Beta Decay Energy of Tritium

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 (Pb²¹²) 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±20 sec.