Answer: Diffraction occurs at all the angles of 2q simultaneously in powder samples.  In order to obtain a diffraction pattern, the detector (in most designs) rotates to various 2q angles to measure diffraction from the sample.

Below is a schematic diagram for a powder X-ray diffractometer, showing the rotating detector.

The source shown is an X-ray tube, which is the most common source of X-rays.   Filters are used to provide a narrow wavelength range for analysis. 

Question:
Why is a monochromatic source desirable for XRD analysis?