Elastic Photon Scattering from HELIUM-4 in the DELTA(1232) Region.

We have measured the differential cross section at laboratory angles 24^circ, 30^circ, 45^ circ, and 60^circ for the reaction ^4He(gamma {,}gamma)^4He at an average lab energy of 320 MeV. This work was performed at the MIT Bates Linear Accelerator using a bremsstrahlung photon beam produced by a 330 MeV electron beam. The scattered photons were detected with a new, high resolution (1.68% FWHM at 330 MeV) NaI(Tl) total absorption scintillation counter. The data were summed over a nine MeV interval below the endpoint of the elastically scattered photon spectrum. Cosmic ray background was rejected by a plastic scintillator veto shield that surrounded the detector and the energy resolution was sufficient to exclude photons from pi^0 decay and inelastic scattering from the region of interest. The results were compared with the predictions of the Delta -hole calculations of Koch, Moniz, and Ohtsuka and were found to be in excellent agreement. This measurement is the first unambiguous test of the Delta -hole formalism for this reaction near the peak of the Delta resonance.