Building Your Own X-Ray Detector Screen

Fluoroscopy is probably the best-known method of X-ray imaging: an X-ray beam passes through the subject to be imaged, and the transmitted X-rays illuminate a phosphor screen. Dense objects, such as metal or bone, cast a shadow on the screen, which provides a real-time image of the subject’s interior. Already having access to X-ray sources, [MarcellF]’s next step was to investigate common phosphor materials, then synthesize his own.

Most common materials that fluoresce under ultraviolet light showed no activity under X-rays: fluorescein, quinine, UV fluorescent paint, and common fluorescent minerals emitted no noticeable glow under 80 kV X-ray stimulation. However, strontium aluminate phosphors did fluoresce well, with a strong afterglow, as did the phosphors in a fluorescent light bulb, some LEDs, and an electroluminescent panel. The electroluminescent panel, which used a zinc sulfide phosphor, was almost as bright as the gadolinium oxysulfide screen from a CT scanner’s detector and had no noticeable afterglow.

One well-known X-ray phosphor is scheelite (calcium tungstate), which [MarcellF] next synthesized. He had previously tested a sample of natural scheelite without success, probably due to impurities. The first step of the synthesis was to melt together potassium nitrate and sodium carbonate, in which [MarcellF] dissolved broken pieces of a tungsten TIG welding rod. This formed sodium and potassium tungstates, which were dissolved and reacted with a calcium chloride solution. This precipitated calcium tungstate, which [MarcellF] annealed to make fluorescent. This produced a blue glow under X-ray stimulation, and doping with lead atoms made it significantly brighter.

We’ve covered several methods of X-ray detection before; most modern fluoroscopes now use a phosphor screen in conjunction with a camera, or sometimes with a photomultiplier tube.