What is the safe distance of X-ray flaw detection?

Generally, the safe distance is 20M, but the safe operating distance is 40M. According to the activity of the source, it is best to calculate and measure with an electric meter. According to the measured values of working time, distance and current distance, and the annual maximum measured value of 0.5 rem, the safe distance can be calculated. (For safe operation, safety protection, lead clothing and protective lead screen should also be taken. )