Answer to Question #4148 Submitted to "Ask the Experts"

A conversion from counts per second (cps) to dose rate (Gy h^-1, Sv h^-1, 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 equivalent dose rate (previously called dose equivalent rate, μSv h^-1) with 662 keV photons from ¹³⁷Cs the count rate will likely be about 5 cps per μSv h^-1. 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 contamination will add pronouncedly to the count rate, and the gamma exposure rate/count rate correlation is not valid.

If the instrument reads in exposure rate units there are fairly simple conversions that can be made to convert to tissue absorbed dose rate or equivalent dose rate by recognizing that an exposure that yields an air charge density of 2.58 x 10^-4 C kg^-1 is approximately equivalent to a tissue absorbed dose of 10 mGy and to a soft tissue equivalent dose of 10 mSv. 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 count rate-to-dose rate conversion factors may be very different from the G-M case noted above. For example, a portable 5.08 cm × 5.08 cm 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.

Hope this response is helpful to you.

George Chabot, PhD, CHP