an X-ray source

Before you can photograph a crystal's diffraction pattern you need a bright, steady beam of X-rays — think of it as a very special lightbulb whose light happens to have a wavelength about the size of an atom. The everyday workhorse that makes this beam is the sealed X-ray tube: a sturdy glass-and-metal cylinder pumped to vacuum, with a hot wire at one end and a lump of metal at the other. Flip it on and X-rays stream out through a thin window.

Here is what actually happens inside. A filament (the cathode) is heated so it boils off electrons, exactly like the element in a toaster glows. A high voltage of about 30 to 45 kilovolts pulls those electrons across the tube and slams them into a metal target (the anode), typically copper. The sudden stop turns their energy into X-rays in two flavours: a broad, smooth spread of wavelengths called bremsstrahlung, German for braking radiation, plus a few very sharp, bright spikes at fixed wavelengths that are characteristic of the target metal (for copper, Cu K-alpha at about 1.5406 angstrom). Only about one percent of the electrons' energy becomes X-rays; the other ninety-nine percent is waste heat, which is why the anode must be water-cooled or spun as a rotating disc.

The sealed copper tube is the beating heart of almost every laboratory diffractometer, and swapping the target metal (molybdenum, cobalt, chromium) changes the wavelength on offer. One honest, practical warning: pick the target to suit the sample. A copper tube shone on an iron-rich sample makes the iron atoms fluoresce, flooding the pattern with background noise — so for iron alloys crystallographers switch to a cobalt or iron target. When you need a beam thousands to billions of times brighter than any tube can give, you go to a synchrotron.