Session HE: Nuclear Structure A~20-60

The influence of $s$ states near threshold on the structure of light nuclei

A recent work [1] identified the role of neutron $s$ states, and their proximity to the neutron separation threshold, on the ordering of the 1$s_{1/2}$ and 0$d_{5/2}$ single-particle levels in light nuclei. A simple Woods-Saxon potential was used to reproduce the systematic data available for these two levels with great success by accounting for the $s$ state binding energy. This talk will explore other noticeable trends in light nuclei involving neutron $s$ states and utilizing simple potential models determine the role binding energy plays. The trends and calculations will aim to provide descriptions of data and predictions of yet to be found two-particle two-hole excited states in $N$=8 and 10 nuclei ranging from $Z$=4-9, as well as the energies of mirror states in neutron deficient Al and Na isotopes. Results will be compared with state-of-the-art calculations. Possible future measurements capable of probing these predictions will be discussed as well.\\[4pt] [1] C. R. Hoffman, B. P. Kay, and J. P. Schiffer, Phys. Rev. C 89, 061305(R) (2014).   

Structure of $^{19}$O from $^{9}$Be + $^{14}$C

The $^{9}$Be($^{14}$C, $\alpha$$\gamma$) reaction at E$_{Lab}$=30 and 35 MeV was used to study excited states of $^{19}$O. The Florida State University (FSU) $\gamma$ detector array was used to detect $\gamma$ radiation in coincidence with charged particles detected and identified with a silicon $\Delta$E-E particle telescope. From $\alpha$-$\gamma$-$\gamma$ coincidences five new states of $^{19}$O have been identified and an additional three new $\gamma$ transitions among known states have been observed. An additional previously unobserved state has been marked tentative from $\alpha$-$\gamma$ coincidences. These results are compared to shell model calculations using the USDA, WBP, and PSDU interactions.   

Two-neutron Sequential Decay of $^{24}$O

A recent experiment performed at the NSCL populated a two-neutron unbound excited state in $^{24}$O through a (d,d') reaction at 82 MeV/nucleon. Using invariant mass spectroscopy, a three-body state was observed in the $^{22}$O + $2n$ system at E = $750^{+100}_{-100}$ keV and $\Gamma = 100^{+300}_{-100}$ keV, placing it at $\sim$7.68 MeV with respect to the ground state of $^{24}$O. Three-body correlations for the decay of $^{24}$O $\rightarrow$ $^{22}$O + $2n$ were examined and found to show strong evidence for a sequential decay through an intermediate state in $^{23}$O. A di-neutron or phase-space model for the three-body breakup is unable to describe these correlations.   

Unbound Excited States of the N=16 Closed Shell Nucleus $^{24}$O

The energies of two low-lying neutron-unbound excited states of $^{24}$O, which were populated by proton-knockout reactions on $^{26}$F, have been measured using the MoNA and LISA arrays in combination with the Sweeper Magnet at the Coupled Cyclotron Facility at the NSCL using invariant mass spectroscopy. The current measurement confirms for the first time the separate identity of 2$^+$ and (1$^+$) neutron-unbound excited states in $^{24}$O with decay energies 0.51(5)~MeV state and 1.20(7)~MeV, respectively, to the $^{23}$O ground state. These measured decay energies are consistent with two previous lower resolution measurements to within 2$\sigma$. The level energies for the two states are computed using the decay energies and the 1-neutron separation energy for $^{24}$O, resulting in 4.70(15)~MeV for the 2$^+$ state and 5.39(16)~MeV for the (1$^+$) state. Errors in the level energies are dominated by uncertainty in the $^{24}$O neutron separation energy, underscoring the need for a higher resolution $^{24}$O ground state mass measurement. Results will be compared with 3 phenomenological and 2 \textit{ab initio} model calculations. \\   

Doppler broadening in the $\beta $-proton-$\gamma $ decay sequence

We report the first observation of Doppler-broadening in $\beta $ delayed proton- $\gamma $ decay. The broadening occurs because the daughter nucleus $\gamma $ decays while recoiling from proton emission. A method to analyze $\beta $ delayed nucleon emission was applied to two Doppler-broadened $^{25}$Al peaks from the $^{26}$P($\beta $p$\gamma)^{25}$Al decay. The method was first tested on the broad 1613 keV $\gamma $-ray peak using known center-of-mass proton energies as constraints. The method was then applied to the 1776 keV $\gamma $-ray peak from the 2720 keV excited state of $^{25}$Al. The broadening was used to determine a $^{26}$Si excitation energy of 13.3 $\pm$ 1.0 (stat.) $\pm$ 0.7 (syst.) MeV. This energy is consistent with proton emission from the known T$=$2 isobaric analog state of $^{26}$P in $^{26}$Si.   

Charge radii of neutron-deficient $^{36, 37}$K

Neutron and proton shell closures have been identified by discontinuities in the chain of charge radii. The shell-closure signature can be seen, e.g. at the $N$ = 28 and 50 neutron-shell closures for the isotopic chains of K through Mn. The signature at $N$ = 20, however is absent in the Ca region. The charge radii of neutron-deficient $^{36, 37}$K isotopes were determined to aid in understanding this abnormal behavior. The experiment was performed at the BEam COoling and LAser spectroscopy (BECOLA) facility at NSCL/MSU. The charge radii were obtained from isotope shifts determined from the hyperfine structure measurements by the optical pumping and subsequent b-decay asymmetry detection. The absence of an unambiguous signature of the shell effect is attributed to a balance of monopole and quadrupole proton-core polarizations above and below $N$ = 20, respectively. The global behavior of the isotopic chain of K charge radii will be discussed.   

Investigation of excited states in $^{47}$Ca through a high-statistics beta-decay study

Recent developments in nuclear many-body calculation methods have extended the application of ab initio interactions to the calcium isotopes (e.g. Refs. [1,2]). Detailed nuclear data for these isotopes are necessary to evaluate the many-body calculation methods and to test the predictive power of the interactions. Transfer reactions from $^{48}$Ca have identified many excited states of $^{47}$Ca, but only four states have been identified in previous beta-decay experiments. High-statistics beta-decay studies using modern detection systems can provide detailed information on level energies, branching ratios, and spin/parity assignments, while comparison to other population methods can yield information about the structure of these states. A recent experiment at TRIUMF-ISAC used the GRIFFIN spectrometer to investigate the levels populated by beta decay in more detail. The decay scheme has been considerably extended and angular correlations between cascading gamma-ray transitions allow spin and parity assignments to be made for some of the observed excited states. An overview of the experimental apparatus as well as a discussion of the results from preliminary analysis will be presented. \\[4pt] [1] G. Hagen et al., PRL 109 032502 (2012)\\[0pt] [2] J. Holt et al., PRC 90 024312 (2014)   

$\gamma-\gamma$ Angular Correlation Measurements With GRIFFIN

When an excited nuclear state emits successive $\gamma$-rays causing a $\gamma-\gamma$ cascade an anisotropy is found in the spatial distribution of $\gamma_{2}$ with respect to $\gamma_{1}$. Defining the direction of $\gamma_{1}$ as the z-axis, the intermediate level, in general will have an uneven distribution of m-states. This causes an anisotropy in the angular correlation of the second $\gamma$-ray with respect to the first. These angular correlations are expressed by the $W(\theta)$ that depends on numerical coefficients described by the sequence of spin-parity values for the nuclear states involved, the multipolarities and mixing ratios. Angular correlations can be used for the assignment of spins and parities for the nuclear states, and thus provide a powerful means to elucidate the structure of nuclei far from stability through $\beta-\gamma-\gamma$ coincidence measurements. In order to explore the sensitivity of the new 16 clover-detector GRIFFIN $\gamma$-ray spectrometer at TRIUMF-ISAC to such $\gamma-\gamma$ angular correlations, and to optimize its performance for these measurements we have studied a well known $\gamma-\gamma$ cascade from $^{60}$Co decay through both experimental measurements and Geant4 simulation. Results will be shown in this talk.   

On the Equivalency of Experimental B(E2) Values Determined by Various Methods

Over the last 60 years a variety of experimental methods have been employed to determine reduced transition probabilities in even-even nuclei. Different methods and data analysis techniques imply a strong need for consistency checks of the reported results. To investigate the equivalence of different measurements we have used a recently-developed B(E2)$\uparrow$ database. For the first time transition probabilities for Doppler Shift Attenuation (DSA), Recoil Distance Doppler Shift (RDDS), Delayed Coincidences (DC), Nuclear Resonance Fluorescence (NRF) and Coulomb Excitation (CE) methods have been analyzed and compared in the Z=6-94 region. The analysis of B(E2;$0_{1}^{+} \rightarrow 2_{1}^{+}$) values of the 100 frequently-studied even-even nuclei indicates these experimental methods produce equivalent results. Possible differences between the DSA and CE values near closed neutron and proton shells could be explained by the experimental deficiencies. Further comparisons of the present data with the inelastic electron scattering (EE$^{\prime}$) results also show agreement. These findings confirm equivalence of the major experimental methods for a wide range of nuclei.