Compton Scattering from Liquid ³He with 61.5-MeV Photons

Abstract

This dissertation reports the first measurement of Compton-scattering differential cross sections from liquid ³He at a photon energy of 61.5 MeV. The work is part of a broader HIγS program aimed at reducing uncertainties on the neutron electric and magnetic polarizabilities, α_E^n and β_M^n, through Compton-scattering measurements on light nuclei where Chiral Effective Field Theory (χEFT) calculations exist. At 61.5 MeV, the modest sensitivity to α_E^n and β_M^n makes the dataset particularly useful as a benchmark for the Baldin sum rule.The experiment was performed at the High Intensity Gamma-ray Source (HIγS) at the Triangle Universities Nuclear Laboratory (TUNL) using a 61.5-MeV circularly polarized γ-ray beam incident on a 20 cm-long cryogenic liquid ³He target maintained at 1.7 K. The beam energy FWHM was 6.2%, and the average flux on target was 1.38 × 10^7 photons/s. Thallium-doped sodium iodide [NaI(Tl)] detectors surrounded the target to cover forward and backward scattering angles, including large backward-angle detectors (DIANA at 150° and BUNI at 115°) and additional detectors at 40°, 55° (×2), and 90° (×2).A total of 112 hours of full-target and 49 hours of empty-target data were collected. Backgrounds were subtracted using flux-scaled empty-target measurements. The target composition was approximately 98.2% ³He with a ⁴He impurity of (1.8 ± 0.25)%. The ⁴He elastic contribution was modeled using simulations weighted by χEFT calculations, and the ³He quasi-elastic contribution was modeled in the impulse approximation. Both were accounted for in extracting ³He Compton-scattering yields.The measured angular distribution is broadly consistent with the scale of theoretical expectations, but differs from the predicted shape in a way not explained by reasonable variations of (α − β) in the range suggested by theory guidance. Accordingly, the primary outcome of this dissertation is the differential-cross-section dataset itself, reported with a transparent and complete uncertainty budget to benchmark improved few-body and χEFT-based calculations. Together with the companion 100 MeV measurement from the same program, these results establish the first liquid-³He Compton-scattering dataset across two beam energies.Keywords: Compton scattering, helium-3, differential cross section, few-body physics, neutron polarizabilities