Radioactive ^{136}Te has two valence protons and two valence neutrons outside of the ^{132}Sn double shell closure, providing a simple laboratory for exploring the emergence of collectivity and nucleon-nucleon interactions. Coulomb excitation of ^{136}Te on a titanium target was utilized to determine an extensive set of electromagnetic moments for the three lowest-lying states, including B(E2;0_{1}^{+}→2_{1}^{+}), Q(2_{1}^{+}), and g(2_{1}^{+}). The results indicate that the first-excited state, 2_{1}^{+}, composed of the simple 2p⊕2n system, is prolate deformed, and its wave function is dominated by excited valence neutron configurations, but not to the extent previously suggested. It is demonstrated that extreme sensitivity of g(2_{1}^{+}) to the proton and neutron contributions to the wave function provides unique insight into the nature of emerging collectivity, and g(2_{1}^{+}) was used to differentiate among several state-of-the-art theoretical calculations. Our results are best described by the most recent shell model calculations.

The $g$ factors of the first-excited states of stable ${}^{124}$Sn and radioactive ${}^{126,128}$Sn were measured by the recoil-in-vacuum method with comparatively high precision. The experiments were performed at the Holifield Radioactive Ion Beam Facility by Coulomb exciting $\ensuremath{\sim}3$ MeV/u beams in inverse kinematics on carbon and titanium targets. The results for ${}^{124}$Sn and ${}^{126}$Sn are in excellent agreement with recent shell-model calculations. For ${}^{128}$Sn the experiment suggests an increase in the magnitude of $g({2}_{1}^{+})$, as predicted by some models. The present results provide a sensitive probe of the valence orbitals that contribute to the ${2}_{1}^{+}$ wave functions as the double-shell closure at ${}^{132}$Sn is approached.

New g-factor measurements by the recoil in vacuum method are discussed. The stable even tin isotopes, neutron-rich Sn-126,Sn-128, and tellurium isotopes including neutron-rich semimagic Te-134, have been studied. The experiments were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) by Coulomb exciting similar to 3 MeV/u beams in inverse kinematics on C and Ti targets, and using the CLARION+HyBall arrays to observe the perturbed particle-angular correlations. The measurements, including those on the neutron-rich radioactive beams, have sufficient precision to distinguish between alternative models.

The Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) is a new generation high-resolution γ-ray spectrometer consisting of electrically segmented high-purity germanium crystals. GRETINA is capable of reconstructing the energy and position of each γ-ray interaction point inside the crystal with high resolution. This enables γ-ray energy tracking which in turn provides an array with large photopeak efficiency, high resolution and good peak-to-total ratio. GRETINA is used for nuclear structure studies with demanding γ-ray detection requirements and it is suitable for experiments with radioactive-ion beams with high recoil velocities. The GRETINA array has a 1π solid angle coverage and constitutes the first stage towards the full 4π array GRETA. We present in this paper the main parts and the performance of the GRETINA system.

Corrigendum to ‘‘Effect of a surface channel on the performance of a P-type Point Contact HPGe detector’’ [Nuclear Instruments and Methods in Physics Research A 680 (2012) 48–55] R.J. Cooper , D.C. Radford , E. Hull , K. Lagergren , P. Mullowney , M.-C. Lin , K. Paul , C. Ahrens , M. Loh d a Joint Institute for Heavy Ion Research, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6371, USA b Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6371, USA c PHDs Co., 3011 Amherst Road, Knoxville, TN 37921, USA d Tech-X Corporation, 5621 Araphoe Avenue, Suite A, Boulder, CO 80303, USA

The $g$ factors of the first-excited states of stable ${}^{124}$Sn and radioactive ${}^{126,128}$Sn were measured by the recoil-in-vacuum method with comparatively high precision. The experiments were performed at the Holifield Radioactive Ion Beam Facility by Coulomb exciting $\ensuremath{\sim}3$ MeV/u beams in inverse kinematics on carbon and titanium targets. The results for ${}^{124}$Sn and ${}^{126}$Sn are in excellent agreement with recent shell-model calculations. For ${}^{128}$Sn the experiment suggests an increase in the magnitude of $g({2}_{1}^{+})$, as predicted by some models. The present results provide a sensitive probe of the valence orbitals that contribute to the ${2}_{1}^{+}$ wave functions as the double-shell closure at ${}^{132}$Sn is approached.

A novel, large-volume High Purity Germanium (HPGe) detector that will provide gamma-ray detection with both sub-mm position resolution and high efficiency is under development. This design is based on a coaxial HPGe geometry and employs a point contact along with segmentation of the outer electrode. Calculations indicate that this device will be capable of achieving values of position sensitivity which are approximately a factor of four or five greater than conventional gamma-ray tracking detectors.

Additional information is reported on single-neutron states above the doubly closed-shell nucleus Sn-132. A radioactive ion beam of Te-134(N = 82) at 565 MeV and a stable ion beam of Xe-136(N = 82) at 560 MeV were used to study single-neutron states in the N = 83 nuclei Te-135 and Xe-137, respectively, by (C-13, C-12 gamma) and (Be-9, Be-8 gamma) direct reactions in inverse kinematics. Particle-gamma and particle-gamma-gamma coincidence measurements using CsI and HPGe arrays allowed determination of decay paths, high-precision level energies, multipolarities of transitions, and relative cross sections. One-neutron transfer with heavy ions is employed to gain selectivity to both low- and high-spin single-neutron states above the N = 82 shell closure. Results are presented for the 13/2(1)(+) states in the N = 83 nuclei Te-135 and Xe-137 at 2108.8(9) keV and 1752.6(3) keV, respectively, and for the 3(1)(-) collective octupole state observed at 3749(5) keV in Te-134(N = 82) inelastic scattering, all previously unknown. While the 13/2(1)(+) state (or nu 1i(13/2) centroid) in Sn-133(Z = 50, N = 83) remains unknown, the present results provide the best empirical prediction of its energy available to date.

Evaporation residue cross sections for Sn-124,Sn-126,Sn-127,Sn-128 + Ni-64 and Sn-132 + Ni-58 have been measured to study the effects of neutron excess in neutron-rich radioactive nuclei on the fusion probability. A comparison of the reduced evaporation residue cross sections for Sn-126 + Ni-64 and Sn-132 + Ni-58, which make the same compound nucleus, shows that the fusion probability is indistinguishable for reactions involving the same atomic elements, Sn and Ni, but different isotope combinations. For the reactions with Ni-64, the fusion probability does not decrease with increasing neutron excess in Sn, contrary to the result of the stable beam Sn + Zr measurement.

The observation of anomalous charge collection in a P-type Point Contact (PPC) High Purity Germanium (HPGe) detector is reported. By studying preamplifier charge signals from the detector, these anomalous events are shown to be consistent with charge carrier drift along the surface of the detector, prior to collection at the contact. It is believed that these events arise from the formation of a surface channel in the detector. By coupling analysis of these experimental signals with field calculations, an estimate of the radial drift velocity of holes at the surface has been made. This value is found to be around 40 times slower than the accepted value for saturated hole drift in the bulk, and is in good agreement with that calculated using a newly developed technique for modelling carrier transport.

High-spin states in 187 Pt have been studied by means of γ-ray spectroscopy techniques. Known bands have been significantly extended and new bands have been found. The band structures are discussed in the framework of the cranking model and negative-parity states are compared with calculations performed with a semi-microscopic axial-rotor plus one-quasiparticle coupling model. Shape coexistence is observed from low excitation energy.

The fusion excitation functions for radioactive (132)Sn + (58)Ni and stable (130)Te + (58,64)Ni were measured at energies near the Coulomb barrier. The coupling of transfer channels in heavy-ion fusion was examined through a comparison of Sn + Ni and Te + Ni systems, which have large variations in the number of positive Q-value nucleon transfer channels. In contrast with previous experimental comparisons, where increased sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels, the reduced excitation functions were equivalent for the different Sn + Ni and Te + Ni systems. The present results suggest a dramatically different influence of positive Q-value transfer channels on the fusion process for the Sn + Ni and Te + Ni systems.

Submitted for the DNP11 Meeting of The American Physical Society The role of transfer couplings in the fusion of Sn+Ni and Te+Ni systems Z. KOHLEY, J.F. LIANG, D. SHAPIRA, R.L. VARNER, C.J. GROSS, J.M. ALLMOND, Oak Ridge National Laboratory, A.L. CARALEY, State Univ. of New York at Oswego, E.A. COELLO, F. FAVELA, Universidad Nacional Autonoma de Mexico, K. LAGERGREN, P.E. MUELLER, Oak Ridge National Laboratory — Evaporation residue and fission cross sections have been measured for the radioactive 132Sn+58Ni and stable 130Te+58,64Ni systems at energies near the Coulomb barrier. Through a comparison with previous Sn+Ni measurements, the role of transfer couplings on the heavy-ion fusion has been examined. While the number of positive Q-value neutron transfer channels varied widely between the different Sn+Ni and Te+Ni systems, the reduced excitation functions were equivalent. This is in contrast to a number of previous studies where large enhancements in the sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels. The present results suggest a significant change in the influence of transfer couplings on the fusion process for the Sn+Ni and Te+Ni systems. This work was supported by DOE Office of Nuclear Physics Zachary Kohley Oak Ridge National Laboratory Date submitted: 28 Jun 2011 Electronic form version 1.4

Several strongly coupled bands in the neutron-deficient nucleus Ta-161 have been identified and quasiparticle configuration assignments have been made on the basis of rotational alignments and cranked shell model calculations. The level scheme elucidated for Ta-161 highlights the competition between the.(h(9/2)) and.(i(13/2)) orbitals to form the yrast spectrum. The band structures in Ta-161 also provide new insights into the structural features of other heavy odd-A nuclei populated with much lower reaction cross sections in this region at the proton drip line.

Excited states in the neutron-deficient odd-Z nuclide Ta-161 were identified for the first time using the Cd-106(Ni-58,3p gamma) reaction at a beam energy of 270 MeV. The pi h(11/2) band, yrast at low spin, was observed up to (47/2(-)) and a further four strongly coupled bands have been established to high spin. Quasiparticle configuration assignments for the new band structures have been made on the basis of cranked shell model calculations. This work suggests that the negative-parity nu(f(7/2), h(9/2)) orbitals are responsible for the first rotational alignment in the pi h(11/2) band.

Excited states in the neutron-deficient odd-$Z$ nuclide $^{161}\mathrm{Ta}$ were identified for the first time using the $^{106}\mathrm{Cd}$($^{58}\mathrm{Ni}$,3$p$$\ensuremath{\gamma}$) reaction at a beam energy of 270 MeV. The $\ensuremath{\pi}{h}_{11/2}$ band, yrast at low spin, was observed up to $(47/{2}^{\ensuremath{-}})$ and a further four strongly coupled bands have been established to high spin. Quasiparticle configuration assignments for the new band structures have been made on the basis of cranked shell model calculations. This work suggests that the negative-parity $\ensuremath{\nu}({f}_{7/2},{h}_{9/2})$ orbitals are responsible for the first rotational alignment in the $\ensuremath{\pi}{h}_{11/2}$ band.

High-precision measurements of matrix elements from the Coulomb excitation of 124,126,128Sn(Z = 50) impinging on a 12C target are presented. The matrix elements and related B(E2) values decrease monotonically as the N = 82 shell closure is approached from N = 74 to 78, despite a near constancy in the first 2+ level energy, E(2_1+). Furthermore, results are presented for the Coulomb excitation of 124,126,128Sn using an enriched 50Ti target, which, combined with the results from the 12C target, provide a measure of the matrix elements and related static quadrupole moments, Q(2_1+) (expected to be ~0 for a spherical shape). These new results indicate that the Sn isotopes have a deformation consistent with zero. The present study marks the first report on measured 2_1+ static quadrupole moments for the unstable Sn isotopes.

In nuclei with equal neutron (N) and proton (Z) numbers, the observed enhanced neutron–proton correlations are predicted to favour isoscalar neutron–proton pairing, an unusual interaction that is distinct from normal nuclear superfluidity. Now, in a major experiment at the GANIL heavy-ion accelerator in France, observations of excited states of the N = Z = 46 palladium nucleus 92Pd provide evidence for spin-aligned isoscalar neutron–proton pairing that is rather different to that predicted, and which has not been seen previously in nuclei. Nuclei with equal neutron (N) and proton (Z) numbers show enhanced correlations that have been predicted to favour an unusual type of pairing, distinct from normal nuclear superfluidity. Here, technically challenging observations are reported of excited states in the N = Z = 46 nucleus 92Pd, from which evidence is inferred for a type of spin-aligned structure in the ground and low-lying excited states, not established in nuclei before and differing from previous predictions. Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work1 that introduced a strong spin–orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron–proton pairing2,3,4,5,6, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus 92Pd. Gamma rays emitted following the 58Ni(36Ar,2n)92Pd fusion–evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron–proton coupling scheme, different from the previous prediction2,3,4,5,6. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling7,8) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron–proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.