Final answer:
High capacity x-ray tubes have copper or molybdenum layered under the tungsten target. These materials are used to manage the heat generated by the X-rays produced due to the high photon energy and deceleration of electrons in the tungsten.
Step-by-step explanation:
High capacity x-ray tubes have copper or molybdenum layered under the tungsten target. The purpose of these materials, which have high thermal conductivity and lower atomic number than tungsten, is to spread out the heat due to the high photon energy absorption. X-ray tubes require materials like tungsten for the anode because when electrons are projected onto them, they produce X-rays as they are decelerated.
The production of these X-rays is due to the filling of inner-shell vacancies, with tightly bound inner-shell electrons, that occur in the tungsten material. This involves significant photon energy that becomes larger for heavier elements as their energy increases approximately as Z² (atomic number squared). Moreover, the heat generated from the absorption of energy by the tungsten necessitates that underlying materials like copper or molybdenum diffuse this heat effectively.