Session D11: Nuclear Reactions & Nuclear Structure

Cluster Structure and Three-Body Decay of ${}^{14}$C

Recent model calculations with most advanced methods for cluster states have shown the need of experimental data to probe the structure of light exotic nuclei, including those with $\alpha$-clustering, such as ${}^{14}$C. The prototype Active Target Time Projection Chamber (pAT-TPC) allows us to investigate these types of structures, giving access to the full excitation function with a single beam energy. This type of experiment measures resonances in ${}^{14}$C that can be compared to the models. Additionally, using a Dalitz-type analysis, three-body decays can be analyzed to determine probabilities of ``democratic" and ``sequential" decay. The measurement was carried out by resonant alpha-scattering of a ${}^{10}$Be beam at 40 MeV delivered by the \textit{TwinSol} facility at the University of Notre Dame. Preliminary results will be presented including event reconstruction using the Random Sample Consensus method.   

Oxygen isotopes beyond the proton drip line

Nuclei at and beyond the drip lines are among the most exotic nuclear species. Beyond the proton drip line, two-proton decay, the most recently discovered nuclear decay channel, occurs. For 2$p$-decaying states in light isotopes, the lifetimes are short and the invariant-mass method is ideal for measuring the decay energy, width, and momentum correlations between the decay fragments. I will present an invariant-mass measurement of the 2$p$-decaying nucleus $^{11}$O, the mirror of $^{11}$Li. I will also present a new measurement of the 2$p$-decaying ground and excited states in $^{12}$O, including a newly observed 2$^{+}$ state. An ambiguity having to do with the possible two-proton decay of this 2$^{+}$ state to the 3.353 MeV excited state in $^{10}$C will also be discussed.   

Intruder Structure in N=21, $^{38}$Cl Isotope

Excited states of the $^{38}$Cl isotope were populated by the $^{26}$Mg($^{18}$O,$\alpha pn\gamma$)$^{38}$Cl fusion evaporation reaction at $E_{lab}=50$ MeV. The $^{18}$O beam accelerated by a Tandem accelerator at John. D. Fox laboratory, Florida State University, was incident on an enriched $^{26}$Mg target. Four clover detectors and one single crystal HPGe detector were used in order to detect the de-exciting $\gamma$ rays, where a $E-\Delta E$ detector was used to select the charged particles emitted from the compound nucleus $^{44}$Ca. The $\gamma-\gamma$ coincidence method, along with the coincidence with charged particles was employed in order to assign new $\gamma$ rays in $^{38}$Cl and to verify the existing ones. The experimental observations will be compared to the shell model calculations with a new interaction currently being developed by the Florida State University Nuclear Physics group.   

Cross-Shell Excitations and High Spins in 41K and 41Ca

The work to be presented here investigates the nuclear structure of 41K and 41Ca, both of which have one or more nucleons in the fp-shell in their ground state configurations. These nuclei play a significant role in exploring the N = 20 shell gap. This work was carried out using fusion-evaporation reactions at Florida State University’s Jon D. Fox Superconducting Linear Accelerator Laboratory: 26Mg(18O,3n)41K and 26Mg(18O,p2n)41Ca. The 18O beam energy was 50 MeV and the target (26Mg) consisted of two foils that were approximately 400 microns thick each. The data analysis resulted in the determination of a number of new energy levels and transitions along with newly proposed spins and parities. This information will be used to aid in refining a shell model interaction for cross-shell nuclei.   

Isoscalar Giant Resonances in Molybdenum Isotopes

It is a well-established question in nuclear structure as to why the incompressibility of nuclear matter calculated from the $E0$ giant resonance strength distributions of open-shell nuclei, such as tin and cadmium isotopes, is lower than that determined using data on closed-shell nuclei such as $^{208}$Pb. To investigate this, giant resonance strength distributions have been extracted for the isotopic chain $^{94,96,97,98,100}$Mo. Angular distributions for 100 MeV/u $\alpha$ particles were obtained using the spectrometer Grand Raiden. Multipole decompositions were carried out on the distributions to isolate the monopole, dipole, and quadrupole strength over the excitation energy range of the spectra. Results of the analysis and implications for the nuclear incompressibility will be discussed.   

Prompt Gamma Activation Analysis of $^{188}$RE

Prompt $\gamma$ activation analysis was used to determine partial $\gamma$ production cross sections from the $^{187}$Re(n,$\gamma$)$^{188}$Re reaction at the Budapest Neutron Centre in Budapest, Hungary. The 10-MW$_{t}$ research reactor provided the source of neutrons, which were cooled to a spectrum of $\sim$140 K and collimated onto a target of powdered rhenium metal enriched to 99.52\% $^{187}$Re. Prompt $\gamma$ spectra were collected with a single Compton-suppressed HPGe detector. Subsequent fitting of the raw spectral data produced precise energies and intensities of the observed transitions. The $^{188}$Re cross sections were determined by standardizing measured peak intensities to well-known $^{35}$Cl(n,$\gamma$)$^{36}$Cl cross sections via a second spectrum from a sample of ReCl$_{3}$. However, the non-trivial volume of the enriched sample resulted in energy-dependent self-attenuation of $\gamma$-rays originating in the sample bulk. This necessitated estimating the effective optical thickness of the enriched sample to derive intensity adjustments, particularly for $\gamma$-rays below $\sim$300 keV. Forthcoming analysis will use statistical decay modeling to predict the population of known levels from the neutron-capture state for comparison with observed cross sections.   

Measuring fusion of $^{41,45}$K and $^{36,44}$Ar ions with $^{28}$Si target nuclei at near barrier energies

Recent measurement of fusion in $^{39,47}$K + $^{28}$Si has demonstrated the feasibility of investigating near barrier fusion with low-intensity ($\approx$ 10$^{4}$ ions/s), reaccelerated, radioactive beams. Such experiments for an isotopic chain allow exploration of the dependence of fusion on neutron number to the limits of stability. To expand the study beyond the closed N=20 and N=28 shells, as well as explore the role of the unpaired proton, an experiment to measure fusion in $^{41,45}$K + $^{28}$Si and $^{36,44}$Ar + $^{28}$Si, 17002, has been approved at NSCL's ReA3 facility. Fusion products will be identified by measuring their Energy and Time-of-flight. The experimental setup, improvements since the previous experiment, and the setup's efficiency, estimated using the statistical decay code evapOR, will be presented. Calculations of the fusion excitation function using the coupled channels model CCFULL will also be shown.