On the measurement of K-40 in natural and synthetic materials by the method of high-resolution gamma-ray spectrometry

In the measurement of natural occurring radioactive material (NORM) concentrations of K-40, Th-232 and U-238 are measured by passive gamma ray spectrometry with either HPGe or NaI(Tl) detectors. K-40 is measured through its 1460.8 kcVy line. However, till now it has been ignored that this line is mixed with the 1459.2 keV line of Ac-228 from the chain of Th-232. A correction must be made for this contribution. All the data in literature has wrong data about the concentration of K-40 although in many cases the error is less than 1%. However, there are cases of considerably higher errors. It should be emphasized that even if the correction in K-40 concentration is large the correction of external dose index is negligible due to the higher weighing factor for Th-232 than for K-40 (by at least a factor of 10).