The Elastic Photoproduction of Neutral Pions from Helium

Abstract

It was shown that in the resulting expression for Ig^o, the term involving B was negligibly small, while the ratio of the coefficient of 0 to that of A was approximately 1/^. Thus the data were essentially a measure of CT total. Since the ratio was not exactly at all energies, a d value had to be assumed for /A, i.e. for the shape of the angular distribution. But the results were claimed to be insensitive to this value. Thus changing this ratio from-0*6 to-1*0 altered ^ total by only 10% at 240 MeV, and by successively smaller percentages at the lower energies. These authors also measured the absolute differential cross-section at 135^, and combined it with the total cross-section data to obtain values of A, B and C. Goldschmidt-Glermont*s results consisted of angular distribution for meson centre-of-mass angles varying between 4 5^ and 160^, for energy bands centred at 220, 260 and 305 MeV; and also total cross-sections between 175 and 325 MeV, obtained by extrapolating the measured angular distribution, assuming no higher power of Cos 0 than the second. I Oakley, Walker and Corson et. al s. experiments which were both done at Cal. Tech., using the same machine, and the same beam calibration, covered the photon energy range 240-480 MeV, but were mutually supplementary. The formers*, data were restricted to pion centre-of-mass angles greater than 7 0^ (7 0^-153^) owing to the higher value for the minimum detectable proton energy. Self-absorption in the target and counters set this low energy limit at about 25 MeV