Actually, the radiation - the "x rays" - in the dentist's x-ray machine is generated with non-radioactive materials. X-rays, which are themselves "radiation" in the sense that they can cause radiation injuries the way that gamma rays from radioactive materials can, are generated when a stream of electrons with a very high velocity is generated at the cathode end of a vacuum tube and then strikes the anode end. Most of the energy of those electrons is turned into heat, but a small percentage of that energy is released as very high energy photons - which are the x-rays.
. X-radiation (composed of X-rays) is a form of electromagnetic radiation
. in medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image. Hence, a thin metal sheet, often of aluminium, called an X-ray filter, is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is called hardening the beam since it shifts the center of the spectrum towards higher energy (or harder) x-rays.A radiograph is an X-ray image obtained by placing a part of the patient in front of an X-ray detector and then illuminating it with a short X-ray pulse. Bones contain much calcium, which due to its relatively high atomic number absorbs x-rays efficiently. This reduces the amount of X-rays reaching the detector in the shadow of the bones, making them clearly visible on the radiograph. The lungs and trapped gas also show up clearly because of lower absorption compared to tissue, while differences between tissue types are harder to see.http://www.sciencedaily.com/releases/2009/03/090303125809.htm