People usually called secondary X-ray produced when X-ray irradiate on the material as X-ray fluorescence (X-Ray Fluorescence), while call the original X-ray as the X-rays which is used to irradiate. So the X-ray fluorescence is still the X-rays.
A typical X-ray fluorescence (XRF) instrument consist of the excitation source (X-ray tube) and the detection system structure. The X-ray tube generates incident X-rays (primary X-rays) to stimulate the sample which is to be tested. Each element of the excited samples will emit secondary X-rays and different secondary X-rays emitted by different elements have a specific wavelength characteristics or energy characteristics. By detecting the specific energy and quantity characteristics of these emitted secondary X-rays on Detection System, and then, the instrument software of the detection system convert the information collected into the type and content of various elements from the sample. And the secondary X-ray generated when X-ray irradiate on other materiasl is referred to X-ray fluorescence.
Using this X-ray fluorescence principle, we can measure every element behind beryllium in the periodic table theoretically. In practice, the effective measurement range of the elements is the elements from 11 (Na) to the 92 (U).