Measurement of low-level radioactive liquid waste with imaging plates

A new method to measure radioactivity in liquid waste is being developed using imaging plates (IPs), which are highly sensitive two-dimensional radiation sensors. This method enables estimation of radioactivity quantitatively after four hours by simple contact of the IP to an acrylic vessel that is filled with the target liquid. No pretreatment is required. The fading effect of quantitative evaluation is estimated by utilizing a fading equation, which was developed by the authors. Sensitivity to radioactivity that is 1/10 of the minimum detection limit set by the Japanese government, 6x10^-2 Bq/cm³ for ¹²⁵I; can be obtained by using this method. To specify a radionuclide in the liquid, several acrylic and lead filters of different thicknesses are affixed to the surface of the vessel. The absorbing patterns of the density of photostimulated luminescence (PSL) (PSL/mm²) as a function of thickness and/or type of filter depend on the type of emission and/or emission energies from radionuclides. The result enables determination of a radionuclide in liquid waste. This new method can be used to measure radioactivity in liquid waste, overcoming many disadvantages of the conventional concentration or evaporation to dryness method. There is no need for pretreatment, thus reducing the handling time and saving labor. (author)