Abstract: A modular solid-state detector cell which intimately associates a scintillator body with a pair of photoresponsive semiconductors so as to present on a single collimator plate all of the elements necessary for transforming incident X-radiation into a measurable electrical signal. The detector is provided with a precision slotted mounting arrangement for receiving the unit cells, and the cells when positioned within the detector assembly have the photoresponsive semiconductors enclosed within the light tight cell. The unit is adaptable to off-line testing of the separate cells for screening before assembly.