Modeling a fan beam X-ray source Package GamBet Input files FanBeam.MIN, FanBeam.DST, FanBeam.GIN, FanBeamPlot.GIN Download FanBeam.zip Description This example illustrates the application of GamBet to determine the conversion efficiency and energy spectrum of a source to produce a fan-shaped X-ray beam. Parameters are based on the COMET MIR-180FB tube. The upper figure shows the geometry to model the interaction and extraction region. A cooled tungsten target of radius 10.0 mm is enclosed in a copper housing inclined at an angle of 20° with respect to an incident electron beam moving in the +z direction. The housing has inner radius 12.5 mm and outer radius 25.0 mm. An extraction slot cut into the housing has of height 4.5 mm in z and covers an angle of 110° in the x-y plane. The slot cut leaves a vacuum window of thickness 1.0 mm. The housing and slot are surrounded by a boundary region assigned the EXCLUDE property in GamBet so that particle trajectories stop when they enter. The right side of the upper figure shows a three-dimensional view of the target and copper surfaces from the direction of the incident electron beam. The model demonstrates several useful features in MetaMesh and GamBet: Building a complex geometry using basic MetaMesh shapes including the EXTRUSION model. Using of a shaped region with the EXCLUDE property to define a non-planar stop boundary. Applying multiple filter criteria to limit entries in the escape file. Defining a GamBet input file with a reduced number of showers to check emitted X-ray trajectories. Using a forcing function for bremsstrahlung interactions to improve output statistics. Results The GenDist input file FanBeam.DST generates a source file FanBeam.SRC representing a 10 mA electron beam at 180 keV uniformly distributed over a rectangle of dimensions 2.3 mm in x and 2.7 mm in y. The 800 model electron trajectories start just below the inclined target and move in the +z direction. The left side of the upper figure shows the dose distribution at z = 0.0 mm in the inclined target. The central figure shows representative trajectories of X-rays projected to the planes x-y and z-x generated from the file FanBeamPlot.GIN. Only a small fraction exit the extraction slot. The production run with NPMult = 500 simulates 400,000 showers. With a Bremsstrahlung forcing factor of 25, about 1700 photons reach the exit of the extraction slot with average energy 93.21 keV. The photon flux is 1.10 × 1013 photons/sec, giving an X-ray power exiting the slot of 0.1639 W. The incident electron power is 1800 W, so the energy conversion efficiency is less than 0.0091%. The lower figure shows the spectrum of photons leaving the slot. The absence of low energy X-rays results from attenuation in the 1.0 mm thick copper window.