論文の公開元へ5. Geochemistry and Environmental Science
概要
INTRODUCTION: Technetium-99 is a pure beta emit- ter with a half-life of 2.11x105 (y) and is a fission product with its yield of 6.0%. Technetium forms anionic TcO - to be mobile species under oxidic condition. Hence, its mi- gration behavior in the environment is an important re- search field for the safety assessment on the disposal of high-level radioactive waste [1]. Investigations on the in- teraction of 99Tc with various materials in the environment require methods other than radiation measurement, such as spectrophotometry, due to its weak radioactivity. In this re- port the production of adequate amount of 99Tc tracer was investigated.
EXPERIMENTS: Technetium-99 was produced by the neutron irradiation of natural isotopic molybdenum. A 2 g of MoO3 powder was encapsulated in a quartz test tube under vacuum and the test tube was placed in an aluminum capsule filled with water. The capsule was irradiated for 47 hours at the reactor power of 1 MW and then for 6 hours at 5 MW in the Hydraulic Conveyer Facility (Hyd.) of the Kyoto University Reactor (KUR). Technetium-99 was re- covered by solvent extraction at 75 days after irradiation. The MoO3 powder irradiated was dissolved with NaOH and then contacted with methyl ethyl ketone (MEK) [2]. The radioactivity of NaOH and MEK phase was measured by -spectrometry. The amount of 99Tc produced was eval- uated from the radioactivity of 99mTc by assuming that 99mTc was in radioactive equilibrium with 99Mo at the end of irradiation.
RESULTS: The gamma ray spectrum of initial NaOH solution in the Fig. 1 shows no 145 keV gamma ray line of 99mTc on the contrary strong lines of 134Cs and 124Sb. The line of 99mTc is found in MEK solution after the solvent extraction, which yielded the distribution ratio of 134Cs and 124Sb was lower than 1/1000. Theses impurities would be completely removed by repeated solvent extraction. Be-sides 99mTc, in Fig. 1 shows the 155 keV line of 188Re, which was determined by its half-life of 0.71 d in Fig. 2. It is reasonable that 188Re was produced from tungsten as an impurity in MoO3. Tungsten is a homologous element of molybdenum; thus, its amount is likely to be larger than other elements. The radioactivity of 99mTc finally de- creased to lower than the detection limit, which shows no 99Mo extracted in MEK phase and high separation effi- ciency of 99mTc and 99Tc. The amount of 99Tc was evaluated from that of 99Mo at the end of irradiation to be 4.6 nmol, if 99Mo and 99mTc is totally decayed. In conclusion, 99Tc can be separated from Mo as a target material and anti- mony, cesium, and tungsten as an impurity by using sol- vent extraction with MEK and the irradiation of as much as 30 g MoO3 would yield an enough amount for an optical analysis.
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