130Teの二重ベーター崩壊半減期

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The half-life of 130Te double β-decay was determined by a mass-spectrometric method. The samples were old tellurium ores (such as tetradymite: Bi2Te2S), obtained from Oya gold mine in Japan. The amount of 130Xe, which was occluded in the tellurium ores as the daughter of 130Te double β-decay, was measured by isotope dilution method using 128Xe as a spike. The mass spectrometer used is 20 cm radius, 90° single focusing type. The detection limit for xenon under static operation is about 105-106 atoms by using a 12-stages Cu-Be electron multiplier followed by a vibrating reed electrometer as a measuring system of ion currents. Th spike 128Xe used was prepared by neutron irradiation of KI in a nuclear reactor. The amount of four samples used were 17.5g (33.8% Te), 10.9g (13.2% Te), 22.3g (22.4% Te) and 12.4g (22.4% Te) . The decay products: 130Xe, occluded in tellurium ores, were extracted by heating them in an electric furnace at about 700-1200 °C. Pure calcium metal, titanium-filament, tangsten-filament and a dry ice trap were used to remove impurity gas components. In the mass spectra, excesses of 129Xe, 130Xe and 131Xe over normal xenon were found, and the excess of 130Xe was predominant among them. The excess amount of 130Xe obtained were(2.09±0.27) × 10-11 ccSTP, (1.01±0.18) × 10-11 ccSTP, (2.65±0.08) × 10-11 ccSTP and(1.27±0.06) × 10-11 ccSTP for above each sample, respectively. On the assumption that the 130Xe excesses obtained above were entirely due to the 130Te doubleβ-decay, the half-lives of 130 Te doubleβ-decay were calculated to be10.7 × 1020 years, 5.38 × 1020 years, 7.29 × 1020 years and 8.27 × 1020 years for the above each case. The half-life of 130Te double β-decay is(7.91±1.11) × 1020 years on the average. This value agrees fairly with the result(1.4×1021 years)of Inghram-Reynolds, the result(3.3×1021 years)of Hayden-Inghram and the theoretical value(2×1021±2 years)of Primakoff-Rosen, based on two neutrino theory. The xenon age of the tellurium ores, which was used for calculation of the above half-lives, was determined from the age of porphyrite, of which the age is expected to be about the same as that of the tellurium ores. The age of porphyrite was determined to be(9.06±0.29)×107 years by K-Ar method with the same mass spectrometer as used for the above 130Xe measurement. The ratio of 129Xe excess to 131Xe excess is obtained about 1.6, and the ratio, estimated from the neutron absorption cross sections for128Te and130Te and their abundance ratios, is about 0.6. These ratios are not in agreement one another. So these excesses in 129Xe and 131Xe seem to be produced, in part, by neutron absorptions of 128Te and130Te and, in part, by different processes.
日本質量分析学会の論文

130Teの二重ベーター崩壊半減期

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