fission products is made negligible by the or each meter volume 101 102 is specially shaped with suitably flared walls in order to optimize the volume to wall area ratio so that the detector 200 can never see the side walls of the meter volumes even when looking through the largest diameter collimator as a result background noise due to back scattered gamma radiation is minimized the 34 high gloss 34 finish of the curved surfaces or walls perpendicular to the detector axis serves to further reduce the background noise the activity of the water in the meter volume 101 and 102 is measured by a detector 200 connected to a measurement and control circuit 300 which may be located at a distance from the measurement device and which may even be common to a plurality of measurement devices a turntable 400 is also provided whose function is explained below the gamma radiation detector is constituted by a hyperpure germanium crystal which transforms the gamma radiation it receives into variations in electrical voltage when it is biased by a very high voltage this detector operates at the temperature of liquid nitrogen and for this purpose it is thermally connected to the contents of a cryostat 201 fed from a liquid nitrogen receptacle valves 20 to 29 serve to feed a selected one of the two volumes 101 and 102 in a manner which depends on the position of the valves and may either be continuous with the fluid flow being immediately recycled downstream or else discontinuous with the volume being filled and once filled being closed off upstream and downstream in order to retain fluid for a certain length of time for the.