Ray Traces and Pinhole Images of the Xoft Axxent™ X-ray Source Steve Axelrod, Thomas W. Rusch, Zirao Zheng – Xoft, Inc Tom Case, Brian Wilfley – Triple Ring Technologies

The Xoft Axxent™ X-ray Tube 2.2 mm diameter Runs at 50 kV, 300 µA Breast brachytherapy X-Ray Tube HV Cable

The Xoft Axxent X-ray Treatment System Controller Source and balloon applicator

Motivation For the Study To develop and manufacture a miniature x-ray tube, one needs to understand the detailed physics Electron beam transport critical performance parameter (Ideal) geometry can be modeled Beam distribution at anode determined with X-ray pinhole images Output compared to actual behavior A vetted computer model is a powerful tool in design and manufacturing

Ray Traces of Electron Trajectories Electrons emitted from hot cathode accelerated by the 50 kV potential, impact the anode, create x-rays Right: very fine mesh defined near filament Modeling program is OmniTrak, by Field Precision LLC Below: less fine mesh transports electrons  anode

Electron Beam Intensity Distributions Filament launch points weighted by a thermal model of the filament temperature profile Followed by a Richardson-Dushman thermionic emission/extraction model Left: side views Below: end view

Modeled Effects of Filament Misalignment Offset filament a few 0.001” crosswise and lengthwise Determine manufacturing tolerances Misaligned crosswise Misaligned lengthwise Above: aligned just right

Pinhole Images - Principle A very good way to see things upside down Key fact: The last Sunday in April is “Worldwide Pinhole Photography Day” - Wikipedia

Pinhole Images – Apparatus Description Source held inside a brass enclosure, with a 30 μm pinhole Can look at either the end or the side of the anode ShadoCam image plane* used to capture x-ray images X-ray sensitivity 10 – 50 kV The x-ray launch points closely match electron distribution Little scatter in the anode Ran sources at various kV and μA operating points Typically integrated for 10 seconds * Shad-o-Snap 1024, Rad-icon Imaging Corp

Sample Images – Side View Model prediction  Actual images

Sample Images – “Typical” End View Central diamond and surrounding oval are clearly visible

Effect of Pinhole Aperture Diameter 400, 100 and 30 μm pinholes Source was an experimental design

Out-takes Not all sources built pass the stringent criteria for human use Polar, azimuthal, depth dose, output level, output stability Not all have pretty pictures…

Rogues’ Gallery of Images Not Human Use Tubes!

Conclusion Beam optics modeling provides important insight into design parameters And allowable ranges of variation Major features of the electron beam distribution predicted by computer modeling are observable in pinhole x-ray images of the sources Variations seen among pinhole images of different sources do not correlate with spatial distributions significantly until the variations are extreme

The End Thanks for sticking around!