Dose-voltage dependence for HELIA/HERMES III bremsstrahlung diodes

The relation D α IV/sup 2.65/ is widely used to estimate the on-axis radiation dose rate, D, downstream of an optimized bremsstrahlung target as a function of diode voltage, V, for flash x-ray sources. This relation is valid only for pencil beams. For diodes having beams with finite spatial and angular extent, the power 2.65 is reduced. This reduction is evaluated via the MAGIC and CYLTRAN codes for diodes operating on the projected 20 MV HERMES III and existing 3 MV HELIA accelerators. For HERMES III, a power of 2.2 is obtained for near-field exposures using a fixed converter optimized at 20 MV. For HELIA, a power of 1.8 is obtained when the change in electron flow with voltage is also taken into account. This voltage dependence, together with the measured voltage and current waveforms on HELIA is used to calculate the expected temporal shape of the radiation pulse. The shape is consistent with that measured using a PIN diode