Systematic errors in electron-photon coincidence measurements of the 3(1)Pstate of helium

The coherence parameters associated with the electron impact excitation of the He 3(1)P state are studied for a Monte Carlo simulation of an experimen t consisting of crossed electron and atom beams. The effects of changes in gas pressure and interaction region geometry are monitored by a fixed polar imeter analysing radiation resulting from the 3(1)P to 1(1)S (53.7 nm) tran sition. This work extends the simulations of Humphrey with the incorporation of a f inite electron detector, a region of background gas and angular dispersion in the electron beam. Humphrey found that with an infinitesimal electron detector, the measured p olarizations were not strongly dependent on interaction region geometry. In this work, however, it is found that the coherence parameters are very str ongly dependent on interaction region geometry, pressure and the solid angl e of the electron detector. Even very small solid angles can adversely affe ct the measured parameters. While angular correlations are not specifically studied, there are indicati ons from simulations of electron scattering angle integrated polarizations that it is most important that any measurements involving a rotation of the photon detector should rotate about a symmetry axis of the gas source.