A method and apparatus for monitoring the level of tritiated water vapor in air using a solid scintillator

Tritium is presently used in large quantities in laboratories for technological studies, as fuel for the process of nuclear fusion, and in the manufacture of radio-luminescent items. It is also produced in fission nuclear plants (with a yield of 10(-4) if the fuel is U-235 and twice as much if the fuel is Pu-239), particularly in those which use D2O as moderator. The weak energy of beta particles (E-mean = 5.7 keV) produced by tritium implies that it is only harmful if internalized. In nature tritium in air is mainly present under two different chemical forms: elementary (HT, DT, T-2) and oxidized (HTO, DTO, T2O). The latter is extremely more dangerous than the former as far as radioprotection is concerned; for this reason the derived air concentration limit (8 x 10(5) Bq m(-3)) for the gaseous form is 25,000 times higher than the value for the oxidized one. The purpose of the work presented here is to realize an area monitor that is able to discriminate in real time between the two chemical forms. Using the properties of zeolite as an absorber and scintillator, it was possible to construct such a detector. In 1 h the instrument can reveal HTO concentrations 40 times below the derived air concentration. A concentration equal to the derived air concentration can be revealed within the first minute of counting and the performance may be further improved. Moreover, the prototype realized is able to work automatically and continuously for 5 h. The capability of discriminating the oxidized chemical form, the sensitivity, and the possibility of obtaining real time information make this instrument a good monitor in those cases where there is a real risk of tritium air contamination.