[en] The invention is for checking a photomultiplier device for use in detecting radiation, in which a covering opaque only to light is provided over the photomultiplier head. It comprises a light source, a calibrated source of the radiation and a means of locating the photomultiplier head so that the opaque covering is exposed to both. For alpha particle detection the opaque covering may be aluminium foil; if this is damaged extraneous light may enter the photomultiplier head and spurious readings result. The radiation source used in the device may be a calibrated uranium disc or may be formed by evaporating a dilute solution of plutonium nitrate onto a stainless steel disc. (U.K.)