Numerical simulation of a reflex triode

Summary form only given. Electron beams are often used to make bremsstrahlung X-rays for a wide variety of applications. In a standard bremsstrahlung converter, electrons are accelerated from a cathode onto a high-atomic-number (high-Z) anode target. Since this target is usually less than the continuous slowing down approximation (CSDA) range, the high-Z target is usually followed by a low-Z beam stop. Some of the X-rays that are created in the converter get absorbed before they can escape. The absorption is spectrum dependent with the softer portions of the spectrum getting preferentially absorbed. The optimum target thickness is determined by making the target thick enough to produce a significant number of X-rays and yet thin enough to minimize X-ray absorption. Coupled Monte-Carlo electron/photon transport analysis shows that the X-ray dose is maximized when the target is about 1/3 the CSDA range and the beam-stop is about 2/3 the CSDA range. In a reflexing converter, electrons are electrically trapped on a high-Z foil by either a virtual cathode (reflex diode) or a second cathode (reflex triode) causing them to make several passes through the foil. Each time the electrons pass through the foil they make X-rays. Since the target foils can be much thinner than 1/3 the CSDA range, the dose from a reflexing converter can be substantially larger than a standard converter. To achieve this dose increase, the product of the number of passes and the foil thickness should be a substantial fraction of the electron CSDA range. Since electrons are trapped on the anode, additional dose gains can be realized by eliminating the beam-stop. The ion current in a reflex diode can be a large portion of the total current. This can result in tremendous X-ray production inefficiency. This paper presents results from 2D LSP particle-in-cell simulations of the reflex diode and reflex triode. The output of LSP has been coupled with the integrated tiger series of radiation transport codes to predict the radiation field. The results show that the X-ray dose can be increased over the non-reflexing diode if ion production is controlled. Most of this increased dose is the result of an increase in the number of soft photons