Answer to Question #5421 Submitted to "Ask the Experts"

I was recently issued a RaeSystems GammaRaeII personal radiation monitor, along with many other responders in the southeastern Virginia area. I am looking over a standard operating procedure (SOP) on how to use the monitor, and in the SOP it indicated that counts per second can be directly converted to microrem/hr. Is this correct? I cannot find any conversion factor for this and I don't think the conversion would be 1:1. Your assistance with this question would be most helpful.

The monitor you are referring to is apparently a pulse-based system, which is the most common type for electronic personal radiation monitors. These kinds of systems typically use silicon diodes, or sometimes small G-M tubes, as the radiation detector, and the dose rate is interpreted from the count rate through use of an appropriate conversion factor. For detectors designed to measure dose and/or dose rate, this conversion is usually accomplished through internal firmware/software. The actual magnitude of the count-rate-to-dose-rate conversion factor depends on a variety of factors, including such things as detector size, detector shielding, threshold setting, and electronic gain. You are correct in your assumption that there is not a 1:1 correspondence between count rate and dose rate. If the conversion factor is required, and it is not available in the literature you have, the manufacturer is likely the best source of such information, although you may get rather frustrated in trying to extract such information from the manufacturer.

There is another option and that is to expose the device to a known dose rate from a source of radiation characteristic of that which you are interested in for field use and to record the count rate. This requires having a source that will deliver a desirable dose rate available and being able to evaluate the dose rate at the distance of interest. The monitoring device should be mounted on an appropriate phantom to simulate the trunk of the body in order to produce a scattered radiation component that is similar to what would be encountered in actual use. If you do not have facilities available to do this you might discuss the situation with people from Virginia's Radiological Health Program to see whether they might be able to assist you. If you have funds you can also arrange for an accepted calibration laboratory to perform the calibration for you. The National Institute of Standards and Technology, NIST, remains the lead laboratory in this country for ionizing radiation calibration.

I'm sorry we do not have sufficient information to be able to provide a definitive answer as to the proper conversion factor to use. Good luck in your pursuits.

George Chabot, PhD, CHP