a blood counting device comprises a capillary conduit for drawing from a subject in which a radiotracer has previously been injected a quantity of blood in the micro - liter range to produce in the capillary conduit a flow of blood from which beta radiation is emitted . at least one direct beta radiation detector is placed closely adjacent to the capillary conduit . the direct beta radiation detector consists of a semiconductor photodiode which detects the beta radiation from the flow of blood when directly hit by this beta radiation .