Normalization of the absolute flux of the bremsstrahlung photon beam for the GlueX experiment relies on a direct counting measurement of the entire beam performed placing a highly efficient calorimeter, the total absorption counter (TAC), directly in the beam near the position of the experimental target. This measurement must be done at very low beam intensity, so as not to overwhelm the TAC with pile-up. The absolute rate measured with the TAC is used to calibrate a pair spectrometer that serves as a continuous relative beam rate monitor, so that the flux can be measured when the TAC is removed. Maintaining stable beam operation when the flux is low enough for efficient operation of the TAC is a challenge because a certain minimum electron beam current is required for the beam stabilization system to function properly. It has been proposed that reducing the primary photon beam collimator diameter to around 1~mm would allow the electron beam current to remain at 10~nA and still achieve sufficiently low photon flux for the happy operation of the TAC. The photon beam has never been simulated with so small a collimator, and so it raises the question of whether it would produce a usable beam, or whether the halo from scraping on the inner walls of the collimator might overwhelm the flux of the beam that passes through
without interactions. This article reports on results of a detailed Monte Carlo simulation of the GlueX beamline with a steel collimator insert inside the standard 5~mm tungsten aperture, with a focus on the spatial and spectral properties of the beam at the GlueX target.
