Convert a radiation reading CPM to dose rate (rads/mrem)
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Author:  hey [ Sun Mar 10, 2013 11:47 am ]
Post subject:  Convert a radiation reading CPM to dose rate (rads/mrem)

Question: Is there a way to convert a radiation reading on a meter from a count-per-minute rate to a rad, rem, or mrem?

Answer: A conversion from counts per minute (cpm) to dose rate (rads/hr, mrads/hr, etc.) is possible, but the conversion factor depends on the kind of detector being used and on the type of radiation being measured and frequently on the energy of the radiation. The relationship between count rate and dose rate is usually established through empirical calibration procedures in which the detector is exposed in a radiation field of the radiation type and energy of interest at a known dose rate, and the count rate is recorded. If the instrument being used has an adjustable discriminator the observed count rate will also be affected by the setting of the discriminator. Changes to the operating voltage and other operating parameters may also affect the observed count rate.

One commonly used Geiger-Mueller (GM) detector is a pancake type with a facial diameter of approximately two inches. When the detector is connected to a standard portable ratemeter and calibrated to interpret exposure rate (mR/hr) with 662 keV photons from 137Cs the count rate will likely be about 3,000 cpm per mR/hr. It should be noted that this correlation between count rate and exposure rate for the cesium (or any other gamma emitter that is used) applies only when the detector is viewing only the gamma radiation; if unshielded contamination is being viewed by the thin window detector, the beta radiation from the cesium or other possible comtamination will add pronouncedly to the count rate, and the gamma exposure rate/count rate correlation is not valid.

The exposure rate in air, in mR/hr, is approximately equal to the soft tissue dose rate in mrads/hr, which may also be used to estimate the dose equivalent rate in mrem/hr. More exact conversions of exposure rate to selected dose equivalent values, such as the 1 cm depth personal dose equivalent, may be made using energy-specific conversion factors (for example, see Report 47 of the International Commission on Radiation Units and Measurements [1992] titled "Measurement of Dose Equivalents from External Photon and Electron Radiations").

If other detectors are used the cpm-to-dose rate conversion factors may be very different from the G-M case noted above. For example, a portable 2" x 2" cylindrical sodium iodide (NaI) scintillation detector will expectedly yield a sensitivity to the gamma radiation from 137Cs that is 10 to 100 times greater than the G-M detector. The NaI detector also exhibits a very strong photon energy dependence so that the cpm-to-dose rate conversion factor will change notably as the photon energy changes. Detectors used for gamma measurements are also sometimes used for beta-dose measurements if the detectors are equipped with thin enough entrance windows. Again, the conversion factors for beta-radiation measurements will likely be different from gamma conversion factors and will also change as beta-particle energies change.

Other related questions have appeared on the Health Physics Society's Ask the Experts Website. Go there and scan down the page to the heading "Conversion of survey meter readings to dose" and then view question numbers 82, 128, and 236.

Hope this response is helpful to you.

George Chabot, PhD, CHP
Health Physics Society

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