Session CE: Nuclear Structure I

New Analysis of Levels in $^{153}$Pr

Previous levels assigned to $^{153}$Pr [1] have recently been called into question and different transitions have been assigned to $^{153}$Pr [2]. Recently prompt gamma-rays in coincidence with isotopically-identified fission fragments using VAMOS$++$ and EXOGAM, produced using $^{238}$U on a $^{9}$Be target, at an energy around the Coulomb barrier have been reported [3]. We have combined the $\gamma -\gamma -\gamma -\gamma $ data from $^{252}$Cf (SF) and those from the in beam mass and Z-gated spectra to assign transitions and levels in $^{153}$Pr. In contrast to the recent work [2], the transitions and levels previously assigned to $^{153}$Pr have been confirmed by the M-Z gated spectra. As described in our other abstract, the levels assigned to $^{153}$Pr [2] are now assigned to $^{151}$Pr. The new evidences for the mass assignment to the transitions will be presented.\\[4pt] [1] J. K. Hwang \textit{et al.}, Phys. Rev. C~\textbf{82}, 034308 (2010)\\[0pt] [2] T. Malkiewicz \textit{et al.}, Phys. Rev. C~\textbf{85}, 044314 (2012)\\[0pt] [3] A. Navin et al. (submitted); A. Navin, 5th Int. Conf. on ``\textbf{\textit{Fission and properties of neutron-rich nuclei, Sanibel 2012}},'' World Scientific, in press.   

An E5 transition in the $^{137}$Cs decay

The beta decay of $^{137}$Cs is mainly to the J$^{\pi}$=11/2$^{-}$ 661.66 keV isomeric excited state in $^{137}$Ba and is usually followed by emission of a single gamma ray as the nucleus relaxes to the J$^{\pi}$=3/2$^{+}$ ground state. It is a well-known standard $\gamma$-ray calibration reference. There is only one intermediate state, with J$^{\pi}$=1/2$^{+}$ at 283.50 keV. The $\gamma$-ray decay branch to this level has never been observed. The transition must be of E5 or M6 multipolarity. The phase space limitation hinders this decay and a $\sim$10$^{-8}$ branch can be anticipated from the few known E5 decay matrix elements. The use of the Gammasphere detector array at Argonne National Lab allows a search for these rare events by selection of an optimal detector opening angle for coincidences, chosen to minimize the effects of Compton cross-scattering in the array. In this manner the E5 cascade transition was observed and the branching ratio measured. Rigorous E5 transitions are only known in four other cases to date, so this measurement adds significantly to the body of knowledge surrounding E5 matrix elements.   

A search for 2-photon emission from the 662 keV state in $^{137}$Ba

Two photon decays from excited nuclear states provide an interesting test both of QED and nuclear structure. It has been extensively studied for cases where one photon decay is forbidden [1]. Two photon decay in direct competition with the first order process has never been convincingly demonstrated. The $^{137}$Ba case is particularly interesting as the decay has high multipolarity, M4, so the 2-photon process can have contributions from both quadrupole-quadrupole and dipole-octupole multipolarities. New calculations and new measurements with NaI(Tl) counters from Brookhaven suggest that the two photon branch is $\sim$ 2 x 10$^{-6}$. Gammasphere is the perfect tool for this investigation, having good energy resolution, high efficiency, broad coverage of angles, and sufficient granularity to minimize pile-up and count-rate difficulties. A very large data set was collected involving more than 10$^{11}$ decays. Evidence for the 2-photon branch has been found, despite the daunting background from Compton scattering. This work was supported by DOE contracts, DE-FG02-94ER40848, DE-AC02-06CH11357, DE-AC02-98CH10886 and DE-AC02-98CH10946.\\[4pt] [1] J. Kramp, et. al, Nucl. Phys. A474 (1987) 412   

Low-Spin TSD Bands in Even-Z nuclei: Hiding in Plane Sight?

Many TSD bands have been reported in the high-spin regime for Dy, Er, Yb, Hf, and W isotopes. However, the low-spin TSD bands, built off the ${\pi}i_{13/2}$ intruder orbital, have gone unreported in these even-Z nuclei, while appearing repeatedly in the nearby Lu, Ta, and Re isotopes. Recent data on $^{169}$W and $^{171}$W, obtained through the $^{118}$Sn($^{55}$Mn,$p3n$) and $^{120}$Sn($^{55}$Mn,$p3n$) reactions respectively, presents evidence for possible low-spin, 2-quasiproton bands, where one of the proton orbitals is suggested to be the ${\pi}i_{13/2}$ intruder. The properties of these two bands are compared to similar structures observed in $^{167}$Hf and $^{169}$Hf, and to ${\pi}i_{13/2}$ bands observed in nearby Ta and Re isotopes.   

Search for Collective Oblate Structures in $^{186}$W

Neutron-rich, A$\approx$180 nuclei exhibit distinctive characteristics that enable a rare transition from prolate to oblate collective rotation at high spins. Recent investigation of prompt rotational structures in $^{180}$Hf provided evidence for a rotational structure that can be associated with collective oblate rotation. Oblate shapes are predicted to become yrast at $\textit{I}\approx$14$\hbar$ in $^{186}$W as compared to $\textit{I}\ge$20$\hbar$ in $^{180}$Hf. Prompt rotational states in $^{186}$W were populated using 725 MeV and 800 MeV $^{136}$Xe beam energies from the ATLAS accelerator incident on a thin enriched $^{186}$W target. Coincident detection of binary reaction fragments and $\gamma$-rays was achieved using the recently upgraded Rochester 4$\pi$ heavy-ion detector array, CHICO2 in conjugation with Gammasphere. Analysis of the data is in progress and will be presented.   

Triaxial Deformation in $^{171}$Re?

Nuclei that display the wobbling mode (associated with the rotation of a triaxially-deformed nucleus) are centered around $N=94$. Currently, wobbling has only been observed in $Z=71$ (Lu) and $Z=73$ (Ta) isotopes, but the role of the proton Fermi surface is not well defined. In order to investigate this, high-spin states were populated in the $N=96$ nucleus $^{171}$Re through the $^{120}$Sn($^{55}$Mn,$4n$) reaction. The $\pi i_{13/2}$ sequence (on which all known wobbling bands have been based) was extended to much higher spin, but no evidence for wobbling was found. A possible explanation for the lack of wobbling will be discussed. In addition, evidence will be shown for a Landau-Zener crossing between two of the negative-parity sequences.   

Conversion electron measurements of $^{195}$Au using ICEBall for Nuclear Structure and Astrophysics at the University of Notre Dame

The Internal Conversion Electron Ball Array (ICEBall)\footnote{M.P. Metlay, J.X. Saladin, I.Y. Lee, and O. Dietzsch, Nucl. Instrum. Meth. A, 336, 162 (1993)} consists of six Si(Li) detectors and it was recently re-comissioned\footnote{A. Battaglia et al. to be submitted} at the University of Notre Dame Nuclear Science Laboratory for spectroscopic studies of heavy nuclei. For the commissioning experiment, a 16 MeV bunched proton beam was used from the FN Tandem for a (p,2n) reaction to populate low spin states of $^{195}$Au. Both conversion electrons and gamma-rays were detected in coincidence between ICEBall and a single high-purity germanium detector. A total of 14 conversion coeffcients were measured. The results will be presented and compared to previous results.\footnote{S. M. Fischer, Spectroscopic Studies of the Nucleus 195-Au, Ph.D. thesis, University of Notre Dame (1994).} This work was supported by the National Science Foundation under contract number NSF PHY-1068192.   

High-Spin Structures in the N$=$153 Nucleus $^{251}$Cf

In continuation of our exploration of band structures in neutron-rich Cf nuclei using inelastic and transfer reactions[1], we report new spectroscopic observations in the $^{251}$Cf nucleus. High-spin states of neutron-rich Cf nuclei were populated using a $^{208}$Pb beam from the ATLAS facility at Argonne, incident on a radioactive target mixture of $^{249,250,251}$Cf. Prompt $\gamma $ rays were detected by the Gammasphere array. Both signatures of the ground state band of $^{251}$Cf were observed for the first time, with enhanced signal-to-noise achieved through appropriate gates on sum energy and fold parameters. Assignment of the band structure to $^{251}$Cf is via coincidence with Cf X-rays as well as the excitation of the $^{208}$Pb beam partner. Configurations are assigned to the observed band from experimental M1/E2 branching ratios from clean decays within the band. Further data analysis is in progress, and the new results will be discussed in the context of physics of the highest neutron orbitals accessible to spectroscopy in the A$=$250 region. \\[4pt] [1] S. S. Hota, Ph.D. Thesis, U. Mass Lowell, 2012   

Investigation of 124Xe nuclear structure with the 8Pi spectrometer at TRIUMF-ISAC

The 124Xe nucleus has been thought to obey O(6) symmetry but a recent Coulomb excitation study has found that while O(5) may be preserved, O(6) appears to be badly broken [1]. To further characterize the structure of this nucleus, a beta-decay experiment was performed at the TRIUMF-ISAC facility. A beam of radioactive 124Cs at a rate of 9.8x10$^{7}$ ions/s was implanted at the center of the 8Pi spectrometer where it underwent $\beta +$/EC decay into stable 124Xe. High-statistics gamma-gamma coincidence measurements have been analyzed to add to the level scheme of 124Xe, which has been extended considerably. The high statistics data set has revealed a new decay branch from a 124Cs high-spin isomer as well as several very-weak transitions between low-spin states in 124Xe. Branching ratios and B(E2) transition strengths have been calculated for the updated level scheme. The results will be important in determining collective properties and nuclear structure of the 124Xe. \\[4pt] [1] G. Rainovski et al. Physics Letters B 683 (2010) 11