Mass Spectrometric Determinations of the Absolute Tritium Activities of NBS Tritiated Water Standards

The tritium content of U.S. National Bureau of Standards tritiated water Standard Reference Materials SRM-4926, -4926C and -4927B have been measured by mass spectrometry, where the accumulation of 3He from tritium decay is measured as a function of time. The averaged results, which are compared with other independent calibrations, are 0.7% higher than the NBS certified values and have an uncertainty ( la) of 0.4%. Additionally, the half life of tritium has been determined from new calculations of the average energy of tritium decay, combined with existing measurements of the tritium decay energy emission rate. The new half life value, 12.38+0.04y, is essentially the same as the 12.38 _+ 0.03 ~ value recently obtained mass spectrometrically with the same analysis system used in the present measurements.