However, the vast majority of the power used in generating X rays produces heat. These tubes are limited in the power that can be applied to the tube by the amount of heat that can be dissipated through water cooling. One way to increase the heat dissipating ability of the system, and thus increase the X-ray beam intensity, is to move or rotate the anode surface so that the beam of electrons continually hits a new region of the anode. The flux of photons of this radioactive material is so low that it is not used as a source of X-rays for diffraction experiments. However, small samples of 55 Fe are often used to test the functioning of X-ray detectors.
Synchrotrons produce the highest flux sources available.
Unfortunately, because synchrotrons are very expensive to build and maintain, there are few such sources available throughout the world. The characteristic wavelengths and excitation potentials for these materials are shown below. Copper radiation is preferred when the crystals are small or when the unit cells are large. The scintillation detectors, typically used in small molecule diffraction, have somewhat higher quantum efficiencies for molybdenum radiation than for copper radiation. Thus many labs use molybdenum radiation exclusively.