Secondary x-ray fluorescence is an effect where a material sample is illuminated by an incoming x-ray source in response to which the sample's constituents emit responsive x-rays of their own. Those secondary, re-radiated x-rays are at wavelengths that are signatures of the sample's constituents.

When I was working at Bertan High Voltage, we were told by our then-client that quantitative analysis of the subject stuff's component materials could be derived from measurements of those secondary emissions.

We made the high-voltage power supply that ran a vacuum tube x-ray source. The high-voltage power supply was simply that, a programmable high-voltage supply that could be set to a desired voltage. The x-ray tube's current was controlled by variable drive to the tube's filament. Feedback control of the beam current, the electron stream, was achieved by way of variable filament power. In short, the x-ray tube was always run at thermionic saturation.

high voltage power supply and current controlFigure 1 High-voltage power supply and current control

Although this arrangement used a grounded cathode configuration, similar arrangements could be made for grounded anode operation with the plate tied to ground, with the high voltage being of negative polarity (think of −50 kV) and the variable filament drive being high voltage isolated.

Eventually, the above system became a standard product at Bertan High Voltage, called the XR-50P. The tube for which this was initially developed was the Amperex RI-50P.

All of that was many years ago and today, I can't find any online references to any of these goodies. However, the operating principles are still valid and there is equipment out there still being sold for this purpose.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).