Answer to Question #5387 Submitted to "Ask the Experts"

Q

After high-dose iodine use we decommission the patient room for regular use. Currently I first do a measurement using our Berthold contamination meter (because it has Bq/cm² units which are specified on my license for contamination limits) and then I follow those measurements up with our Ludlum survey meter that measures in microsieverts. (I have exclusions in our license related to the patient washroom.) Am I using the meters correctly? People have suggested that the Berthold is too sensitive, but if I only use the Ludlum and there is any reading beyond background I will have to use the Berthold anyhow. Any advice or suggestions?

A

Thank you for your question pertaining to the measurement of residual iodine contamination. I am assuming that the isotope of iodine used is ¹²⁵I. This radionuclide emits very low-energy x rays and gamma rays. It is important that the contamination meter that you use to perform surface activity measurements be calibrated to similar radiation types and energies. Once it has been properly calibrated to measure ¹²⁵I residual radioactivity you can determine the minimum detectable concentration (MDC) for a particular count time (the MDC will be in units of Bq/cm²). A comparison of the MDC to the contamination limits specified in your license will address the issue of whether the contamination meter is too sensitive. As a general rule, the MDC should be 10 to 50% of the contamination limits. Two references that specifically address decommissioning surveys and instrument calibration issues are the Multiagency Radiation Survey and Site Investigation Manual (MARSSIM) and NUREG-1507. While the survey meter that measures in microsieverts provides additional assurance that relatively large concentrations of ¹²⁵I have not been missed, it is unlikely that this meter is sufficiently sensitive to meet the contamination limits in your license.

Hope this helps.

Eric W. Abelquist, CHP