57 The absolute counting efficiency of each instrument was determined as a function of y-ray energy by counting a series of standards prepared in the Same geometry used to count the samples. Each standard contained a known amount of a radienuclide whicn was obtained from the N.B.S. or a supplier. commercial An aliquot of each standard solution was added to an acrylic casting resin and homogenized by stirring until the resin set. Each encapsu- lated standard was thus uniformly distributed in the volume of the counting container at a standard density of 1.1 g/cc. The results of these calibra- tions are shown in Fia. 10. The results of interlaboratory comparisons on the concentrations of gamma emitting radionuclides in environmental samples measured by this, and other, laboratories are shown in Table 12. The concentrations measured in a seaweed sample agreed well with the reported probable concentration, The 1376, measured jn the sediment sample was in slight disagreement with the reported probable concentration. In spite of the cause of the discrepancy in the 3 Cs values, the agreements are within 11% in these environmental samples. Because each of the bulk sediments which were y-counted in this work had varying bulk densities (average density of 75 samples was 1.12 g/cc), the cerversion factors used to calculate the concentration of each isotope in ech sample varied somewhat from those used to calculate the concentrations ‘- radionuclides in the IAEA standard samples shown in Table 12. a€ Since the to dpm conversion factors which are needed to calculate the absolute "é-lenuclide concentrations from raw counting data are complex functions of ‘*eeral variables, including gamma-ray energies and bulk densities, the ‘ . =e ‘ions relating these variables were not determined. Instead, new cpm to conversion factors were determined, using standards prepared at a bulk “""3ty of 1.35 by adding NaCL to the acrylic casting resin. The appropriate