Answer to Question #463 Submitted to "Ask the Experts"

Q

We have been using GM (pancake) probes for the end-of-the-day surveys and package surveys for many years. Our new RSO says that we should use "ion chamber" instruments for the surveys. I am confused; I talked to a couple of other RSOs and they all use GM probes but they say "opinons vary." My point is, why should we change unnecessarily (NRC has been happy with us)? Are there any specific regulatory guidelines to say what to use in a hospital setting (nuclear medicine department)?

A

Proper selection and use of instrumentation is critical for proper radiological monitoring and is required by all regulatory bodies—although specific detectors are not usually specified for the applications mentioned. Geiger-Mueller pancake detectors are designed for the detection and measurement of surface contamination and are primarily intended for beta-emitter measurements—but are also sensitive to all other primary radiation types under proper exposure conditions. Pancakes could also be used for exposure-rate-type measurements, but would require careful consideration when matching calibration data with the applied use. As such, these detectors are not typically used for exposure-type measurements but are used extensively for evaluating surface-deposited radioactivity. Again, this does not preclude alternative uses when properly calibrated and used. Ion chambers are primarily intended for exposure-rate measurements, but can be used for other applications. Atmosphere-vented ion chambers are typically designed for measuring intermediate- to high-exposure rates with a detection threshold above a few mR/h—depending on the specific design—and for estimating beta dose rates or relatively high surface activity levels. These instruments would be a poor choice for measuring free-release level surface activity. Robert L. Coleman Oak Ridge National Laboratory