Apparatus and methods for detecting evaporation conditions in an evaporator for evaporating metal onto semiconductor wafers, such as GaAs wafers, are disclosed. One such apparatus can include a crystal monitor sensor configured to detect metal vapor associated with a metal source prior to metal deposition onto a semiconductor wafer. This apparatus can also include a shutter configured to remain in a closed position when the crystal monitor sensor detects an undesired condition, so as to prevent metal deposition onto the semiconductor wafer. In some implementations, the undesired condition can be indicative of a composition of a metal source, a deposition rate of a metal source, impurities of a metal source, position of a metal source, position of an electron beam, and/or intensity of an electron beam.