THE DETERMINATION OF ZERO‐THICKNESS ACTIVITY IN GEIGER COUNTING OF C 14 SOLUTIONS USED IN MARINE PRODUCTIVITY STUDIES

Liquid scintillation counting methods are described for the determination, with a precision of ± 2%, of the absolute activities of C 14 stock solutions to be used in marine productivity studies. The efficiency of a windowless Geiger counter was found to be 58% by counting very thin films of labelled plastic whose absolute activities were then determined by liquid scintillation counting. The zero‐thickness Geiger activities of several C 14 stock solutions were determined with a precision of ± 3%, and an accuracy of ± 5%. This method was compared with the method of extrapolating self‐absorption curves of BaC 14 O 3 planchets of varying thicknesses. It was found that the self‐absorption curve between 0.5 and 6.0 mg/cm 2 conformed equally well to an exponential as to a hyperbolic function. Extrapolation of these functions to 0 mg/cm 2 gave zero‐thickness activities 24% and 17% respectively lower than those obtained by the new method described, showing that the self‐absorption curve varied considerably from both the exponential and hyperbolic function at thicknesses below 0.5 mg/cm 2 . This is attributed largely to back‐scattering. Empirical self‐absorption curves were found to be unsatisfactory due to the impossibility of preparing planchets of BaCO 3 in the critical range of 0 to 0.5 mg/cm 2 .