Session A1: Photonuclear Physics

A Study of the Nucleon Electromagnetic Response: the Compton@HIGS Program

The electromagnetic properties of the nucleon arise from its composite nature. External electric and magnetic fields induce dipole moments described by the scalar polarizabilities ($\alpha_N$ and $\beta_N$), while the response of the nucleon spin is described by four spin polarizabilities. Consequently, studying these observables provides a window into the interaction among the constituent charges and currents in the nucleon, described theoretically by QCD. High precision measurements of the scalar and spin polarizabilities provide critical benchmarks for studying the low-energy, non-perturbative regime of QCD and emerging calculations from lattice QCD and Chiral Effective Field Theories. To this end, a program of Compton scattering experiments on light nuclei is underway at the High Intensity Gamma Source (HI$\gamma$S) at Duke University, with the aim of providing the world's most precise measurements of the polarizabilities. We report preliminary measurements of the Compton scattering differential cross section for $^4$He, $^2$H, and $^1$H and discuss the sensitivity of these data to the polarizabilities.   

Photofission measurements using the HI$\gamma$S facility at TUNL

In an effort to understand and compare the effect of the incoming probe on the fission product yield distribution (neutron $\it{vs.}$ photon), a program has been initiated to measure fission product yields in photoinduced fission. Photofission measurements were performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) at Triangle Universities Nuclear Laboratory on $^{235}$U, $^{238}$U, and $^{239}$Pu at E$_{\gamma}$ = 13 MeV. Results on the first ever photofission product yield measurements obtained with monoenergetic photon beams will be discussed during the meeting.   

CALIOPE: A Test of CPT-invariance in o-Ps Decay

We propose a test of CPT-invariance in ortho-positronium (o-Ps) decays with CALIOPE, or CP(T) Aberrant Leptons in o-Ps Experiment. CPT-violation could manifest itself in angular cor- relations between the directions of the three gamma rays emitted in the decay of o-Ps. We use an array that provides 75\% of 4$\pi$ angular acceptance, increasing our statistics over previous such experiments. The array is composed of 24 NaI(Tl) bars arranged cylindrically. A cylinder of aero- gel adjacent to a tagged source supplies positronium at the center of the array. We will present a systematics study and the first detection of o-Ps. A search for CP-violation is also possible with the addition of an electromagnet.