Compton polarimeter has been constructed, and its sensitivity to γ-ray linear polarisation determined in the energy range 0.4 to 4.4 MeV. An example is given describing its application to resolve a spin ambiguity for the 4868 keV level in 33 S populated by the reaction 30 Si(α,n) 33 S at an α-particle bombarding energy of 9.8 MeV.

The statistical analysis of γ-ray angular correlation experiments

Abstract The statistical analysis of γ-ray angular correlation experiments is discussed with the emphasis on experiments where the reaction mechanism does not lead to exactly known alignment. If a model of the reaction producing the alignment is made then it can be used to estimate the most probable value of the alignment and the confidence in that prediction. The inclusion of such estimates in the analysis is described. A procedure for including many types of relevant data into the analysis is described. Particular emphasis is given to the general technique of estimating the error to quote in association with a mixing ratio measurement. A cautious recipe for the estimate is suggested which follows the procedures of linear least squares regression analysis.

Coulomb excitation of 226Ra

Abstract Radioactive targets of 226 Ra were Coulomb excited by 4 He projectiles (particle spectroscopy), and 16 O, 32 S and 208 Pb projectiles (particle-γ coincidences). The K = 0± rotational bands could be followed up to spin 18 + and 17 − , respectively. All experiments were simultaneously analyzed in terms of E1, E2, E3 and E4 matrix elements coupling the positive- and negative-parity rotational states. The systematics of level energies, as well as dipole, quadrupole and octupole matrix elements are discussed in the framework of models assuming stable octupole deformation.