A simple method using standard radiation survey meters is given for monitoring drinking water against bomb fallout. The y-field associated with a suitable volume of water is measured and compared with the fields expected from permitted concentrations of fission products of known age. The fields produced by these permitted concentrations have been tabulated, and the concentrations are themselves chosen consistent with U.S. and British recommendations. The validity of the technique is examined from a number of points of view and has been checked by experimental studies. The method is generally applicable with minor limitations whether or not water treatment has been undertaken.

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