Determination of 129I in 131I-pharmaceuticals produced in THOR

131I is one of the most important radionuclides used in nuclear medicine. The accompanying isotope 129I with insignificant activities in 131I-pharmaceuticals, produced in THOR, were determined in terms of 129I/131I ratio by neutron activation analysis. The detection limit of 129I can be lowered to order of 0.1 Bq, superior to conventional radiometric methods. The 129I/131I ratios in the 131I-pharmaceuticals, were measured to be in the range from 3.9 to 8.3.