Session CM: Nuclear Structure A>140

New levels and $\gamma$-rays in $^{164}$Gd

Levels in $^{164}$Gd were populated by two separate experiments: spontaneous fission of $^{252}$Cf and $\beta$-decay of $^{164}$Er. The $^{164}$Er was produced by uranium fission, and transported via tape collector through the LeRIBSS station at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The data were taken by four Clover HPGe $\gamma$-ray detectors, and two plastic scintillating $\beta$ detectors. This setup allows for $\gamma-\gamma$ and $\gamma-\gamma-\beta$ coincidence measurements. The other experiment consisted of placing a 62 $\mu$Ci source of $^{252}$Cf in the center of the GAMMASPHERE detector array for four weeks producing $5.7\times10^{11}$ $\gamma-\gamma-\gamma$ and higher coincidence events, with $1.9\times10^{11}$ of them being $\gamma-\gamma-\gamma-\gamma$ coincidence or higher. These two data sets were used to analyze the level scheme structure of $^{164}$Gd, uncovering new levels and $\gamma$'s.   

Lifetime Measurements in 170Er

The nature of excited 0⁺ bands in nuclei remains the topic of much debate in nuclear structure physics. Obtaining level lifetimes of well-deformed nuclei in the rare-earth region will help guide theoretical interpretations of these bands. This work studies ¹⁷⁰Er, which lacks lifetime data for low-lying 0⁺ states. The lifetimes of low-lying 0⁺ states in ¹⁷⁰Er below 3.4 MeV were determined using the Doppler-shift attenuation method (DSAM) at the University of Kentucky Accelerator Laboratory. Preliminary results of level lifetimes in ¹⁷⁰Er, including 0⁺ states, will be presented.   

Wobbling motion in A$\sim$190 region

The rare phenomenon of nuclear wobbling motion has already been established in the A$\sim$160 and the A$\sim$130 regions. Based on observations of significant triaxiality at low spins in the A$\sim$190 region, we have extended our investigation of wobbling motion to the $^{187}$Au nucleus. The experiment was performed using the Gammasphere array at the Argonne National Laboratory and the $^{174}$Yb($^{19}$F,6n)$^{187}$Au reaction was used to populate the levels of interest. A longitudinal wobbling band has been identified by establishing the characteristic $\Delta$I = 1, E2 nature of the $n_{w+1} \rightarrow n_{w}$ linking transitions. Calculations in the framework of the Particle Rotor Model (PRM) are found to be in good agreement with the experiment. This observation opens a new mass region where nuclear wobbling motion may be found.  

  

Search for the ultra-low energy isomer state of thorium-229 nuclei

The nuclear isomeric first excited state of Thorium-229 (^229mTh) has drawn attention from many fields. In general, nuclei are insensitive to ambient disturbance due to the shielding by their core electrons. In addition, the extraordinary-low energy level (7.8 eV) of ^229mTh can be accessed by laser spectroscopy techniques. Since its insensitiveness may enable us to achieve longer decoherence time in comparison to atomic states, it attracts much interest from atomic clock, fundamental physics, and other fields. However large energy uncertainty of 0.5 eV makes the laser spectroscopy difficult. In spite of much effort, a transition between the isomer state and the ground state has not been observed directly by optical methods.
The aim of our experiment is a determination of the energy of the isomeric transition precisely by nuclear resonant scattering (NRS) with synchrotron X-ray beam in SPring-8. We have succeeded in developing a fast X-ray detector system and a dense ²²⁹Th target which are the essential parts of the experiment. We will present a detail of the developed system and the current status of the experiment especially the latest beam-time result taken in April 2018.   

Angular momentum selectivity of the beta-Oslo method to measure gamma-ray strength and nuclear level density

Recently, the beta-Oslo Method was developed, a new application of the Oslo Method to nuclei formed by beta decay, allowing strength function and level density measurements in neutron-rich nuclei. However, in the case of Gamow-Teller beta decay being dominant, the angular-momentum range of states populated in the daughter nucleus is within 1$\hbar$, much more narrow than for charged-particle reactions used in the Oslo method.

To investigate the effect on the Oslo Method of this angular momentum selectivity, artificial nuclear level schemes for a range of neutron-rich strontium isotopes were generated by the Monte Carlo codes DICEBOX and RANIER. Gamma ray cascades were simulated from distributions of states with angular momentum that can be populated by beta decay and analyzed with the Oslo Method.  The strength functions and nuclear level densities extracted were then compared to both the known models used to generate the cascades and those parameters extracted from a broader initial angular momentum distribution.