The extrapolated end point from Kurie plots of the tritium beta spectrum between 14 and 18 kev has been measured as 18.61±0.02 kev (internal consistency) on the Argonne double-lens beta spectrometer. Composite sources of tritiated estradiol and 10-hr thorium B (${\mathrm{Pb}}^{212}$) and its daughters allow calibration at 25 kev (the "$A$" line) as well as some check on the effect of the source thickness on the "$A$" line. A review of other measurements, including mass difference data, average energy determination, and proportional counter work, shows agreement with the present magnetic spectrometer result, 18.6±0.1 kev (this error includes some estimate of possible systematic effects). Two notable exceptions give lower values, 18.2 and 18.0 kev—one an average energy determination, the other a magnetic spectrometer result. The 18.6-kev decay energy and a half-life of 12.43 years give $ft=1137\mathrm{}\pm 20$ sec.

• Received 20 February 1959

DOI:https://doi.org/10.1103/PhysRev.115.450