Session T17: Nuclear Structure

New Isospin-Breaking ``USD'' Hamiltonians and their Predictions for Nuclei in the $sd$-shell

Two new USD-type Hamiltonians, USDC and USDI, have been developed that directly incorporate Coulomb and other isospin-breaking interactions. Starting from \textit{ab initio} interactions, linear combinations of two-body matrix elements were constrained by experimental energy levels in \textit{sd}-shell nuclei. With this method, binding energies and excitation energies of proton-rich nuclei in the shell can be added to the data set used in the fit. USDC and USDI contain an analytic Coulomb interaction with Miller-Spencer short range correlations and an effective isotensor interaction. Also presented are the modified interactions, USDCm and USDIm, that have had the Coulomb interaction constrained to better reproduce experimental mirror energy differences. These Hamiltonians are used to provide new predictions for the proton-dripline, to examine isospin level mixing matrix elements, and to calculate the \textit{b-} and \textit{c-}coefficients of the IMME. Several examples of states with large Thomas-Ehrman Shifts are modeled using USDC and a single-particle interaction.   

On Demand

The modelling of astrophysical processes is central to predicting the abundance patterns of matter in the universe. Specifically, the behavior of reaction rates at the astrophysically relevant temperatures is a key input to reaction network calculations. Ab initio theory of nuclear reactions may provide predictions for such rates without resorting to phenomenology. However, general applicability is limited to rather light systems due to the complexity of the many-body problem in the energy continuum, and in particular to reactions where the lighter of the two fragments consists of up to two nucleons. We will outline the basics of the no-core shell model with continuum and present a method extending its applicability to the description of $\alpha$ clustering and $\alpha$-induced reactions. First attempts for the description of $\alpha$-$\alpha$ scattering with chiral two- and three-nucleon forces will also be presented.   

On Demand

A bright neutron source generating 10$^{\mathrm{11}}$ incident particles, produced in inertial confinement implosions\footnote{ T. R. Boehly \textit{et al.}, Opt. Commun. \textbf{133}, 495 (1997).} by the OMEGA laser, were used to irradiate an isotopically enriched $^{\mathrm{7}}$Li target. Absolute yields and energy spectra were measured for neutrons emitted from interactions of 14-MeV neutrons with $^{\mathrm{7}}$Li in a forward-angle geometry ($\theta $~$=$~0\textdegree ~to 7.4\textdegree ) with a neutron time-of-flight spectrometer. The data were analyzed with an R-matrix calculation using a single-level, single-channel approximation to interpret the level structure of $^{\mathrm{7}}$Li. The inferred differential neutron cross section reveals several $^{\mathrm{7}}$Li resonances in the range 4~MeV $\le $ E* $\le $ 12~MeV, attributed to inelastic $^{\mathrm{7}}$Li$\left( {\mbox{n,{n}'}} \right)^{\mathrm{7}}$Li scattering. In addition, a neutron line with significant cross section is observed at E$_{\mathrm{n}}$~$=$~3.5~MeV and a width of E$_{\mathrm{\Gamma }}\le $~0.9~MeV, which may correspond to excitation of a $^{\mathrm{7}}$Li state at E* $=$ 10.2 MeV. It is tempting to identify this state with a narrow resonance at E* $=$ 10.2 MeV, predicted\footnote{ M. Vorabbi \textit{et al.}, Phys. Rev. C \textbf{100}, 024304 (2019).} by a no-core shell model with continuum (NCSMC) to exist just above the threshold for the mass partition of p $+$ $^{\mathrm{6}}$He reaction. However, the measured linewidth is larger than that (E$_{\mathrm{\Gamma }}$~$=$~0.13 MeV) predicted by the NCSMC model. This material is based upon work supported by the DOE NNSA under Award Number DE-NA0003856.   

On Demand

Isospin symmetry is an approximate symmetry between hadrons and can be useful in the description of atomic nuclei, especially in the prediction of atomic mass with the isobaric multiplet mass equation (IMME). Isobaric quintets provide the best test of the IMME and can identify higher order corrections suggestive of isospin symmetry breaking effects in the nuclear Hamiltonian. Only two quintets have all five members’ ground state masses measured to high-precision by Penning trap mass spectrometry. We present the first Penning trap measurement of neutron-deficient $^{36}$Ca, marking the completion of the $A = 36$ quintet. A beam of $^{36}$Ca was produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory. The beam was stopped and the mass of atomic $^{36}$Ca$^+$ and $^{36}$Ca$^{2+}$ measured by the Time of Flight - Ion Cyclotron Resonance method in the LEBIT Penning trap. The measurement is considered with evaluated nuclear data to update the IMME and search for isospin symmetry breaking effects for $A = 36$.   

Not Participating

The nuclei of interest, $^{41}$K and $^{41}$Ca, straddle the N/Z = 20 shell gap and subsequently give insight into the evolving shell structure surrounding this region. The addition to the high-spin structure of these nuclei results from the augmented FSU high purity germanium detector array. The nuclei were produced by bombarding a $^{26}$Mg target with a 50 MeV $^{18}$O beam from the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University. Several new levels and gamma decays have been observed. Additionally, spins and parities have been measured. The structure of these nuclei will be compared with predictions of the spsdpf cross-shell FSU shell model interaction.   

Detailed study of beta-decay schemes in Bi-214 and Po-214 with GRETINA

Bi-214 and Po-214 are part of the decay chain of Ra-226, which is one of the radioisotopes discovered in the late 1800s. The studies of their decay properties go back to the early days of nuclear physics. However, despite the detailed knowledge on the energies and intensities of the gamma radiations emitted in the decay of Ra-226, the beta-decay level schemes in Bi-214 and Po-214 are still not well established. Specifically, only about 20{\%} of the levels have their spin and parity assigned with confidence, and more than 30 gamma rays are not able to be placed in the level schemes. This unsatisfactory situation is a result of the equipment used for the studies which were not sufficient for measuring the subtle properties of observed gamma rays. A measurement was performed at NSCL at Michigan State University with a 1-micro-Ci Ra-226 source in the target position of the GRETINA array. The high spatial resolution of GRETINA will not only allow an angular correlation measurement to high precision, it will also permit the measurement of the linear polarization of gamma rays of interest. The decay schemes of Bi-214 and Po-214 will be presented with more complete level schemes including confident spin and parity assignments.   

Spectrometer Optics Calibration for PRex-II Experiment

Neutron density as a fundamental parameter is important in neutron star structure, heavy iron collision, and atomic parity violation experiment. However, the measurement of neutron density is much harder in electron scattering since neutron is an uncharged particle. PRex-II experiment measuring the neutron density by measuring the parity-violating in electron scattering since neutron weak charge is much large than that of a proton. The PRex-II experiment was performed from Jun to September 2019. In this report, I will give a preliminary Spectrometer Optics Calibration result for the PRex-II experiment.