How2Day: Chemicals That Glow Under X-Rays

Many metallic crystals glow under x-rays. ballyscanlon/Stockbyte/Getty Images
X-rays are part of the continuous spectrum of light waves, having wavelengths about 50 times shorter than visible light. The short wavelength gives x-rays high energy and the ability to penetrate most solid materials. The energy makes some chemicals glow, much as black light paints glow under ultraviolet light. You can use this glow to detect x-rays and produce images made by x-rays passing through objects.

Crystals of sodium iodide, mixed with traces of thallium, glow blue under an x-ray source. These crystals are usually used in scintillators---electronic detectors that depend on flashes of light they produce when exposed to radiation. Sodium iodide detectors are often used in research and nuclear medicine. This chemical is hygroscopic, that is, it absorbs moisture and this degrades it. A detector using this crystal must be tightly sealed.

Another metallic crystal, cadmium tungstate, glows a blue-green color under x-rays. Its glow is not as strong as sodium iodide, but unlike other chemicals, it has a very short afterglow. This feature makes more finely-detailed x-ray pictures possible. Computer assisted tomography (CAT) machines use cadmium tungstate crystals to make high-quality x-ray images.

Cesium iodide crystals produce different colors depending on traces of other materials used in the chemical. With thallium, it glows green, with sodium, violet. Cesium iodide crystal is softer and less brittle than sodium iodide, making it suitable for detectors used in mining and other rugged environments.

Zinc sulfide, a powder that looks white under normal light, glows yellow-green when exposed to x-rays. Scientists have used this common, easy-to-make crystal powder since the earliest days of x-ray research in the early 1900s.

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