Session KG: Mini-Symposium on Decay Spectroscopy of Exotic Nuclei II

Decay spectroscopy of exotic fission products

The beta decay studies of exotic fission products have been performed at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory. The scientific program was focused on the beta-strength function measurements and resulting new half-lives [1,2] and beta-delayed neutron properties. These observables are important for nuclear structure analysis and modeling of the nucleosynthesis within rapid neutron capture process. The highlights include ten new beta half-lives and several Pn branching ratios including an observation of beta-delayed two-neutron emitter 86Ga[2]. In addition, the measurements of the beta-strength within beta-gamma emission window were performed using a Modular Total Absorption Spectrometer for 22 fission products. These MTAS results are also important for the analysis of reactor anti-neutrino anomaly.\\[4pt] [1] M. Madurga et al., Phys. Rev. Lett. 109, 112501, 2012.\\[0pt] [2] C. Mazzocchi et al., Phys. Rev. C 87, 034315, 2013.\\[0pt] [3] K. Miernik et al., Phys. Rev. Lett. 111,132502,2013.   

The Beta-Delayed Proton and Gamma Decay of 27P for Nuclear Astrophysics

The destruction of 26Al can be accomplished by proton capture on either the ground state or the metastable-state. The indirect method used here was the study of beta-delayed gamma and proton decay of 27P. The states that are populated above the proton threshold in 27Si can then decay by proton emission to 26mAl. These states represent the resonances of interest in the direct proton capture process. While no new proton lines were observed, a slightly higher total proton branching ratio was estimated. Several new gamma lines were seen, mostly gamma's emitted from the IAS, which itself had a new and more accurate energy value assigned.   

Total absorption spectroscopy of N $=$ 51 nucleus 85Se

An experimental campaign utilizing the Modular Total Absorption Spectrometer (MTAS) was conducted at the HRIBF facility in January of 2012. The campaign studied 22 isotopes, many of which were identified as the highest priority for decay heat analysis during a nuclear fuel cycle, see the report by the OECD-IAEA Nuclear Energy Agency in 2007 [1]. The case of 85Se will be discussed. 85Se is a Z$=$34, N$=$51 nucleus with the valence neutron located in the positive parity sd single particle state. Therefore, its decay properties are determined by interplay between first forbidden decays of the valence neutron and Gamow-Teller decay of a 78Ni core. Analysis of the data obtained during the January 2012 run indicates a significant increase of the beta strength function when compared with previous measurements, see ref [2].\\[4pt] [1] OECD-IAEA, NEA No 6284, vol. 25, ISBN 978-92-64-99034-0, 2007.\\[0pt] [2] Zendel et al, J. inorg, nucl. Chem. Vol.42, pp. 1387-1395 Pergamon Press Ltd., 1980.   

Study of Beta-Delayed Neutrons From $^{77}$Cu 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 during the decay of $^{77,78}$Cu. 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. \\[4pt] [1] C. Matei et al., Proceedings of Science, NIC X, 138 (2008)\\[0pt] [2] S. V. Paulauskas et al., NIMA 797, 22 (2014)   

Monte Carlo simulations of VANDLE for reaction and beta-delayed neutron decay studies

The Versatile Array of Neutron Detectors at Low Energy (VANDLE) is a plastic scintillator array designed for various experimental studies including beta-delayed neutron spectroscopy and (d,n) transfer reactions in inverse kinematics. We have developed a Geant4 simulation of VANDLE to determine neutron detection efficiency in realistic environments and optimize the array geometry for different types of experiments. The experimental assembly including VANDLE bars, beta plastic scintillators, HPGe detectors, along with the detector support structure was modeled to assist in the fine-tuning of the setup and give a detailed understanding of the array performance. Scattering off of the different parts of the experimental apparatus contributes to a background and the ability of simulation to identify various components of the background makes it critical for the data analysis. The simulation was validated by comparing to available experimental data and will serve as an important guide for the design of future experiments.   

Search for the heaviest N$\sim$Z alpha emitters

The enhancement of an alpha-decay probability for nuclei above $^{100}$Sn is expected since valence protons and neutrons above $Z=N=50$ occupy the same single-particle orbitals. The search for new alpha emitters in this region has started at JAEA Tandem Laboratory at the Advanced Science Research Center (ASRC) at Tokai, Japan. We have performed the commissioning experiments at the Recoil Mass Separator (RMS) using two reactions: $^{58}$Ni+$^{54}$Fe and $^{58}$Ni+$^{58}$Ni with the $^{58}$Ni beams. The data-acquisition technique developed at UTK/ORNL enabled us to identify alpha-particles and protons from the decay of $^{108,109}$Te, $^{109}$I and $^{113}$Cs, and to detect the $^{109}$Xe-$^{105}$Te-$^{101}$Sn decay chain. The summary of the results will be presented to demonstrate the capabilities of the ASRC's RMS.   

A new opportunity: coincident spectroscopy in neutron-deficient actinides

Due to high $\gamma$-ray background rates heavy element production facilities are usually not sensitive to the electron capture decay of neutron deficient actinides. We have developed new capabilities at the Berkeley Gas Filled Separator (BGS) that allow us to study these isotopes. The highly selective and efficient separation of compound nucleus evaporation residue products using the BGS couple with a rapid delivery to a low-background detector facility, opens up many new possibilities for nuclear decay and structure studies in the neutron deficient actinides. The decay of these actinides produces vacancies in the K-shell resulting in x-rays uniquely identifying the Z of the decay products. We present the first results of this new methodology in studying the nuclear structure of fermium-254 by observing the gamma rays in coincidence with fermium x-rays. Coincident gamma-decay spectroscopy gives us a new tool to study the nuclear structure of previously inaccessible systems.   

$\beta$-decay of $^{33}$-$^{35}$Mg near the island of inversion

$^{31-35}$Mg nuclei lie in or around the island of inversion near the $N=20$ shell closure. These nuclei exhibit a peculiar behavior with their ground states (gs) dominated by deformed configurations. The daughter nuclei $^{31-35}$Al are suggested to have mixed gs configurations of normal and intruder type and thus serve as a transition from intruder dominated Mg isotopes to the normal gs configuration in Si isotopes. An experiment was performed in the ISAC-I facility at TRIUMF with the goal of populating states in $^{33-35}$Al via the beta decay of $^{33-35}$Mg. 500MeV protons on a UC$_{x}$ target with laser-ionization produced pure beams of Mg ions, which were then implanted on a moving Mylar tape at the center of the 8$\pi$ facility. Surrounding the implantation point was a set of 11 plastic scintillators used for beta tagging and 20 Compton-suppressed HPGe detectors for gamma-spectroscopy. First results from this experiment will be presented. The level schemes produced for states in $^{33,34}$Al will be compared to shell model calculations to understand the influence of intruder states in the neutron-rich $^{33-35}$Al isotopes. Additionally, disagreements between previous experimental results of $^{33}$Mg and $^{33}$Al will be discussed.   

Lifetimes in the A=109 isobaric chain

Nuclear structure is known to evolve rapidly within the neutron-rich region around 50 $<$ N $<$ 82, A$\sim$110. A reliable characterization of this evolution, and especially the onset of deformation, is important for both structure and for nuclear astrophysics. However, the structure of nuclei within this region remains an open question. This work focuses on gamma spectroscopy and lifetime measurements, specifically for the A=109 chain of nuclei including $^{109}$Tc, $^{109}$Rh and $^{109}$Pd, as well as the A=105, 107 and 111 $\beta$-decay chains. We present the results from a fast-timing lifetime experiment performed at the Univ. of Jyv\"{a}skyl\"{a} IGISOL facility for the A=109 chain.   

Half-Lives of $^{101}$Rh and $^{108m}$Ag

Half-lives of short-lived nuclei can easily be measured by direct counting techniques, whereas those of long-lived naturally-occurring nuclei are usually determined by specific activity measurements. However, half-lives in the range of 1 -- 1,000,000 years are notoriously difficult to determine. For example, published values for the half-life of $^{101}$Rh range from 3.0 $\pm$ 0.4 years to 10 $\pm$ 1 years, and for $^{108m}$Ag published values range from 127 $\pm$ 21 years to 438 $\pm$ 9 years. In order to resolve the issues of what the half-lives of these isotopes actually are, we set up two separate long-term gamma-ray counting experiments. Gamma-ray data were collected in time bins using high-purity Ge detectors and ORTEC PC-based data acquisition systems. We counted in this manner for a period of approximately 5 years for $^{101}$Rh and 3 years for $^{\mathrm{108m}}$Ag. In this talk we will describe the details of these experiments and will present the final results for the half-lives of $^{101}$Rh and $^{\mathrm{108m}}$Ag determined from these measurements.