Compton@HIGS: neutron electromagnetic polarizabilities with helium-3 nuclear Compton scattering*

The scalar electric and magnetic polarizabilities of the neutron are currently only known to ±10% and ± 30%, respectively. These quantities offer powerful tests of predictions of effective field theories and lattice QCD in the non-perturbative regime where the building blocks of everyday matter exist. The Compton@HIGS collaboration is embarking on a series of absolute cross-section measurements for elastic Compton scattering from ¹H, ²H, ³He, and ⁴He over a wide angular range. We use the mono-energetic and polarized photons with energies up to ~ 120 MeV from the free-electron-laser-based High-Intensity Gamma-ray Source (HIGS) at the Triangle Universities Nuclear Laboratory (TUNL). Our goal is to reach ~ ± 2% uncertainty in measuring these very small cross sections (~nanobarns) to improve determination of the neutron’s electromagnetic polarizabilities by a factor of two. A cryocooler-based cryogenic target system capable of liquefying and cooling our flammable or valuable target materials in a 0.3-L-volume thin-walled Kapton cell with windows for the gamma beam and visual access has been developed. We recently added an additional recirculating cooling stage that can reach our desired minimum temperature of 1.7 K with ~ 120 mW of cooling power. This summer, we completed our first runs with liquid ³He. We acquired sufficient statistics, remained at our 1.7 K operating temperature with stability of ~ 20 mK (correspond to < 0.2% systematic effect), and successfully demonstrated handled our full 350 STP-bar-L inventory of ³He.