Search

We have studied the β decay of the Tz=−1, f7/2 shell nuclei Ni54, Fe50, Cr46, and Ti42 produced in fragmentation reactions. The proton separation energies in the daughter Tz=0 nuclei are relatively large (≈4–5 MeV) so studies of the γ rays are essential. The experiments were performed at GSI as part of the Stopped-beam campaign with the RISING setup consisting of 15 Euroball Cluster Ge detectors. From the newly obtained high precision β-decay half-lives, excitation energies, and β branching ratios, we were able to extract Fermi and Gamow-Teller transition strengths in these β decays. With these improved results it was possible to compare in detail the Gamow-Teller (GT) transition strengths observed in beta decay including a sensitivity limit with the strengths of the Tz=+1 to Tz=0 transitions derived from high resolution (3He,t) reactions on the mirror target nuclei at RCNP, Osaka. The accumulated B(GT) strength obtained from both experiments looks very similar although the charge exchange reaction provides information on a broader energy range. Using the “merged analysis” one can obtain a full picture of the B(GT) over the full Qβ range. Looking at the individual transitions some differences are observed, especially for the weak transitions. Their possible origins are discussed. (Less)

Search for a new kind of superfluidity built on collective proton-neutron pairs with aligned spin is performed studying the Gamow-Teller decay of the T=1, Jπ=0+ ground state of Ge62 into excited states of the odd-odd N=Z nucleus Ga62. The experiment is performed at GSI Helmholtzzentrum fur Schwerionenforschung with the Ge62 ions selected by the fragment separator and implanted in a stack of Si-strip detectors, surrounded by the RISING Ge array. A half-life of T1/2=82.9(14) ms is measured for the Ge62 ground state. Six excited states of Ga62, populated below 2.5 MeV through Gamow-Teller transitions, are identified. Individual Gamow-Teller transition strengths agree well with theoretical predictions of the interacting shell model and the quasiparticle random phase approximation. The absence of any sizable low-lying Gamow-Teller strength in the reported beta-decay experiment supports the hypothesis of a negligible role of coherent T=0 proton-neutron correlations in Ga62. (Less)

Y. Oktem,1 D. L. Balabanski,2 B. Akkus,1 L. Amon Susam,1 L. Atanasova,2 C. W. Beausang,3,4 R. B. Cakirli,1,5 R. F. Casten,3 M. Danchev,6,7 M. Djongolov,6,7 E. Ganioglu,1 K. A. Gladnishki,7 J. Tm. Goon,8 D. J. Hartley,6,9 A. A. Hecht,3,10 R. Krucken,3,11 J. R. Novak,3 G. Rainovski,7 L. L. Riedinger,6 T. Venkova,2 I. Yigitoglu,12 N. V. Zamfir,3,13 and O. Zeidan6 1Department of Physics, Istanbul University, Vezneciler, 34134, Istanbul, Turkey 2Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, BG-1784 Sofia, Bulgaria 3Wright Nuclear Structure Laboratory, Yale University, New Haven, Connecticut 06520, USA 4Department of Physics, University of Richmond, Richmond, Virginia 23173, USA 5Max-Planck-Institut fur Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany 6Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 7Faculty of Physics, St. Kliment Ohridski University of Sofia, BG-1164 Sofia, Bulgaria 8Louisiana State University Department of Physics and Astronomy 202 Nicholson Hall, Tower Drive, Baton Rouge, Louisiana 70803-4001, USA 9Department of Physics, U.S. Naval Academy, Annapolis, Maryland 21402, USA 10Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA 11TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada 12Gaziosmanpasa University, Science Faculty, Turkey 13National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, Romania (Received 14 July 2012; published 12 November 2012)

Excited low-spin, nonyrast states in {sup 170,172,174}Hf were populated in {beta}{sup +}/{epsilon} decay and studied through off-beam {gamma}-ray spectroscopy. New coincidence data allowed for a substantial revision of the level schemes of {sup 170,172}Hf and a confirmation of the level scheme of {sup 174}Hf. The Hf isotopes represent a unique situation in which a crossing of collective intrinsic excitations occurs, enhancing significantly the effects of mixing. Using branching ratios from excited 2{sup +} states, this mixing is followed and studied. The resulting mixing matrix elements are found to be {approx}30 keV - an order of magnitude larger than estimated previously for nearby nuclei. In the case of {sup 170}Hf, the 2{sub {beta}}{sup +} and 2{sub {gamma}}{sup +} level are shown to be completely mixed.

The $g$ factor of the ${2}_{1}^{+}$ state of $^{170}\mathrm{Hf}$ was measured by perturbed \ensuremath{\gamma}-\ensuremath{\gamma} angular correlation in a static external magnetic field. The result, $g({2}_{1}^{+})=0.28$(5), extends the systematics of $g$ factors of even-even Hf isotopes to $N=98$ and enables a better test of theoretical models. The $g({2}_{1}^{+})$ experimental values of these isotopes exhibit a remarkable constancy as a function of neutron number. This phenomenon, which was also observed for other isotopic chains in the Gd--W range, is explained in terms of a recently proposed empirical model.

High-spin states of {sup 191,193}Au and {sup 192}Pt have been populated in the {sup 186}W({sup 11}B, xn) and {sup 186}W({sup 11}B, p4n) reactions, respectively, at a beam energy of 68 MeV and their {gamma} decay was studied using the YRAST Ball detector array at the Wright Nuclear Structure Laboratory at Yale University. The level scheme of {sup 193}Au has been extended up to I{sup {pi}}=55/2{sup +}. New transitions were observed also in {sup 191}Au and {sup 192}Pt. Particle-plus-Triaxial-Rotor (PTR) and Total Routhian Surface (TRS) calculations were performed to determine the equilibrium deformations of the Au isotopes. The predictions for oblate deformations in these nuclei are in agreement with the experimental data. Development of nonaxial shapes is discussed within the framework of the PTR model.

High-spin states of $^{187}\mathrm{Ir}$ have been populated in the $^{176}\mathrm{Yb}(^{15}\mathrm{N},4n)$ reaction and measured with the YRAST Ball spectrometer. The $\ensuremath{\pi}{h}_{9∕2}$ rotational band has been extended beyond the first alignment crossing, which was found at rotational frequency $\ensuremath{\hbar}{\ensuremath{\omega}}_{c}\ensuremath{\approx}0.39\phantom{\rule{0.3em}{0ex}}\text{MeV}$. Two different scenarios for describing this crossing are considered: the alignment of an ${h}_{9∕2}$ proton or ${i}_{13∕2}$ neutron pair and it is concluded that a proton band crossing is more likely. A systematic study of the rotational alignment crossings in the $\ensuremath{\pi}{h}_{9∕2}$ bands in the $N=104,106,108$ isotopes of $_{73}\mathrm{Ta},_{75}\mathrm{Re},_{77}\mathrm{Ir},$ and $_{79}\mathrm{Au}$ is presented.