Bremmstrahlung target optimization for reflex triodes

Coupled particle-in-cell (PIC) and Monte Carlo simulations of the reflex triode have been performed with tantalum foil thicknesses varying between 2.5 mum (0.0056 times the CSDA range at 1 MeV) to 250 mum (0.56 the CSDA range at 1 MeV). The PIC/Monte Carlo simulations are in good agreement with reflex triode experiments on Gamble II at 1 MV, 1 MA. Experimental measurements and simulations both show that the dose is maximized for a foil thickness of about 20 mum. For foils thicker than 20 mum, the analysis shows that fewer of the 10-100 keV photons escape the foil reducing the dose. For foils thinner than 20 mum, the dose decrease is due to a loss of electron confinement to the foil allowing electrons to drift radially outward and strike a low-atomic-number foil holder which causes the dose to decrease. An examination of the electron orbits shows that for all foil thicknesses electrons initially flow radially inward under the influence of the strong self-magnetic field. If the foil is thick, then electrons lose a significant amount of energy with each interaction with the foil and are absorbed close to the point where they initially interact with the foil. If the foil is thin, electrons lose very little energy with each pass. For very thin foils, the simulations show that, with each pass, the electrons move outward in radius a distance of approximately twice the Larmor radius. Therefore, for thin foils, there are a limited number of passes the electrons can make before moving out of the diode where they strike the foil holder. Based on these results, a formula is derived that is able to predict fairly well the anode thickness that optimizes the dose.