Tritium Scale (New Half-Life)
Tritium concentrations are expressed in TU, where 1 TU indicates a T/H abundance ratio of 10-18. The values refer to the tritium scale recommended by U.S. National Institute of Science and Technology (NIST, formerly NBS), and International Atomic Energy Agency (IAEA). The TU-numbers are based on the NIST tritium water standard #4926E. Age corrections and conversions are made using the recommended half-life of 12.32 years, i.e., a decay rate of λ = 5.626 % year-1. In this scale, 1 TU is equivalent to 7.151 dpm/kg H2O, or 3.222 pCi/kg H2O, or 0.1192 Bq/kg H2O (Bq = disint/sec). counting TU values are calculated for date of sample collection, REFDATE in the table, as provided by the submitter. If no such date is available, date of sample arrival at our laboratory is used. The stated errors, eTU, are one standard deviation (1 sigma) including all conceivable contributions. In the table, QUANT is quantity of sample received, and ELYS is the amount of water taken for electrolytic enrichment. DIR means direct run (no enrichment). Remark: From 1 Jan 1994 through 31 Dec 2001 we used the previously recommended value for the half-life, 12.43 years. The use of the new number, 12.32 years, will in practice increase the reported TU-values by 0.9 %. This is insignificant since our reported values carry 1 sigma uncertainties of 3 %.
Very low tritium values
In some cases, negative TU values are listed. Such numbers can occur because the net tritium count rate is, in principle the difference between the count rate of the sample and that of a tritium-free sample (background count or blank sample). Given a set of "unknown" samples with no tritium, the distribution of net results should become symmetrical around 0 TU. The negative values are reported as such for the benefit of allowing the user unbiased statistical treatment of sets of the data. For other applications, 0 TU should be used.
Reliability of results
Refer to Services Rendered (Tritium), Section II.8, in the “Tritium Laboratory Price Schedule; Procedures and Standards; Advice on Sampling”. Tritium efficiencies and background values are different in the nine counters and values are corrected for cosmic intensity, gas pressure and other parameters. For tritium, the efficiency is typically 1.00 cpm per 100 TU (direct counting). At 50Ç enrichment, the efficiency is equivalent to 1.00 cpm per 2TU. The background is about 0.3 cpm, known to about – 0.02 cpm. Our reported results include not only the Poisson statistics, but also other experimental uncertainties such as enrichment error, etc.
References
Lucus L.L. and M.P. Unterweger, Comprehensive review and critical evaluation of the half-life of tritium, J. Res. Natl. Inst. Stand. Technol., 105, 541-549, 2000.
Lucas, L.L., Massic activity ratios of the NBS/NIST tritiated-water standards issued between 1954-1999, J. Res. Natl. Inst. Stand. Technol., 105, 535-539, 2000.
