The internal-conversion and internal-pair-production decays of the first excited 0 + state in 68 Ni are studied following the β decay of 68 Co. A novel experimental technique, in which the ions of 68 Co were implanted into a planar germanium double-sided strip detector and which required digital pulse processing, is developed. The values for the energy of the first excited 0 + state and the electric monopole transition strength from the first excited 0 + state to the ground state in 68 Ni are determined to be 1605(3) keV and 7.6(4) × 10 −3 , respectively. Comparisons of the experimental results to Monte Carlo shell-model calculations suggest the coexistence between a spherical ground state and an oblate first excited 0 + state in 68 Ni.

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Study of β+ decay of the exotic Tz=-3/2 nucleus 55Cu, via delayed γ rays, has revealed a strongly isospin mixed doublet (4599-4579 keV) in 55Ni, which represents the fragmented and previously unknown isobaric analog of the ground state of 55Cu. The observed small log ft values to both states in the doublet confirm the superallowed Fermi β decay. The near degeneracy of a pair of 3/2- levels in 55Ni results in the strong isospin mixing. The isospin mixing matrix element between the T=3/2 and T=1/2 levels is inferred from the experiment to be 9(1) keV, which agrees well with the matrix element of the charge symmetry breaking shell model Hamiltonian of Ormand and Brown. A precise value of the half-life of 55Cu at 57(3) ms was also obtained.

Excited states were investigated in 21 F and 25 Na using the 9 Be( 14 C,pnγ) reaction at 30, 35, and 45 MeV and the 9 Be( 18 O,pnγ) reaction at 35 MeV. Protons were detected and identified in an E-∆E telescope at 0 o in coincidence with one or more γ radiations in the FSU Compton-suppressed Ge detector array. Many new levels and electromagnetic decays were observed, especially among the higher spin states. Angular distributions and mean lifetimes were measured wherever possible in both nuclei. The energy levels of the positive-parity states in the two nuclei agree rather well with shell model calculations using both the USDA and WBP interactions up to the highest spins observed of 13/2h. Both a weak coupling approximation and shell model calculations using the WBP interaction generally reproduce the negative-parity states in 21 F. The shell model calculations reproduce relatively well the measured M1 and E2 transitions in both nuclei, but overpredict the parity-changing E1 transitions in 21 F, the only nucleus in which negative-parity states were observed in the present experiment.

The 9 Be( 14 C, αγ) reaction at E Lab =30 and 35 MeV was used to study excited states of 19 O. The Florida State University (FSU) γ detector array was used to detect γ radiation in coincidence with charged particles detected and identified with a silicon ∆E-E particle telescope. Gamma decays have been observed for the first time from six states ranging from 368 to 2147 keV above the neutron separation energy (S n =3962 keV) in 19 O. The γ decaying states are interspersed among states previously observed to decay by neutron emission. The ability of electromagnetic decay to compete successfully with neutron decay is explained in terms of neutron angular momentum barriers and small spectroscopic factors implying higher spin and complex structure for these intruder states. These results illustrate the need for complementary experimental approaches to best illuminate the complete nuclear structure. 1The decay mode(s) of a nuclear quantum state is (are) one of its most important properties after energy. Usually nuclear decay follows the hierarchy of the fundamental forces of nature.Decay by emission of one or more particles mediated by the strong nuclear interaction is normally the dominant mode for those states unbound to particle emission. Bound excited states usually decay by emission of electromagnetic radiation to the ground state. The ground state, in turn, decays much more slowly by β decay mediated by the weak nuclear interaction until the lowest energy neutron-to-proton ratio has been reached. Of course, there are exceptions: lower energy charged particle decay from unbound states is inhibited by the Coulomb barrier, and large spin change or low emission energy can inhibit γ decay so much that β decay occurs first. Even decay by neutrons which face no Coulomb barrier can be inhibited by large spin change, as is well known in high-spin spectroscopy of medium and heavy nuclei. However, based on the simple picture of barrier penetrability, neutron decay is usually assumed to dominate over radiative decay when angular momentum barriers are not very high. But Fermi's Golden Rule [1] states that the decay rate is the product of the coupling strength, the density of final states, and the matrix element between the wavefunction of the parent state and the daughter state (usually called spectroscopic factor (S)). What is often overlooked is that this latter factor due to the nuclear structure may be more instrumental than the angular momentum barrier in limiting neutron decay rates below those of electromagnetic decay. Examples are states with complex structure often involving intruder configurations.The role of factors beyond barrier penetrability is highlighted by observations reported in this paper of the decays of unbound states in 19 O populated in the 9 Be( 14 C,αγ) reaction which favors higher-spin states, which are typically more complex and may involve intruder configurations. The importance of 19 O with a closed major proton shell and an exactly halffilled neutron d 5/2 orbital has been recognized wi...