F. S. Stephens’s research while affiliated with Lawrence Berkeley National Laboratory and other places

In rotating nuclei, backbending appears to be a large increase in the moment of inertia of a band, accompanied by a decrease in the rotational frequency. This has been shown to be a band crossing where one band (e.g. the ground or vacuum band) is crossed by a band in which the Coriolis force has unpaired two high-j nucleons and aligned their angular momentum with the rotation axis of the nucleus. With increasing spin this force gradually quenches the nuclear pairing correlations, with one or two of these band crossings in the process. © 2013 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.

Over a period of several years we have performed three separate experiments at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron in which 235U (thick target) was Coulomb-excited. The program involved stand-alone experiments with Gammmasphere and with the 8pi Spectrometer using 136Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with a 40Ca beam at 184 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g., [631]1/2, [624]7/2, and [622]5/2) strongly populated by E3 excitation. The CHICO data have been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many M1 matrix elements could be extracted from the γ-ray branching ratios. A number of new features have emerged, including the unexpected attenuation of magnetic transitions between states of the same Nilsson multiplet, the breakdown of Coriolis staggering at high spin, and the effect of E3 collectivity on Coriolis interactions.

Recently it has become possible to perform detailed spectroscopy on nuclei beyond Z = 100 with the aim of understanding the underlying single-particle structure of superheavy elements. A number of such experiments have been performed at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory using the Berkeley Gas-filled Separator (BGS), coupled with delayed γ-ray and electron-decay spectroscopy. Experiments have been performed on 254No (Z = 102), 257Rf (Z = 104), and 261Sg (Z = 106). The results provide new information on the properties of transactinide nuclei, which is important for testing models of the heaviest elements.

We report results from an experiment on the decay of the high-K isomers in 254No. We have been able to establish the decay from the known high-lying four-quasiparticle isomer, which we assign as a Kπ=16+ state at an excitation energy of Ex=2.928(3) MeV. The decay of this state passes through a rotational band based on a previously unobserved state at Ex=2.012(2) MeV, which we suggest is based on a two-quasineutron configuration with Kπ=10+. This state in turn decays to a rotational band based on the known Kπ=8− isomer, which we infer must also have a two quasineutron configuration. We are able to assign many new gamma-rays associated with the decay of the Kπ=8− isomer, including the identification of a highly K-forbidden ΔK=8 E1 transition to the ground-state band. These results provide valuable new information on the orbitals close to the Fermi surface, pairing correlations, deformation and rotational response, and K-conservation in nuclei of the deformed trans-fermium region.

Two isomeric states have been identified in 255Lr. The decay of the isomers populates rotational structures. Comparison with macroscopic-microscopic calculations suggests that the lowest observed sequence is built upon the [624]9/2+ Nilsson state. However, microscopic cranked relativistic Hartree-Bogoliubov (CRHB) calculations do not reproduce the moment of inertia within typical accuracy. This is a clear challenge to theories describing the heaviest elements.

Excited states in Rf256 were populated via the Pb208(Ti50,2n) fusion-evaporation reaction. Delayed γ-ray and electron decay spectroscopy was performed and three isomeric states in Rf256 have been identified. A fourth low-energy nonyrast state was identified from the γ-ray decay of one of the higher lying isomers. The states are interpreted as multi-quasiparticle excitations.

New techniques in the use of highly-segmented HPGe detectors enable the tracking of the path of a scattered gamma ray in the detector. This enables precision Doppler correction of gamma rays emitted from fast-moving sources for high-resolution spectroscopy at radioactive beam facilities. Critical to these applications is a knowledge of the position resolution to which the scattering points can be determined. We directly measured the position resolution of using a highly collimated Cs source. Signal decomposition was used to determine the position and charge deposition of the interaction points from the scattered gamma ray in the crystal, followed by tracking to identify the first interaction point. The set of first interaction points form a line through the detector and their dispersion gives the position resolution of the crystal in two dimensions. Such a measurement was performed with the 36-way segmented GRETINA P3 prototype detector the position resolution was found to be sigmax = 1.5 mm and sigmay = 1.7 mm.

The structure of the neutron-rich N=20 nucleus 35P has been investigated through nucleon transfer experiments using the 208Pb(36,Xγ) reaction at 230 MeV. The level structure, of mainly 1ℏω excitations in 35P, has been significantly expanded. The measurements are compared with shell model calculations. Experimental branching ratio limits are reported for predicted transitions to the 2ℏω bandheads in 35P and 34Si.