Abstract: In this paper,the formula of the low detection limit of the net count rate in radioactive measurement is derived from the point of non-equality,the actually probability of making type II errors corresponding to the two typical low detection limit is calculated.In the absence of an approximation,the number of radionuclides that have decayed follow a binomial distribution.When the average of the net count rate is the low detection limit of the net count rate,the probability that the random value of the net count rate is less than the decision threshold is the probability value of making type II errors.Combining the number of radioactive nuclei at initial moment,the number of radioactive nuclei in the whole measurement time that has decayed and the probability of radioactive nuclides decay,the actual probability value of making type II errors can be calculated.It is found that the maximum practical probability value of the type II errors is 1 to 2 orders of magnitude less than the nominal value in most case.Therefore,the low detection limit can be modified,and an appropriate correction coefficient can be given,so that the actual probability value of making type II errors is close to and slightly less than the nominal value.Through calculation,it is found that the correction coefficient given by k_α=1.645 and k_β=1.645 can reduce low detection limit by 22 percent.

Key words: net count rate, low detection limit, correction coefficient