Evidence For Triaxiality Research Articles

Shape coexistence and the role of axial asymmetry in 72Ge

The quadrupole collectivity of low-lying states and the anomalous behavior of the $0^+_2$ and $2^+_3$ levels in $^{72}$Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2. A total of forty six $E2$ and $M1$ matrix elements connecting fourteen low-lying levels were determined using the least-squares search code, gosia. Evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, is presented. These are interpreted using a simple two-state mixing model as well as multistate mixing calculations carried out within the framework of the triaxial rotor model. The results represent a significant milestone towards the understanding of the unusual structure of this nucleus.

New level schemes with high-spin states ofTc105,107,109

New level schemes of odd-$Z$ $^{105,107,109}\mathrm{Tc}$ are proposed based on the $^{252}\mathrm{Cf}$ spontaneous-fission-gamma data taken with Gammasphere in 2000. Bands of levels are considerably extended and expanded to show rich spectroscopic information. Spin/parity and configuration assignments are made based on determinations of multipolarities of low-lying transitions and the level analogies to the previously reported levels, and to those of the neighboring $\mathrm{Rh}$ isotopes. A non-yrast negative-parity band built on the $3∕{2}^{\ensuremath{-}}[301]$ orbital is observed for the first time in $^{105}\mathrm{Tc}$. A positive-parity band built on the $1∕{2}^{+}[431]$ intruder orbital originating from the $\ensuremath{\pi}({g}_{7∕2}∕{d}_{5∕2})$ subshells and having a strong deformation-driving effect is observed for the first time in $^{105}\mathrm{Tc}$, and assigned in $^{107}\mathrm{Tc}$. A positive-parity band built on the excited $11∕{2}^{+}$ level, which has rather low excitation energy and predominantly decays into the $9∕{2}^{+}$ level of the ground state band, provides evidence of triaxiality in $^{107,109}\mathrm{Tc}$, and probably also in $^{105}\mathrm{Tc}$. Rotational constants are calculated and discussed for the $K=1∕2$ intruder bands using the Bohr-Mottelson formula. Level systematics are discussed in terms of the locations of proton Fermi levels and deformations. The band crossings of yrast positive-parity bands are observed, most likely related to ${h}_{11∕2}$ neutron alignment. Triaxial-rotor-plus-particle model calculations performed with $\ensuremath{\epsilon}=0.32$ and $\ensuremath{\gamma}=\ensuremath{-}22.5\ifmmode^\circ\else\textdegree\fi{}$ on the prolate side of maximum triaxiality yielded the best reproduction of the excitation energies, signature splittings, and branching ratios of the positive-parity bands (except for the intruder bands) of these $\mathrm{Tc}$ isotopes. The significant discrepancies between the triaxial-rotor-plus-particle model calculations and experiment for the $K=1∕2$ intruder bands in $^{105,107}\mathrm{Tc}$ need further theoretical studies.

Dynamics of the Galactic Bulge

Recent work on the intrinsic shape, the internal kinematics of stars and gas, and the dynamics of the Galactic Bulge is discussed. Starcounts, measurements of the integrated light and the kinematics of the atomic and molecular gas all provide strong evidence that the Bulge is triaxial, and is rotating fairly rapidly. To date, there is little evidence for triaxiality in the stellar kinematics: the available stellar velocities are consistent with Kent's (1992) oblate model. This unsatisfactory situation is expected to improve rapidly.

Nuclear spectroscopy ofOs189

Forty-three states have been observed up to an energy of 1474 keV in $^{189}\mathrm{Os}$ using the reactions $^{188}\mathrm{Os}(d,p)^{189}\mathrm{Os}$ and $^{190}\mathrm{Os}(d,t)^{189}\mathrm{Os}$, produced with 12 MeV deuterons, and the $^{188}\mathrm{Os}(n,\ensuremath{\gamma})^{189}\mathrm{Os}$ reaction using thermal neutrons and observing $\ensuremath{\gamma}$ rays from 36 to 5933 keV. The neutron separation energy of $^{189}\mathrm{Os}$ was determined to be 5920.8 \ifmmode\pm\else\textpm\fi{} 2.0 keV in agreement with the value of the ($d,p$) reaction. The $Q$ value for the $^{190}\mathrm{Os}(d,t)^{180}\mathrm{Os}$ reaction was measured to be -1530 \ifmmode\pm\else\textpm\fi{} 4 keV. Using $l$-transfer results, spin-parities were assigned to 14 states. A number of states up to 818 keV were qualiatively interpreted in terms of the Nilsson model. Anomalous Nilsson systematics particularly of the 1/2-[510] and 3/2-[512] bands involving anomalously large ($d,p$) and ($d,t$) cross sections populating the 5/2- and 3/2- states at 69.6 and 95.3 keV, respectively, are interpreted in terms of decreasing deformation and the consequent increase in Coriolis coupling. The low lying 5/2- state at 276 keV is interpreted as the lowest rotational state built on the 9/2-[505] level at 30.8 keV. This implies triaxiality for this configuration.NUCLEAR REACTIONS $^{188}\mathrm{Os}(d,p)$, $^{190}\mathrm{Os}(d,t)$, $E=12.0$ MeV, measured $\ensuremath{\sigma}(\ensuremath{\theta})$, DWBA analysis; $^{188}\mathrm{Os}({n}_{\mathrm{th}},\ensuremath{\gamma})$; deduced energies, $l$, $j\ensuremath{\pi}$ of $^{189}\mathrm{Os}$ levels. NUCLEAR STRUCTURE Nilsson model applied to $^{189}\mathrm{Os}$; evidence for triaxiality.