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Answer to Question #5421 Submitted to "Ask the Experts"Category: Instrumentation and Measurements — Instrument Calibration (IC) The following question was answered by an expert in the appropriate field: Q
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. A
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.
Answer posted on 19 April 2006. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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