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Multi-quasiparticle high-K states, several of which are isomeric, were observed in Hf180 with the Gammasphere array. Lifetimes in the ns-μs range were determined using centroid-shift and decay measurements within a μs coincidence time window. The configurations of high-K states involve two and four quasiparticles, with states up to Kπ=(18−) established. High-K excitations are found to be progressively more favored with increasing excitation energy. The K quantum number is quite robust up to the highest spins observed, as evidenced by the large values of the reduced hindrance for isomeric decays. Rotational bands built on three high-K states are identified, and the measured branching ratios in these sequences enable the assignment of underlying configurations. Multi-quasiparticle calculations using the Lipkin-Nogami approach for pairing, with blocking included, reproduce the observed high-K energies quite well.

We report on a study of the structure of the unbound nucleus He-7 utilizing the proton-removal reaction H-2(Li-8,He-3)He-7. Combining the present results with those of our prior measurements of the neutron-adding reaction H-2(He-6,p)He-7, a consistent picture emerges for the low-lying excitations in He-7. Specifically, the negative-parity sequence of resonances, in order of excitation energies, is consistent with 3/2(-),1/2(-), and 5/2(-). The stable-beam reactions H-2(Li-7,t)Li-6 and H-2(Li-7,He-3)He-6 were also measured. The results are compared with the predictions of nuclear structure models, including those of ab initio quantum Monte Carlo calculations.

The 31-yr isomer of {sup 178}Hf at excitation energy 2.446 MeV, and with J{sup {pi}}, K= 16+, 16, has been the object of several studies for possible mechanisms that might trigger isomer decay to the ground state. Potential for the control of nuclear energies (MeV) with atomic energies (keV) is the driving interest. The 178Hf isomer is a favorite nucleus in the search for triggered decay because this isomer is long-lived, available in microgram quantities, the decay scheme is well known, the excitation energy is high, and a sample enriched in the isomer can be fabricated for irradiation. In fact, accelerated decay of the {sup 178}Hf isomer when irradiated with photons from a dental x-ray machine has been reported by Collins et al. The triggering x-ray energies were reported to be in the 20-60 keV range. Using the synchrotron radiation at the Advanced Photon Source (APS) we published limits on such accelerated emission approximately 5 orders of magnitude lower than those in ref. 1. Very recently, a new measurement of triggering has been reported by Collins et al., using monochromatic x rays from the SPring-8 synchrotron but this time for (several) much lower incident x-ray energies in the 9-13 keV region. In order to verify this observation, we undertook a new measurement at the APS, with significantly increased focus on incident x-rays below 20 keV, as described below.

A new measurement of the β-delayed α decay of N16 has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a N16 beam, yielding very clean α spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21 keV b. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation.

The excitation energies and spins of the levels in the yrast superdeformed band of $^{191}$Hg have been determined from two single-step $\gamma$ transitions and the quasi-continuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included., Comment: 8 pages, 11 figures

Enhanced decay of the 31-yr isomer of 178Hf induced by x-ray irradiation has been reported previously. Here we describe an attempt to reproduce this result with an intense "white" x-ray beam from the Advanced Photon Source. No induced decay was observed. The upper limits for the energy-integrated cross sections for such a process, over the range of energies of 20–60 keV x rays, are less than 2×10^-27cm^2 keV, below the previously reported values by more than 5 orders of magnitude; at 8 keV the limit is 5×10^-26 cm^2 keV.

Spectroscopic studies of weakly populated proton i13/2 bands in superdeformed Tl nuclei (around mass 190) and in the normally deformed, very light 183Tl nucleus are discussed. Among the results presented, the first measurement of a superdeformed quadrupole moment in an odd-Z nucleus, 191Tl, is reported. The experiments were conducted with the Gammasphere array as "stand-alone" device and coupled with the Argonne Fragment Mass Analyzer.

Five superdeformed ~SD! bands have been observed in 149Tb, using the Gammasphere spectrometer. The measurement was performed with a backed target to obtain lifetime information from a Doppler-shift attenuation method analysis. It is proposed that the yrast SD band corresponds to the 148Gd(yrast)^p63 intruder configuration. The first excited band ~band 2! is found to be identical to the 150Tb yrast SD band. Band 3 is proposed to correspond to the p@301#1/2(a521/2)!@651#3/2(a521/2) proton-hole excitation coupled to the p64n 71(n@651#1/2)21 intruder configuration. Bands 4 and 5, the weakest SD bands seen in the present work, show similarities with band 3 and are most likely associated with a similar intruder configuration.

Quadrupole moments of the six known superdeformed ~SD! bands of 193Hg and the yrast SD band of 192Hg have been determined by a Doppler-shift-attenuation-method measurement utilizing the gammasphere array. The quadrupole moments of all 193Hg SD bands were found to be similar, suggesting the active single-particle orbitals in the mass-190 region exhibit only small shape-driving effects. Additionally, there is evidence for an unexpected difference in the quadrupole moments of SD bands in 192Hg and 193Hg.

High-precision lifetime measurements have been performed in superdeformed ~SD! bands of 192,194Hg with the Doppler-shift attenuation method. Intrinsic quadrupole moments Q0 were extracted for three SD bands in 194Hg and for the yrast SD band in 192Hg. Within experimental uncertainties, all four SD bands have equal Q0 values. These results provide constraints on differences in Q0 values between the ‘‘identical’’ SD bands 194Hg~3! and 192Hg~1!.

Discrete γ rays directly connecting states of a superdeformed (SD) band in 194Hg to the yrast states have been discovered. Thus, the excitation energies and spins of all members of the lowest SD band are established for the first time, together with their likely parity. The SD band decays from its 10+ and 12+ states, which lie 4204.8 and 4407.4 keV above the normal-deformed yrast states of the same spins.