axial homogeneity as well as excellent agreement with isotope dilution ma55 spectrometry, these results are assumed t o be typical ofthe bulk. Since track counting may be considered t o be a special case of general nuclear counting where residual effects are observed, as opposed t o more conventional detection technique, the uncertainty due t o counting varies directly a s the square root of the number of events observed. Therefore for low concentrations of uranium and/or boron, counting statistics will be the limiting factor in precision. The accuracy appears t o be competitive with other analytical techniques. Uranium analyses were also made by isotope dilution mass spectrometry by two independent laboratories, NBS and the United States Geological Survey Laboratory in Denver. The comparison of the results by the three laboratories is given in Table V. Since the neutrons used t o irradiate the sample were highly thermalized, ~ 2 subcadmium 4 t o 1 epicadmium, the thorium contribution t o tracks production was negligible (
