Elastic Compton Scattering on ³He at 61 MeV: Commissioning the New Cryotarget and Benchmarking EFT

Elastic Compton scattering on light nuclei provides a powerful probe of nucleon structure and a stringent test for theoretical descriptions based on chiral Effective Field Theory (χEFT). While nucleon polarizabilities (α_E1 and β_M1) have a more significant effect on the scattering cross section at higher photon energies, scattering data at lower energies—where sensitivity to α_E1 and β_M1 is reduced—play a crucial role in validating the baseline assumptions of these EFT calculations and in verifying the Baldin sum rule.

In May 2024, the world’s first measurements of Compton scattering from liquid ³He were performed at the High Intensity Gamma-ray Source (HIγS) facility at the Triangle Universities Nuclear Laboratory (TUNL) located at Duke University. Measurements were taken using a circularly polarized photon beam at energies of 61 and 100 MeV.

For the 61 MeV campaign, the average beam flux on target was 1.4 x 10⁷ Hz. A total of 100 hours of full-target data and 44 hours of empty-target background data were collected. Scattered photons were detected at laboratory angles between 40° and 150° using large coincidence-shielded NaI detectors.

This run marked the first successful use of the newly upgraded cryogenic target system to liquefy ³He. The target was maintained at 1.7 K in a 0.3-liter thin-walled Kapton cell. The experiment completes several key technical and safety developments required for future high-precision photonuclear studies with ³He.