Activated charcoal filter counting for radioiodine effluent concentration determination in protein iodinations

Regulatory agencies have recently placed emphasis upon quantification of I released to the environment during protein iodinations at radioiodination facilities. This necessitates air sampling in order to determine the concentration of ¹²⁵I in the effluent. Air sample trapping mechanisms generally employed are activated charcoal filters. Difficulty arises in quantifying the activity of ¹²⁵I trapped because of the attenuation of the ¹²⁵I decay photons by the charcoal. Evaluation of the activity incident upon commercially available filters using a scintillation detector and large detector source separation is considered here. It is demonstrated that the activity in the filter may be treated as an exponential distribution within an attentuating matrix. This treatment essentially adds a constant correction factor to the counting efficiency of a given geometry for a filter-affluent flow rate combination. Finally, it is shown that an approximation assuming a uniform distribution of activity produces a large error in correction factor to the counting efficiency for the filters examined.