Session K03: Low-Energy Nuclear Physics

Updates on Kinematically Complete Photodisintegration of $^3$He

Kinematically complete measurements of the three-body photodisintegration of $^3$He were performed with nearly monoenergetic 15 MeV photons using the High Intensity Gamma-ray Source (HI$\gamma$S) at the Triangle Universities Nuclear Laboratory. The experimental technique was based on two-particle coincidence measurements involving both proton-proton (pp) and neutron-proton (np) pairings, with proton-deuteron coincidences from the two-body reaction used as an in-situ luminosity monitor. There was good agreement between ab-initio theoretical calculations and measurements in the vicinity of the np collinear point (proton at rest). The measured np final state interaction (FSI) peak included low-energy neutrons that were not predicted by the GEANT4 simulation. This discrepancy between data and the simulated experiment in the kinematic region of the np FSI is being investigated and will be discussed in the talk. The pp coincidence cross-section data are consistently about 40$\%$ larger than the predictions across the measured kinematic range. This talk will present an overview of the methods employed in the measurement and an update on the analysis.   

Neutron-neutron quasifree scattering in neutron-deuteron breakup

The neutron-deuteron (nd) system is a rich environment for testing models of the nucleon-nucleon interaction and three-nucleon (3N) forces. Rigorous ab-initio calculations of 3N scattering observables accurately describe most experimental data. However, some exceptions remain, including the cross section for neutron-neutron (nn) quasifree scattering (QFS) in nd breakup. Recent measurements of this cross section reveal that theory underpredicts the data by more than 15\%. These results imply charge-symmetry breaking at a level higher than expected. We are conducting new measurements of the nn QFS cross section in the tandem accelerator laboratory at the Triangle Universities Nuclear Laboratory. Our measurements are carried out using two substantially different experimental setups to assess systematic uncertainties. The first measurement, which is performed at an incident laboratory neutron energy of 10.0 MeV, uses heavily shielded detectors with an open neutron source. The second measurement utilizes unshielded detectors with a collimated 16.0 MeV neutron beam. Time-of-flight techniques are used to determine the energies of the breakup neutrons detected in coincidence. Our measurements differ from previous experiments in that the beam-target luminosity is determined from the nd elastic scattering yields measured simultaneously with the nd breakup yields. A description of experimental methods and results will be presented.   

Elastic and Inelastic Compton Scattering From $^{12}$C at HIGS

A program of Compton scattering experiments is underway at HIGS with the goal of determining the nucleon polarizabilities. As part of that work, the HINDA detector array has been supplemented with a pair of very large NaI detectors, BUNI and DIANA, from Boston University and the University of Kentucky, respectively. Both have large acceptance and high resolution for gamma-ray energies above 50 MeV, where the sensitivity of Compton experiments to nucleon polarizabilities is largest. Two approved experiments aim to study the neutron electric and magnetic polarizability through measurements of elastic Compton scattering on targets of deuterium and $^3$He at energies from 60 to 120 MeV. In both cases, inelastic processes must be resolved in the measured scattering spectra, and this requires not only the detectors but also the HIGS beam to operate in high resolution mode. As part of the development towards that goal, we have measured Compton scattering spectra from a carbon target with both BUNI and DIANA at 62 MeV. Both detectors exhibit clearly resolved elastic peaks, allowing for cross section measurements at both 115 and 150 degrees, which can be compared with previous results. In addition, due both to the suppression of atomic backgrounds at 150 degrees, as well as the exceptional stability of the HIGS beam, the back-angle spectrum collected with DIANA exhibits resolved transitions not only to the first, but also possibly to the second excited state of $^{12}$C. This improved ability to quantify inelastic contributions contrasts with previous Compton measurements, which used a tagged bremsstrahlung beam. We will discuss the motivation and the methods used to collect these new data, and offer a preview of the neutron polarizability experiments expected to run soon at HIGS.   

Compton Scattering and Nucleon Polarizabilities at HI$\gamma$S

The electromagnetic polarizabilities of the nucleon are fundamental quantities revealing the internal dynamics of the nucleon. In the past decade, effective field theories have successfully established a bridge between QCD and low-energy description of the nucleon and made predictions for the polarizabilities. Lattice QCD calculations are also eminent on electromagnetic polarizabilities. High precision data are now needed to benchmark these calculations. To this end, the Compton scattering program at the High Intensity $\gamma$-ray Source (HI$\gamma$S) has provided the Compton scattering cross sections on the proton at 78.7 MeV to extract the electromagnetic polarizabilities of the proton. In addition, the cross sections of elastic Compton scattering from $^4$He have also been measured at high precision to provide a complementary approach to determine the nucleon polarizabilities. In this talk, I will present the details of the experimental method, the analysis and the results of the 78.7 MeV Compton scattering data on $^4$He and the proton.