Radiography with noble gas containing detector cells

Radiographic apparatus comprising means defining a patient position, a source of penetrating radiation, such as X-radiation, arranged to project said radiation through said position along a plurality of substantially co-planar beam paths, means for scanning said source relative to said patient position so as to project said radiation through said position along further beam paths, detector means for detecting the radiation emergent from said position along said paths, including a plurality of cells containing a noble gas and means for sequentially deriving, from each cell, output signals indicative of the amount of radiation emergent from said position along a respective group of said paths, said amount being indicated in each case by the number of electrons and ions of said gas generated by radiation projected through the position along the respective path, means being provided for periodically interrupting the incidence of said radiation upon said cells to provide for the clearance from each cell of ions generated by radiation projected through said position along one of said group of paths before the cell is exposed to the radiation projected through said position along the next path of its respective group.