X-RAYS

Penetrating electromagnetic radiations having wavelengths shorter than those of visible light. They are usually produced by bombarding a metallic target with fast electrons in a high vacuum. In nuclear reactions it is customary to refer to photons originating in the nucleus as gamma rays, and those originating external to the nucleus as x-rays. These rays are sometimes called Roentgen rays after their discoverer, W. C. Roentgen.