Optimization of Bremsstrahlung Energy Deposition

Thick-target bremsstrahlung energy deposition in a thin, gold calorimeter has been measured as a function of converter foil thickness for Mo, Sn, La, Ce, Pr, Nd, and Ta converters at source electron energies of 0.5, 0.75, and 1.0 MeV. The thickness-maximized deposition is not a monotonically increasing function of converter atomic number, but instead exhibits a broad maximum near the type 4f rare-earth region. This maximum is substantially greater than the deposition from the Ta converter--Ta being a material routinely employed in flash x-ray sources. Theoretical models also predict higher deposition from La converters than from Ta converters and are used to show that this enhancement is a consequence of higher characteristic x-ray production in La. Nevertheless, there remain significant discrepancies between theoretical predictions and measured data, even when improved numerical cross sections which have just become available are employed in the calculations.