Manufacturing and the use of the positron-emitting radionuclide keep expanding as the number of domestic PET facilities increases. On the other hand, it was difficult to measure accurately such a gassy short half-life nuclide as15O whose half-life is 2.037 minutes. Then a detector using GSO scintillator for activity concentration in air was developed from the view point of the characteristic of the positron-emitting radionuclide. The detector has cubic structure, four planes of which consist of a cubic scintillator and the other planes are open. This detects two annihilation photons emitted in the opposite direction simultaneously by facing two scintillators with energy discrimination. The number of coincidence signals from each detector pair is counted in a moment. Consequently, only the positron-emitting radionuclide that exists in the space (125 mL in case of this detector) becomes detectable, and the radioactive concentration of the unit volume can be measured in real time. Postulating15O2, it was confirmed that 5.2×10-2Bq/cm3 was detectable in 60 seconds by using and evaluating 511 keV photon from22Na.
The activity ratios of224Ra/228Ra and228Th/232Th in leachate from euxenite increased with increasing pH, while the activity ratio of 230Th/232Th decreased. The leaching behavior of224Ra which is formed through four decay processes of232Th, was different from that of228Ra which is formed by a single decay of232Th. Thorium-228, decay products of232Th, was also leached more effectively than232Th. However, 230Th which is formed through four decay processes of238U showed different leaching behavior from224Ra and228Th. Variation in the activity ratio of 228Ra/226Ra in leachate from euxenite sample in metamict state with pH of the solution in contact showed no apparent tendency and was observed to be almost as same as that in the euxenite sample partly because of so small activity ratio in the sample. The leaching efficiency of Ra isotopes and228Th was estimated to be mainly dependent on the difference in the history of decay processes which have formed the nuclides. This estimation was consistent with the observation with the monazite sample reported previously.
To study the effect of rhizosphere pH on the cadmium (Cd) uptake, soybean seedlings were grown for 4 days in 0.2 mM CaC12solution containing l μM Cd under pH 4.5 or pH 6.5 using109Cd as a tracer. The concentration of Cd in root at pH 6.5 was 3.4 times higher than that at pH 4.5. Cadmium amount adsorbed on root surface at pH 6.5 was 1.3 times higher than that at pH 4.5. With high accumulation of Cd in root, Cd toxicity such as brownish of the tissue and the suppression of lateral root development was only observed under pH 6.5 condition. Transpiration rate was found to have no relation to high accumulation of Cd in shoot.
A simple source preparation method was developed for α-ray spectrometry to determine U and Th isotopes in silicate rock samples. Samples were decomposed by the mixed acid of HF-HCIO4-HNO3, and U and Th was isolated by use of UTEVA Spec. resin. Isolated U and Th were then extracted into TTA-benzene solution for purification. The organic phase was evaporated on a hot stainless steel planchet to dryness. This method is effective for the speedy preparation of thin sources of U and Th for α-ray spectrometry.