Session KE: Nuclear Structure A~70-100

Excited State Properties in Neutron-rich Nuclei near N = 40

The neutron-rich nuclei near N = 40 have recently been the focus of many experimental and theoretical efforts. In this region, the competing energy cost for promoting pairs of nucleons across either Z = 28 or N = 40 and the energy gain from residual nucleon-nucleon interactions gives rise to several low-energy 0$^{+}$ states and is a hallmark of shape coexistence. Low-energy 0$^{+}$ states have been observed in $^{68}$Ni, and predicted for other nuclei in the region. Recent theoretical calculations are able to reproduce the energies of known states in $^{68}$Ni and stress the importance of the tensor component of the monopole interaction. Yet, while energies of the levels are a useful comparison, a more stringent test is the reproduction of level lifetimes, where the predicted half-lives can vary by several orders of magnitude depending on the interaction. To further benchmark theoretical calculations in this region, a setup designed to measure level lifetimes has been constructed. A description of the array and preliminary results will be presented.   

Shape Coexistence in Neutron-Rich Nickel Isotopes around $N =$ 40

Shape coexistence is a fascinating phenomenon in atomic nuclei characterized by multiple states with different intrinsic shapes coexisting at similar excitation energies. In even-even nuclei, a hallmark of shape coexistence is low-energy 0$^{+}$ states. In $^{68}$Ni, the Monte-Carlo Shell Model (MCSM) employing the A3DA interaction, utilizing the \textit{fpg}$_{9/2}d_{5/2} $model space for protons and neutrons, predicts triple shape coexistence with three 0$^{+}$ states below 3 MeV. Transitioning to $^{70}$Ni, the energy of the prolate-deformed 0$^{+}$ state is predicted to drop precipitously from 2511 to 1525 keV. This is due to strengthening of the attractive $\nu g_{9/2}-\pi f_{5/2}$ and repulsive $\nu g_{9/2}-\pi f_{7/2}$ monopole interactions of the tensor force altering the effective single-particle energies of the $\pi f_{7/2}$ and $\pi f_{5/2}$ single-particle states, thereby reducing the spherical $Z =$ 28 shell gap. Recent beta-decay spectroscopy experiments at the National Superconducting Cyclotron Laboratory (NSCL) have discovered a new excited 0$^{+}$ state at 1567 keV in $^{70}$Ni. This result supports MCSM predictions extending the picture of shape coexistence to $^{70}$Ni and demonstrates the importance of the tensor force for describing the nuclear structure of neutron-rich nuclei. Results of the latest NSCL experiments will be presented.   

New data on excited states in very neutron rich nickel isotopes

The vicinity of $^{78}$Ni still remains elusive. Several experimental studies show evidence that $^{78}$Ni is a doubly magic nucleus, but deformed phenomena have been reported in this region. Spectroscopic studies around the region are of interest to clarify the shell evolution toward $^{78}$Ni. Beta decay studies of neutron-rich Co isotopes have been performed at the NSCL. The isotopes were produced in the fragmentation of the 140 MeV/u $^{82}$Se beam. These isotopes were implanted in a planar Germanium DSSD. The measurement of gamma-rays using clover detectors revealed excited states in neutron-rich Ni isotopes at high excitation energies. Preliminary results will be presented.   

Gamow-Teller decays near 78Ni

Decays of neutron-rich nuclei close to 78Ni such as 82,83Zn and 82,83Ga produced in proton-induced fission of 238U were studied at the Holifield Radioactive Ion Beam Facility. New gamma-ray transitions were identified and level schemes, which include states at high excitation energies were constructed. The high energy levels were populated through allowed Gamow-Teller decays of the 78Ni components of the wave function, and were interpreted with new shell model calculations.   

Study of Beta-delayed Neutrons near $^{78}$Ni using VANDLE

As nuclei become more neutron rich, the nuclear structure changes their properties. For example, beta decays will access increasingly more neutron unbound states. The measurement of neutrons emitted from these states is critical, as beta-delayed neutron emission becomes a dominating decay mode. To this end, the Versatile Array of Neutron Detectors at Low Energy (VANDLE)[1,2] measures the energy of neutrons emitted from excited states above the neutron separation energy populated through beta decay or transfer reactions. The time-of-flight technique determines the energy, which requires a time resolution on the order of 1 ns. In addition, the detector requires a low detection threshold to measure neutron energies of 100 keV or lower. A successful experimental campaign at the Holifield Radioactive Ion Beam Facility, using ions produced via proton induced fission on $^{238}$U, has yielded results on beta-delayed neutrons emitted from isotopes near $^{78}$Ni. Of particular interest, is the observation of low-energy neutrons emitted from states well above the neutron separation energy. Results from this experiment will be presented. [1] C. Matei et al., Proceedings of Science, NIC X, 138 (2008); [2] S. V. Paulauskas et al., NIMA 797, 22 (2014)   

Beta-Delayed Neutron Spectroscopy Using VANDLE at CARIBU

Measurement of spectroscopic information on beta-delayed neutrons of neutron rich fission fragments is of interest to the areas of astrophysics, reactor design, nuclear structure and stockpile stewardship. Using the Time of Flight (TOF) method, the Versatile Array of Neutron Detectors at Low Energy(VANDLE)[1,2,3] measured fission fragments of $^{252}$Cf provided by CARIBU at Argonne National Lab. $^{135,136}$Sb and $^{85}$As isotopes were measured to explore the nuclear structure around doubly magic nuclei $^{132}$Sn and $^{78}$Ni. A new TOF start detector was developed for this experiment using new Silicon Photo-Multipliers from SensL to allow for a lower beta particle energy detection threshold and better timing resolution compared to previous VANDLE experiments.   

Total Absorption Spectroscopy of 85Se, 85Br

Two experimental campaigns utilizing the Modular Total Absorption Spectrometer (MTAS) were conducted at the HRIBF facility in January of 2012 and March 2015. The cases of 85Se and 85Br will be discussed in concert with shell model predictions. 85Se is a Z$=$34, N$=$51 nucleus, therefore its decay properties are determined by interplay between first forbidden decays of valence neutrons and Gamow-Teller decay of 78Ni core. Analysis of the 85Se and 85Br data indicate a significant modification of the beta strength function when compared with previous measurements for both nuclei, see ref [1].\\[4pt] [1] Zendel et al, J. inorg, nucl. Chem. Vol.42, pp. 1387-1395 Pergamon Press Ltd., 1980.   

Reducing Ambiguities in Spectroscopic Factors with Combined Measurements and the $^{86}$Kr(d,p) Reaction at 35MeV/u

Spectroscopic information for low-lying states above shell closures depends on the shape of the bound-state potential, which greatly affects the extracted spectroscopic factors. To mitigate this uncertainty, Mukhamedzhanov and Nunes [1] have proposed a combined method; the external portion is fixed with a peripheral reaction, and is combined with a higher energy measurement with a larger contribution from the interior. This will constrain the single-particle ANC, and should enable spectroscopic factors to be deduced with uncertainties dominated by cross-section measurements rather than the bound-state potential. Published measurements of $^{86}$Kr(d,p) at 5.5MeV/u [2] were used for the external contribution of this reaction. An ANC analysis shows that the reaction is peripheral at this energy and the ANC has been extracted. At less-peripheral energies, $^{86}$Kr(d,p) at 35MeV/u has been measured in inverse kinematics at the NSCL using the ORRUBA and SIDAR arrays of silicon strip detectors. Results of the ANC analysis and preliminary results from $^{86}$Kr(d,p) at 35MeV/u will be presented. This work is supported in part by the NSF and the U.S. DOE. [1] Phys.Rev.C 72, 017602 (2005) [2] Phys.Rev.C 1,938 (1970)   

Total Absorption Spectroscopy of the $^{137}$Xe, $^{137}$I, and $^{92}$Rb $\beta$-Decays

The NaI(Tl) based Modular Total Absorption Spectrometer (MTAS) was constructed to measure improved $\beta$-decay feeding patterns from neutron-rich nuclei. It is difficult to measure $\beta$-decay feeding intensities with high precision $\gamma$-ray measurements due to the low efficiency of high precision detectors. There are several important applications of improved measurements of $\beta$-decay feeding patterns by total absorption spectroscopy; improve understanding of elemental abundances in the universe, help with stockpile stewardship, contribute to understanding of underlying nuclear structure, and improve $\beta$-decay feeding measurements to calculate accurately the $\bar{\nu}_e$ spectra needed to evaluate precisely reactor neutrino measurements. We present $\beta$-decay feeding results for two ``priority one" measurements, $^{137}$Xe and $^{137}$I, and for $^{92}$Rb, which is a large individual contributor to the $\bar{\nu}_e$ uncertainty of the reactor anomaly. In addition to $\beta$-$\gamma$ decays, $^{137}$I has a $\beta$-neutron decay channel which is measurable in MTAS. We will demonstrate techniques for analyzing MTAS $\gamma$-decay data. We will also describe $\beta$ and neutron spectroscopy in MTAS.