X-Ray Diffractometer

Overview

An X-ray diffractometer identifies what a crystalline material is made of and how its atoms are arranged. When a monochromatic X-ray beam strikes a crystal, the regularly spaced planes of atoms scatter it, and at certain angles the scattered waves reinforce one another, Bragg's law. The pattern of angles and intensities is a fingerprint: every crystalline phase has its own, so a powder diffraction scan tells you whether a sample is quartz or calcite, which polymorph of a drug crystallized, or how much of an alloy is one phase versus another.

The measurement is fundamentally geometric. The instrument's job is to deliver a clean beam to the sample and to record, with great angular precision, the angles at which diffraction peaks appear.

How it works

X-rays come from the X-Ray Tube & Generator. Inside the X-Ray Tube, electrons accelerated by tens of kilovolts slam into a metal anode and knock out the element's characteristic X-ray lines; a high-voltage generator powers it and a shutter gates the beam. Before reaching the sample the beam passes through the Optics & Monochromator, where slits set its divergence and a monochromator and filter isolate a single wavelength.

The heart of the instrument is the Theta-Theta Goniometer. In theta-theta geometry the tube and detector swing through equal, opposite angles about a stationary sample, driven by geared servo motors and read by optical encoders to a thousandth of a degree. The sample sits on the Sample Stage, which spins it to average over many crystal orientations.

Diffracted X-rays land on the Strip Detector, a silicon-strip detector that counts photons at many angles at once, turning a slow point-by-point scan into a fast parallel measurement. The whole goniometer lives inside the Shielded Enclosure, whose door interlocks and safety relays shut off the beam the instant it is opened. The Control & Computer sequences generator, motion and detector and inverts the recorded pattern into phase identifications, while the Cooling System system carries heat away from the anode.