| The differential cross section for coherent photoproduction of neutral pi-mesons from helium has been measured for three energies at a total of five laboratory angles in the kinematic range where delta resonance production is expected to be important. The measurements were made using a twin arm pi0 spectrometer. The experiment employed bremsstrahlung photon beams derived from electron beams having energies of 338 MeV, 260 MeV and 220 MeV. The cross sections were measured at laboratory angles of 32 and 55 degrees using the 338 MeV beam, 72.5 and 45 degrees using the 260 MeV beam and 50 degrees using the 220 MeV beam. The results are compared with previous measurements and an isobar-doorway model calculation at 330 MeV. The data at 330 MeV fall below the values predicted by the isobar-doorway model with one point (at 32 degrees) coming within experimental errors of the theory. Total cross sections at each energy are estimated from the data. These cross sections differ from the theoretical values both in magnitude and in the shape of the dependence of the total cross section upon incident photon energy. The experiment was carried out at the Bates Linear Accelerator Center at Middleton, Massachusetts in June of 1984. |
