238U, 234U, 228Ra and 226Ra radioisotopes are always existed in nature materials as well as in under groundwater. It is poison when they entered to human body and their radio-activity is assessed from mBq/l to kBq/l by WHO and UNSCEAR. Their radio-activity is high but the concentration of them is very low, and when simultaneous spectral measurement will be gotten interference, so it is difficult to precisely determine by conventional methods. |