B. E. Tomlin

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.

Half-lives and branchings for {beta}-delayed neutron emission for neutron-rich Co-Cu isotopes in the r-process

The {beta} decays of very neutron-rich nuclides in the Co-Zn region were studied experimentally at the National Superconducting Cyclotron Laboratory using the NSCL {beta}-counting station in conjunction with the neutron detector NERO. We measured the branchings for {beta}-delayed neutron emission (P{sub n} values) for {sup 74}Co (18{+-}15%) and {sup 75-77}Ni (10{+-}2.8%, 14{+-}3.6%, and 30{+-}24%, respectively) for the first time, and remeasured the P{sub n} values of {sup 77-79}Cu, {sup 79,81}Zn, and {sup 82}Ga. For {sup 77-79}Cu and for {sup 81}Zn we obtain significantly larger P{sub n} values compared to previous work. While the new half-lives for the Ni isotopes from this experiment had been reported before, we present here in addition the first half-life measurements of {sup 75}Co (30{+-}11 ms) and {sup 80}Cu (170{sub -50}{sup +110} ms). Our results are compared with theoretical predictions, and their impact on various types of models for the astrophysical rapid neutron-capture process (r-process) is explored. We find that with our new data, the classical r-process model is better able to reproduce the A=78-80 abundance pattern inferred from the solar abundances. The new data also influence r-process models based on the neutrino-driven high-entropy winds in core collapse supernovae.

Probing Shell Structure and Shape Changes in Neutron-Rich Sulfur Isotopes through Transient-Field g-Factor Measurements on Fast Radioactive Beams of 38S and 40S

The shell structure underlying shape changes in neutron-rich nuclei near N = 28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

Shell structure underlying the evolution of quadrupole collectivity in 38 S and 40 S probed by transient-field g-factor measurements on fast radioactive beams

The shell structure underlying shape changes in neutron-rich nuclei between N=20 and N=28 has been investigated by a novel application of the transient field technique to measure the first-excited state g factors in {sup 38}S and {sup 40}S produced as fast radioactive beams. Details of the new methodology are presented. In both {sup 38}S and {sup 40}S there is a fine balance between the proton and neutron contributions to the magnetic moments. Shell-model calculations that describe the level schemes and quadrupole properties of these nuclei also give a satisfactory explanation of the g factors. In {sup 38}S the g factor is extremely sensitive to the occupation of the neutron p{sub 3/2} orbit above the N=28 shell gap as occupation of this orbit strongly affects the proton configuration. The g factor of deformed {sup 40}S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

Nuclear magnetic moment of the 57Cu ground state.

The nuclear magnetic moment of the ground state of (57)Cu(Iota(pi) = 3/2(-), T(1/2) = 196.3 ms) has been measured to be /mu((57)Cu)/ = (2.00 +/- 0.05)mu(N) using the beta-NMR technique. Together with the known magnetic moment of the mirror partner (57)Ni, the spin expectation value was extracted as = -0.078 +/- 0.13. This is the heaviest isospin mirror T = 1/2 pair above the (40)Ca region for which both ground state magnetic moments have been determined. The discrepancy between the present results and shell-model calculations in the full f p shell giving mu((57)Cu) approximately 2.4mu(N) and approximately 0.5 implies significant shell breaking at (56)Ni with the neutron number N = 28.

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.

Ground state magnetic dipole moment of {sup 35}K

The ground state magnetic moment of {sup 35}K has been measured using the technique of nuclear magnetic resonance on {beta}-emitting nuclei. The short-lived {sup 35}K nuclei were produced following the reaction of a {sup 36}Ar primary beam of energy 150 MeV/nucleon incident on a Be target. The spin polarization of the {sup 35}K nuclei produced at 2 deg. relative to the normal primary beam axis was confirmed. Together with the mirror nucleus {sup 35}S, the measurement represents the heaviest T=3/2 mirror pair for which the spin expectation value has been obtained. A linear behavior of g{sub p} vs g{sub n} has been demonstrated for the T=3/2 known mirror moments and the slope and intercept are consistent with the previous analysis of T=1/2 mirror pairs.

Nuclear magnetic moment of the Cu57 ground state

The nuclear magnetic moment of the ground state of (57)Cu(Iota(pi) = 3/2(-), T(1/2) = 196.3 ms) has been measured to be /mu((57)Cu)/ = (2.00 +/- 0.05)mu(N) using the beta-NMR technique. Together with the known magnetic moment of the mirror partner (57)Ni, the spin expectation value was extracted as = -0.078 +/- 0.13. This is the heaviest isospin mirror T = 1/2 pair above the (40)Ca region for which both ground state magnetic moments have been determined. The discrepancy between the present results and shell-model calculations in the full f p shell giving mu((57)Cu) approximately 2.4mu(N) and approximately 0.5 implies significant shell breaking at (56)Ni with the neutron number N = 28.

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.

Nuclear Magnetic Moment of the {sup 57}Cu Ground State

The {sup 4}He total photoabsorption cross section is calculated with the realistic nucleon-nucleon potential Argonne V18 and the three-nucleon force (3NF) Urbana IX. Final state interaction is included rigorously via the Lorentz integral transform method. A rather pronounced giant resonance with peak cross sections of 3.0 (3.2) mb is obtained with (without) the 3NF. Above 50 MeV strong 3NF effects, up to 35%, are present. Good agreement with experiment is found close to threshold. A comparison in the giant resonance region is inconclusive, since data do not show a unique picture.The nuclear magnetic moment of the ground state of {sup 57}Cu(I{sup {pi}}=3/2{sup -},T{sub 1/2}=196.3 ms) has been measured to be vertical bar {mu}({sup 57}Cu) vertical bar =(2.00{+-}0.05){mu}{sub N} using the {beta}-NMR technique. Together with the known magnetic moment of the mirror partner {sup 57}Ni, the spin expectation value was extracted as =-0.78{+-}0.13. This is the heaviest isospin T=1/2 mirror pair above the {sup 40}Ca region for which both ground state magnetic moments have been determined. The discrepancy between the present results and shell-model calculations in the full fp shell giving {mu}({sup 57}Cu){approx}2.4{mu}{sub N} and {approx}0.5 implies significant shell breaking at {sup 56}Ni with the neutron number N=28.The intermediate valence compound YbAl{sub 3} exhibits a broad magnetic excitation in the inelastic neutron scattering spectrum with characteristic energy E{sub 1}{approx_equal}50 meV, equal to the Kondo energy (T{sub K}{approx}600-700 K). In the low temperature (T<T{sub coh}{approx}40 K) Fermi liquid state, however, a new peak in the scattering occurs at E{sub 2}{approx_equal}33 meV, which lies in the hybridization gap that exists in this compound. We report inelastic neutron scattering results for a single-crystal sample. The scattering at energies near E{sub 1} qualitatively has the momentum (Q) dependence expected for interband scattering across the indirect gap. The scattering near E{sub 2} has a very different Q dependence: it is a weak function of Q over a large fraction of the Brillouin zone and is smallest near (1/2,1/2,1/2). A possibility is that the peak at E{sub 2} arises from a spatially localized excitation in the hybridization gap.