Abstract
Abstract The decay of 138La, the middle member of a naturally occurring isobaric triplet, is studied by means of heavily shielded scintillation and proportional tube spectrometers. The existence of only two of three γ-rays reported by earlier investigators is confirmed, their energies bcing (0.81±0.01) and (1.44±0.02)MeV. No annihilation quanta were observed. Coincidence studies show that these γ-rays are not in cascade, and that the 1.44 MeV transition belongs to the electron capture branch. A previously unobserved β-ray spectrum with maximum energy (205±10) kev is detected. The experimental log ft value of 19.0 and known spin change ΔI = 3 indicate a 3rd forbidden transition. No evidence of a reported 1.0 Mev β-transition is found. A new decay scheme is proposed and agrees with the results of independent investigators on the β× and β+ activity of 138Cs and 138Pr respectively. The partial half-life of tlie electron capture branch is found to be (1.64±0.06) × 1011 yr, and of the β branch (4.1±0.7)×1012yr. and (3.5±0.3)×1011 yr, measured by direct counting of the β-rays and by calculation of the total counts due Lo the 0.81 MeV γ-ray respectively. 70% of the total disintegrations are estimated to proceed by electron capture and 30% by β emission.
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