No abstract available

[en] A γ -ray spectroscopic measurement of the N=Z nucleus ⁶⁸Se has been made following the ¹²C( ⁵⁸Ni, 2 n) reaction at 185 and 220 MeV using Gammasphere and the Argonne Fragment Mass Analyzer. Despite a very low production cross section of 200(50) μb , two distinct rotational bands were found; the ground state band consistent with oblate collective rotation, and an excited band consistent with prolate rotation. These observations support long-standing predictions that nuclear ground states with substantial oblate (β₂∼-0.3) deformation should exist in this region. (c) 2000 The American Physical Society

[en] The ground state rotational bands of the N=Z nuclei ⁷²Kr , ⁷⁶Sr , and ⁸⁰Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. ⁷²Kr shows the most marked effect. It has been widely suggested that these ''delayed alignments'' arise from np -pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation

[en] Isomer decays, β-γ and electron-γ spectroscopy, α-γ fine structure studies, and Coulomb excitation are cases of nuclear structure experiments where the ideal detector is a compact, highly segmented, very efficient germanium (Ge) detector box. In order to develop such a structure, we are working on the R and D of large, segmented, High Purity planar Ge strip detectors (HPGeDSSD) which form the walls of such a box. We have developed a 92mm x 92mm x 20mm HPGeDSSD, which has 16 x 16 orthogonal strips of 5mm width. We are in the process of designing a focal plane detector for the Argonne Fragment Mass Analyzer (FMA) which consists of a 5 sided box, each side having a HPGeDSSD backed by a large segmented clover HPGe detector. MCNP simulations indicate this detector would have an efficiency of ∼60% for 122 keV gamma rays and ∼15% for 1.33 MeV radiation, which is ideal for studying the decays of nuclei far from stability that are usually produced with very low cross sections

[en] The excitation energies and spins of the yrast superdeformed band in ¹⁹¹Hg have been determined by analyzing the quasicontinuum spectrum connecting the superdeformed and normal-deformed states. The results from this analysis, combined with that given by one-step decay lines, give confident assignments of the spins and energies of the yrast superdeformed band in ¹⁹¹Hg

[en] Using the GAMMASPHERE array in conjunction with the Fragment Mass Analyzer, the level structure of the near drip-line nucleus ¹⁷⁸Hg has been considerably expanded with the recoil-decay tagging technique. Of particular interest is a new rotational band which exhibits a complex decay towards the low spin states arising from both the prolate-deformed and the nearly spherical coexisting minima. It is proposed that this band is associated at low spin with an octupole vibration which is crossed at moderate frequency by a shape driving, two quasiproton excitation. (c) 1999 The American Physical Society

[en] Charge symmetry breaking effects that perturb analog symmetry between nuclei are usually small but are important in extracting reliable Fermi matrix elements from ''superallowed'' β decays and testing conserved vector current theory, especially for the heavier cases. We have used the ⁴⁰Ca(³⁶Ar, pn)⁷⁴Rb and ⁴⁰Ca(⁴⁰Ca,αpn)⁷⁴Rb reactions at 108, 123 and 160 MeV, respectively, to populate ⁷⁴Rb and determine the analog distortion through comparison of T=1 states in ⁷⁴Rb with their corresponding ⁷⁴Kr levels. We have traced the analogs of the ⁷⁴Kr ground-state band in ⁷⁴Rb to a candidate spin J=8 state and determined the Coulomb energy differences. They are small and positive and increase smoothly with spin. New T=0 states were found that better delineate the deformed band structure and clarify the steps in deexcitation from high spin. A new T=0 band was found. No evidence was found for γ decay to or from a low-lying J^π=0⁺ state in ⁷⁴Rb despite a careful search

[en] We have produced a secondary radioactive beam of ⁷⁸Rb and Coulomb re-excited it. The beam was produced in the reaction ²⁴Mg(⁵⁸Ni,3pn)⁷⁸Rb at a beam energy of 260 MeV, using the ANL ATLAS accelerator. The residues of interest were separated from other reaction products and non-interacting beam using the Fragment Mass Analyzer (FMA). The beam leaving the FMA was ⁷⁸Kr and ⁷⁸Rb^gs,m1,m2, which was refocused onto a ⁵⁸Ni secondary target. We have extracted a spectrum of γ-rays associated with re-excitation of A=78 isobars. The re-excitation of stable ⁷⁸Kr was observed, which serves as a reference. γ-rays associated with excitation of ⁷⁸Rb^gs,m1,m2 were also seen. The measured yields indicate that all the ⁷⁸Rb states are highly deformed

[en] ⁸³Nb has been studied in order to locate the upper extent of large deformation in the N∼Z∼40 region and ascertain its sign and magnitude. The decay scheme has been considerably extended by studying gamma decay following the ⁵⁸Ni(²⁸Si, p2n)⁸³Nb reaction at 204 and 215 MeV and using Gammasphere. Both signature partners of two rotational bands have been observed. The band properties support a I^π=(5/2)⁺ ground state spin assignment. The in-band branching ratios and multipole mixing ratios were found to be consistent with significant prolate deformation, as found in other N=42 isotones. The bands are particularly similar to the isotone ⁸¹Y. A projected shell model analysis has been performed which provides good reproduction of the features of both bands. The calculations indicate the nucleus to be nearly axially symmetric. ^82,83Nb may be the heaviest nuclei in the A∼80 region with large deformation. The deformation is predicted to fall in heavier systems approaching ¹⁰⁰Sn. The conditions appear to be right for the occurrence of K isomers

[en] The fusion-evaporation reaction involving a 175 MeV ³⁶S beam and a ¹²⁴Sn target was performed, and the emitted γ rays were observed with the Gammasphere spectrometer. Significant additions to the level scheme of ¹⁵²Gd were made in spite of the relative weakness of the α4n exit channel, being only ∼2% of the total fusion cross-section. The high-spin behavior of ¹⁵²Gd was compared with that of other N=88 nuclei. A striking similarity was observed with ¹⁵⁴Dy and it is therefore suggested that the angular-momentum-induced shape changes that take place in ¹⁵⁴Dy also occur in ¹⁵²Gd in the 30-40h spin range. This is supported by Cranked Nilsson-Strutinsky calculations which were used to interpret the high-spin bands. It is found that a better agreement between calculation and experiment is obtained if the Z=64 shell gap increases with a decreasing number of valence particles outside the doubly-closed ₆₄¹⁴⁶Gd₈₂ nucleus