Studies of beam formation and rest gas attenuation in atomic beam sources

At the SpinLab test stand (University of Ferrara), the elastic scattering cross section for molecular hydrogen has been determined for relative velocities in the range 1500-3000 m/s from the measured attenuation of a beam traversing a chamber of rest gas. This cross section is a fundamental input for simulations of gas flows in an atomic beam source with the Direct Simulation Monte Carlo method. Both the predicted beam intensity and width of the beam's velocity distribution are sensitive to the cross section value, and agreement with the measured values is demonstrated for the first time. The simulations also predict properties of the expansion that are difficult to access experimentally, namely a peaking factor of 1.45 +/- 0.05 that is essentially independent of input flow in the range 25-250 sccm, and an increase in the beam divergence at the skimmer exit for higher input flows.