The ash content of coal can be determined using the pair production technique based on the simultaneous measurement of the annihilation radiation and compton scattered radiation. The proper correction of the influence of changes in mass per unit area with reference to the measured intensities is an important problem using this technique. It can be solved using the time interval distribution method. The application of this method allows the correct determination of the normalization factors needed. The solution of this problem is an important supposition for application the pair production technique on conveyor belts without any coal preparation.