A. C. Morton

Half-life of the doubly magic r-process nucleus 78Ni.

Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r process). 78Ni is the only doubly magic nucleus that is also an important waiting point in the r process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110(+100)(-60) ms. In the same experiment, a first half-life was deduced for 77Ni of 128(+27)(-33) ms, and more precise half-lives were deduced for 75Ni and 76Ni of 344(+20)(-24) ms and 238(+15)(-18) ms, respectively.

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Beta counting system for fast fragmentation beams

A new beta counting system has been developed at the National Superconducting Cyclotron Laboratory to study the beta decay of exotic nuclei produced by fast fragmentation. This system uses a double-sided silicon strip detector to detect both fragment implants and their subsequent beta decays; these events are correlated on a pixel-by-pixel basis, providing a direct measurement of the decay time with specific particle identification information regarding the parent nucleus. The experimental capabilities of this system are described, and future plans discussed.

Beta decay studies of nuclei near 32Mg: Investigating the ν(f7/2)-(d3/2) inversion at the N = 20 shell closure

Abstract 32 Mg lies within a region of deformed nuclei commonly referred to as the “island of inversion”. The β decay of 33 Al and 33 Mg has been studied to learn about nuclear structure near 32 Mg. Decay curves and precise half-life measurements are presented for both species. Gamma-ray spectra from correlated 33 Al decay events are also presented. The β -decay properties of 33 Al are shown to be well-described by an sd shell model calculation, suggesting that the ground state of 33 Al lies primarily outside the island of inversion.

Half-life and spin of {sup 60}Mn{sup g}

A value of 0.28{+-}0.02 s has been deduced for the half-life of the ground state of {sup 60}Mn, in sharp contrast to the previously adopted value of 51{+-}6 s. Access to the low-spin {sup 60}Mn ground state was accomplished via {beta} decay of the 0{sup +} {sup 60}Cr parent nuclide. New low-energy states in {sup 60}Mn have been identified from {beta}-delayed {gamma}-ray spectroscopy. The new, shorter half-life of {sup 60}Mn{sup g} is not suggestive of isospin-forbidden {beta} decay, and new spin and parity assignments of 1{sup +} and 4{sup +} have been adopted for the ground and isomeric {beta}-decaying states, respectively, of {sup 60}Mn.

Decay studies at the end of the rp-process

The rapid proton-capture process (rp-process) produces proton rich nuclei on the surface of accreting neutron stars. The light curves of type I X-ray bursts are an observable result of the energy produced. In reaction networks, a series of (p, γ) reactions and β-decays proceed along a path close to the proton drip-line culminating, for some bursts, in a Sn—Sb—Te cycle. To address uncertainties in the masses near the proton drip-line, we have observed the β-decay of neutron-deficient Sb isotopes at the National Superconducting Cyclotron Laboratory at Michigan State University. A mixed beam of unstable nuclei was produced at the Coupled Cyclotron Facility and isolated in the A1900 fragment separator. Nuclei were then implanted and decays were observed in the NSCL β-decay end station. We discuss preliminary results on β-decay properties and the search for proton radioactivity.

High-precision branching ratio measurement for the superallowed {beta}{sup +} emitter {sup 62}Ga

A high-precision branching ratio measurement for the superallowed {beta}{sup +} decay of {sup 62}Ga was performed at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. The 8{pi} spectrometer, an array of 20 high-purity germanium detectors, was employed to detect the {gamma} rays emitted following Gamow-Teller and nonanalog Fermi {beta}{sup +} decays of {sup 62}Ga, and the SCEPTAR plastic scintillator array was used to detect the emitted {beta} particles. Thirty {gamma} rays were identified following {sup 62}Ga decay, establishing the superallowed branching ratio to be 99.858(8)%. Combined with the world-average half-life and a recent high-precision Q-value measurement for {sup 62}Ga, this branching ratio yields an ft value of 3074.3{+-}1.1 s, making {sup 62}Ga among the most precisely determined superallowed ft values. Comparison between the superallowed ft value determined in this work and the world-average corrected Ft value allows the large nuclear-structure-dependent correction for {sup 62}Ga decay to be experimentally determined from the CVC hypothesis to better than 7% of its own value, the most precise experimental determination for any superallowed emitter. These results provide a benchmark for the refinement of the theoretical description of isospin-symmetry breaking in A{>=}62 superallowed decays.

Development of new shell structure in pf-shell nuclei

β-delayed γ-ray measurements have been part of an experimental program at the NSCL to understand the role of the πf7/2 - νf5/2 proton-neutron monopole interaction in neutron-rich pf-shell nuclei above 48Ca. Central to this study has been an attempt to observe the development of new shell structure at N = 32, 34 through the systematic observation of E(2+1 ) as a function of neutron number. Additionally, the ground state spin and parity of odd-odd and odd-A nuclei were interpreted in an extreme single-particle model to follow the monopole migration of the νf5/2 as protons are removed from the πf7/2state.

High precision measurements of {sup 26}Na {beta}{sup -} decay

High-precision measurements of the half-life and {beta}-branching ratios for the {beta}{sup -} decay of {sup 26}Na to {sup 26}Mg have been measured in {beta}-counting and {gamma}-decay experiments, respectively. A 4{pi} proportional counter and fast tape transport system were employed for the half-life measurement, whereas the {gamma} rays emitted by the daughter nucleus {sup 26}Mg were detected with the 8{pi} {gamma}-ray spectrometer, both located at TRIUMFs isotope separator and accelerator radioactive beam facility. The half-life of {sup 26}Na was determined to be T{sub 1/2}=1.07128{+-}0.00013{+-}0.00021 s, where the first error is statistical and the second systematic. The logft values derived from these experiments are compared with theoretical values from a full sd-shell model calculation.

High precision measurements of Na-26 beta(-) decay

High-precision measurements of the half-life and {beta}-branching ratios for the {beta}{sup -} decay of {sup 26}Na to {sup 26}Mg have been measured in {beta}-counting and {gamma}-decay experiments, respectively. A 4{pi} proportional counter and fast tape transport system were employed for the half-life measurement, whereas the {gamma} rays emitted by the daughter nucleus {sup 26}Mg were detected with the 8{pi} {gamma}-ray spectrometer, both located at TRIUMFs isotope separator and accelerator radioactive beam facility. The half-life of {sup 26}Na was determined to be T{sub 1/2}=1.07128{+-}0.00013{+-}0.00021 s, where the first error is statistical and the second systematic. The logft values derived from these experiments are compared with theoretical values from a full sd-shell model calculation.