Calculating an unknown source activity using modeled and experimental results

A method is presented that provides a way to calculate the unknown activity of a source by using experimental exposure rate measurements from an ion chamber and exposure rates calculated using the MCNP radiation transport code. The method consists of fitting experimental data to MCNP results with both data sets in the form of (Equation is included in full-text article.)where r is the distance from the source at which the measurement was taken, X˙ is the exposure rate, and An is an assumed nominal activity of the source. The fit is done by calculating a correction factor for the nominal activity that shifts the experimental data to match the MCNP results. The actual activity of the source in question is found by multiplying the assumed nominal activity by the activity correction factor. The method was used to calculate the activities of the three Cs sources used in the Ohio Emergency Management Agency's instrument calibration range. It was found that the activities were less than the decay-corrected nominal activities by factors ranging from 3% to 10%.