2001 年 121 巻 5 号 p. 327-344
The disintegration rates of 222Rn and its daughters in natural water were determined successfully by the use of the integral counting method with a liquid scintillation spectrometer. A significant advantage of this method is its freedom from the quenching effect. Moreover, when plural α-, and β-emitters are present, their total amounts can be determined. The simple extrapolation of integral counting curve to zero pulse-height, however, do not give the true disintegration rate for the soft β-emitters (Emax<200 keV), because the liquid scintillator (LS) has a relatively high detection threshold. Therefore, the zero detection threshold of the liquid scintillation spectrometer was determined by measuring standard 3H samples, and a modified integral counting method which extrapolates the integral counting curve to the zero detection threshold was proposed. The method has been successfully applied to various β-emitters, 222Rn samples, and coloured samples of β-emitters, giving more accurate absolute disintegration rate than the conventional integral counting method and the efficiency tracing method. In the course of the study determining 222Rn by liquid scintillation counting, we observed unexpected phenomena; the air luminescence from gaseous space above LS, and the temperature dependence of pulse-height spectra. As for the former phenomenon, we proposed a method for correcting errors due to air luminescence, a method for determining α-emitters by the air luminescence, and a rapid calibration method for 222Rn detectors. As for the latter phenomenon, we observed that the pulse-height spectra for α, and β-emitters in LS are shifted toward higher pulse-height with decreasing temperature. We found that the fluorescence intensities of the solvent of LS (toluene) is promoted at lower temperatures, and that not only toluene, but also the fluorescence intensity of a number of aromatic hydrocarbons and aliphatic hydrocarbons show the same effect as toluene. Other unexpected results are existence of metals in number of enzymes, and discrepancies between the experimental value for Kurie plot of allowed β-emitters and the value which would be expected according to Fermi’s theory, the results of which would affect the transmission probability of potential barrier for α-particles.