The Bremsstrahlung Beam Shape

Abstract

During the April 1998 run with 4GeV photons, the Radphi experiment performed diagnostic studies of the photon beam that included scanning the target and a small scintillator simultaneously through the beam in the horizontal direction and recording the rates as a function of position. In this note these data are compared with expectations based upon the ideal bremsstrahlung angular distribution. An excellent agreement is obtained once the geometry and position of the target relative to the beam are properly taken into account. The implications of these data for an absolute normalisation of the photon flux using pairs measured downstream of the Radphi target are explored, and the sensitivity to deviations in beam position on the target is quantified. The proposal that the target be fitted with an aluminum collar to reduce this sensitivity is addressed.