Session KH: Nuclear Structure III

Cross Section Measurements of the $^{76}$Ge$(n, n'\gamma)$ Reaction

Neutrinoless double-beta decay (0$\nu\beta\beta$) is a topic of great current interest and, as such, is the focus of several experiments and international collaborations. Two of these experiments, Majorana and GERDA, are seeking evidence of 0$\nu\beta\beta$ in the decay of $^{76}$Ge, where the signal would appear as a sharp peak in the energy spectrum at the Q-value of the reaction plus a small amount of recoil energy, or 2039 keV. Due to the high sensitivity of such a measurement, knowledge of background lines is critical. A study of $^{76}$Ga $\beta^{-}$ decay into $^{76}$Ge [1] revealed a 2040.70(25)-keV transition from the 3951.70(14)-keV level, which, if populated, could potentially be a background line of concern. In addition to $\beta^{-}$ decay from $^{76}$Ga, a potential population mechanism could be cosmic-ray-induced inelastic neutron scattering. Measurements of the neutron-induced cross section of the 3951.70-keV level have been performed utilizing the $^{76}$Ge$(n, n'\gamma)$ reaction at the University of Kentucky at neutron energies ranging from 4.3 to 4.9 MeV.\\[4pt] [1] D.C. Camp and B.P. Foster, Nucl. Phys. \textbf{A}177 (1971) 401-417.   

Investigation of $^{110}$Cd with the $(n,n^{\prime}\gamma)$ reaction

Excited states in $^{110}$Cd have been investigated with the $(n,n^{\prime}\gamma)$ reaction using monoenergetic neutrons. Excitation functions, using neutron energies in the range of 2 MeV to 3.4 MeV were collected, and $\gamma$-ray angular distributions at neutron energies of 2.6, 2.9, and 3.4 MeV were performed. The $(n,n^{\prime}\gamma)$ results were complemented with $\gamma\gamma$ coincidences from a $^{110}$In $\beta$-decay experiment (see contribution by A. Diaz Varela) performed at TRIUMF. This has resulted in a comprehensive level scheme for $^{110}$Cd below $\approx 3$ MeV, including stringent limits on unobserved transitions. Level lifetimes were extracted using the Doppler shift attenuation technique. Comparison of the results with simplified model expectations suggests that the low-lying structure of $^{110}$Cd is that of a deformed $\gamma$-soft rotor, rather than a spherical vibrator.   

Collective quadrupole behavior in $^{106}_{46}$Pd: deficit of E2 strength of the three-phonon levels

The low-lying excited states in $^{106}$Pd exhibit a structure that resembles a 3-phonon quituplet, thus making $^{106}$Pd an excellent candidate for a ``good quadrupole vibrator.'' To examine this possibility, excited states in $^{106}$Pd were investigated using the (n,n$^{\prime}\gamma$) reaction at the University of Kentucky. Level lifetimes, spins, transition multipolarities, and multipole mixing ratios were determined. The feeding to the proposed two-phonon triplet of states $J^{\pi}$(E$_{x}$,keV) = 4$^{+}$(1229), 2$^{+}$(1128) and 0$^{+}$(1134) was studied for states up to $\sim$3~MeV, and observed $E$2 decay strength sums were \textless\ 50\% of that expected for low-energy quadrupole vibrational collective behavior. This deficiency of strength cannot be explained by considering the fragmentation of the three-phonon states. {\it This material is based upon work supported by the U.S. NSF under Grant No. PHY-0956310.}   

Study of low-spin states in $^{154}$Gd via the $(p,p^{\prime}\gamma)$ reaction

Located in a region where nuclear shapes change rapidly, nuclei at $N=90$ have long been a challenge for nuclear structure models. Competing ideas involve shape coexistence, a critical point of a shape phase transition, and tetrahedral symmetries. For example, it has been predicted that $^{154}$Gd is a tetrahedral double-magic nucleus. Despite previous studies of $^{154}$Gd, there remains no definite explanation for its structure. Much discriminatory information between nuclear models comes from low-spin non-yrast states, so probing states below the pairing gap and their properties may lead to a clearer interpretation of the structure of $^{154}$Gd. An ideal reaction to achieve this is the $(p,p^{\prime}\gamma)$ reaction. We performed the $^{154}$Gd($p,p^{\prime}\gamma$) reaction at the laboratory of the University of Jyv\"{a}skyl\"{a} using 12 MeV protons and the JUROGAM II and LISA spectrometers. Preliminary results and analysis will be presented.   

Lifetime Measurements of Levels in $^{160}$Gd

The rare earth region of nuclei has been well established as a region of deformation for decades. However, the nature of vibrations built on a deformed ground state remain far from understood and present an oustanding challenge to nuclear structure physics. Studies of $^{158}$Gd has shown a preponderance of excited 0$^{+}$ states with varying degrees of collectivity. We have measured level lifetimes, reduced transition probabilities and angular distributions of gamma-rays excited by inelastic neutron scattering and the use of the Doppler Shift Attenuation Method (DSAM) at the University of Kentucky 7 MV Van de Graaff Accelerator Facility. Low lying excited states of $^{160}$Gd were populated up to an excitation energy of E $<$ 2 MeV. We will present and discuss the measured level lifetimes of $^{160}$Gd and their implied degrees of collectivity. This work was supported by the NSF under contract numbers PHY-1068192, PHY-12-05412, and PHY-0956310.   

Search for one- and two-phonon octupole vibrational states in the spherical nuclei near $^{132}$Sn

Excited high spin states in $^{135}$I, $^{136}$Xe, $^{137}$Cs, $^{138}$Ba, $^{139}$La, $^{140}$Ce and $^{142}$Nd with N$=$82 are reorganized and interpreted in a different way to find one- phonon octupole vibrational (POV) bands. Two nearly identical (similar) bands with $\Delta $I$=$3 are found in these nuclei. From the presence of two nearly identical excited bands with $\Delta $I$=$3 in these nuclei, one-POV bands are proposed. Also, high spin states of $^{134}$Sb, $^{134,135}$Te, $^{135,136}$I, $^{137}$Xe and $^{139}$Ba near $^{132}$Sn are reanalyzed in order to search for one- and two-POV states. New spins and parities are tentatively assigned to the 2203.9 keV state in $^{137}$Xe and the 1976.6 and 2091.7 keV states in $^{139}$Ba from the state energy plots of the $N =$ 82 and 83 nuclei. High spin states of $^{134}$Sb, $^{134,135}$Te, $^{135,136}$I, $^{137}$Xe and $^{139}$Ba connected by E1, E3$/$M2 and E3 transitions are proposed, for the first time, as zero-, one- and two-POV states. One- and two-POV states in $^{134}$Sb and $^{135}$Te are built on a 7$^{-}$ ($\pi $g$_{7/2}\nu $f$_{7/2})$ state and a 19$/$2$^{-}$ ($\nu $f$_{7/2}\otimes $6$^{+}_{1})$ state, respectively. One-POV states built on the 19$/$2$^{-}(\nu $f$_{7/2}\otimes $6$^{+}_{1}$ ) and the 21$/$2$^{-}$ ($\nu $h$_{9/2} \quad \otimes $6$^{+}_{2}$ ) states coexist in $^{137}$Xe. Then, one- and two-POV states in $^{139}$Ba are built only on the 21$/$2$^{-}$ ($\nu $h$_{9/2}\otimes $6$^{+}_{2})$ state. One- and two-POV states in $^{134}$Te are built on the 6$^{+}_{2}$ state with some mixing with the 6$^{+}_{1}$ state.   

Level Lifetimes in $^{132,134}$Xe from Inelastic Neutron Scattering

The stable isotopes of xenon span a region which exhibits an evolution from spherical to gamma-soft behavior; thus the structure of these nuclei may provide insight into the nature of this transition. Highly enriched ($>$99.9\%) $^{132}$Xe and $^{134}$Xe gases were converted to solid $^{132}$XeF$_2$ and $^{134}$XeF$_2$, which were used as scattering samples for inelastic neutron scattering measurements at the University of Kentucky Accelerator Laboratory. Lifetimes of levels up to 3.5 MeV in excitation energy in $^{132}$Xe and $^{134}$Xe were determined using the Doppler-shift attenuation method. New transitions and levels have been observed and reduced transition probabilities have also been determined. This new information will be examined in an effort to elucidate the structure of these two transitional nuclei.   

Probing the structure of $^{130}$Xe and $^{136}$Xe with inelastic neutron scattering

Xenon isotopes in the vicinity of A = 130 provide an opportunity to study the transition in nuclear character from $\gamma$ soft to vibrational. Although these even-mass Xe isotopes are stable, they prove difficult to investigate directly due to their gaseous nature and thus remain relatively lightly studied. To shed new light on the low-lying structure of these isotopes, experiments have been performed at the University of Kentucky where samples of \textit{solid} $^{130}$XeF$_{2}$ and $^{136}$XeF$_{2}$ were excited via the (n,n'$\gamma$) reaction. Excitation function measurements (between 1.8 and 3.3 MeV) allow the placement of new levels and transitions and provide information about the J$^{\pi}$ of levels. Gamma-ray angular distribution data allow the determination of transition multipolarities, $\gamma$-ray branching ratios, and level lifetimes (using DSAM techniques). We will present new information concerning the low-spin structure of these transitional nuclei. This material is based upon work supported by the U.S. National Science Foundation under grant no. PHY-0956310.   

High-resolution study of excited $0^+$ states in mercury isotopes

In recent years, much effort was invested in systematic studies of low-lying $0^+$ excitations in medium- to heavy-mass nuclei, ranging from $^{152}$Gd to $^{194}$Pt. This region is particularly interesting, as the structure of these nuclei changes from transitional nuclei in the Gd region, over well-deformed nuclei in the Yb region, to $\gamma$-soft nuclei in the Pt region. Recently, we moved further towards the $^{208}$Pb proton-neutron shell closure by investigating $0^+$ excitations in $^{198}$Hg, $^{200}$Hg, and $^{202}$Hg at the Q3D magnetic spectrograph in Munich. This allows us to test if the $0^+$ density can be used as a signature for the prolate-oblate shape-phase transition in the Hf-Hg region. We present and discuss the results of our high-resolution study on excited $0^+$ states in the mercury isotopes.