The Radioactive Heavy Hydrogen Isotope

Tritium is the heavy isotope of hydrogen (section 5.2). Its symbol is 3H, or T. Tritium atoms are unstable and disintegrate radioactively, forming stable 3He atoms. The radioactive decay is accompanied by the emission of [1 particles, measurable in specific laboratories  

The rate of radioactive decay is by convention expressed as the half-life, T1/2, defined as the time span during which a given concentration of the radioactive element atoms decays to half their initial value. T1/2 of tritium is 12.3 years. Thus, after 12.3 years one-half the initial concentration of tritium atoms is left, after 24.6 years only one-quarter is left, and so on. A radioactive decay curve of tritium is given in Fig. 10.1. Using the decay curve it is possible to determine, for example, how many years it takes for a given amount of tritium to decay to 20% of the initial amount. The answer, obtained from Fig. 10.1, is 29 years. Similarly, one can determine what percentage of an initial amount of tritium will be left after 20 years. The answer is 32% (read from Fig. 10.1). 

The concentration of tritium in water is expressed by the ratio of T atoms to H atoms  

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