Tritiated water detection in the 2.17 µM spectral region by cavity ring down spectroscopy

Nuclear waste containers are intended to be stored in dedicated disposal sites. For the inside and environmental safety of the disposal sites, the tiny outgassing rates leaking out the containers are measured. Presently, the radioactive HT gas is measured by liquid scintillation. However an alternative method-cavity ring down spectroscopy, an isotopically selective laser technique based on molecular spectroscopy-is currently developed and evaluated for tritium measurement in its oxidized form, HTO. Applying this method, the number density of the gaseous HTO sample hold in the optical cavity cell, is derived from the laser beam absorption by vibrational symmetric stretching 2ν1 (R) HTO lines in the 4590 and 4600 cm−1 spectral range. To ensure a future accurate HTO measurement, the theoretical line intensities are confronted to the experiment: two tritiated water standards are measured with a dedicated CRDS set-up. Compared to the theoretical database, the line positions are correct (−0.067 to −0.128 cm−1), their relative intensities are in agreement with the database, but their absolute intensities are 20% weaker. Among the seven intense lines, the 4596.485 cm−1 line (intensity 8.22 10−22 cm/molecule) and the 4592.407 cm−1 line (intensity 9.83 10−22 cm/molecule) are isolated and intense for a sensitive detection. The HTO detection limit with the present set-up is 3 kBq (10 min), equivalent to 1.8 1012 molecules in the 111 cm3 CRDS cell. This detection limit could be improved by a factor 3 by reducing the detection noise.