Systematic errors in angularcorrelation measurements of the 3 1P state of helium

Abstract

A Monte Carlo study is made of the effect of severalexperimental variables on the results of angular correlationexperiments. The simulation examines the intensity of the53.7 nm radiation resulting from the decay of the electronimpact excited 3 1P helium state to the 1 1S stateat an impact energy of 30 eV.Crossed electron and atomic beams define the target, which isviewed in coincidence by electron and photon detectors.Previous work (Humphrey I 2001 Meas. Sci. Technol. 12 246-63) has examined systematic effects in polarisationcorrelation measurements of this atomic state. The current workextends that work to angular correlation measurements.The code used in this paper has been improved considerably overthat used in the previous study, with much more accuratesimulations of the angular distribution of radiated andre-radiated photons. It also uses what is generally consideredto be a more reliable data set (the 111 state close couplingcalculation results of Fursa and Bray), and a very much largernumber of iterations.Chief amongst the results of this paper are marked differencesbetween the pressure dependencies of polarisation and angularcorrelations, severe distortion of the extracted parameters dueto non-coincidence of detector rotation and gas jet symmetryaxes, and a strong dependence of the angular correlationparameters on electron detector solid angle.