Efficient x-ray tube simulations.

This article describes an efficiency study of directional bremsstrahlung splitting (DBS) for x-ray tube modeling. DBS is shown to be up to five or six orders of magnitude more efficient at 50 or 135 kV tube potential than a simulation without splitting, and 60 times more efficient compared to uniform bremsstrahlung splitting. A methodology is presented to determine the optimum splitting number for a given situation using a second degree polynomial expression derived from theoretical considerations. Very large optimum splitting numbers are found for small fields (5 mm radius) at 1 m from the x-ray source, which are relevant for half-value layer (HVL) calculations and for simulations related to primary air kerma standards. Two approaches for the calculation of kerma at a plane and inside a volume using track-length estimation are implemented in BEAMnrc, a user-code from the EGSnrc Monte Carlo simulation system for photon and electron transport. A practical application of DBS to HVL calculations for a Comet MXR-320 x-ray tube is reported. The agreement with measured HVLs at different constant tube potentials is found to be better than 2.3%.