Answer to Question #1055 Submitted to "Ask the Experts"

Q

What is mRad? How do you convert mR/hr to mRad/hr?

A

In practice of radiation safety and control, one is concerned about relating a given radiation field (a radiometric or physical quantity) to an actual or potential biological effect for radiation workers or members of the public. Thus, over the years operational and protection quantities and units of measure have been developed and defined. ICRU Reports 51 and 57 provide definitions and excellent discussions of the basis for currently recommended quantities and units. Despite official acceptance of the SI (that is, international) system of quantities and units, here in the United States many radiation measurements and dose limits are still expressed in the older "special units." One older special physical quantity is "exposure," which had the "roentgen" (R) for its unit of measure. The roentgen was defined for photons only, related to ionization of dry air and the charge collected of one sign, and defined as 1 R = 0.000258 coulombs per kilogram. Today, we relate the physical quantities of fluence (number of particles per unit area), kerma (kinetic energy released in material), and absorbed dose (mean energy imparted to matter) to the protection quantity of "effective dose" and operation quantities of "dose equivalent." Different radiations may vary in effectiveness for causing biological damage, and the spatial distribution of irradiation (that is, partial or whole body) will impact the probability of an effect (for example, cancer induction). Therefore, one must modify absorbed dose in an organ or tissue by a radiation weighting factor and then apply organ/tissue weighting factors to allow summation and comparison to uniform whole-body irradiation. A guide to the various radiation protection quantities and units can be found on the inside cover of the Health Physics Society's publication Operational Radiation Safety. Those related to this question are as follows: The SI unit for absorbed dose is the "gray" (Gy), which replaced the "rad." 1 Gy = 1 J/kg = 100 rad (Note in the cgs system: 1 rad = 100 ergs/g) The SI unit for dose equivalent and effective dose is the "sievert"(Sv), which replaced the "rem." 1 Sv = 100 rem For photons only (that is, x and gamma rays), the old special unit roentgen corresponds to an absorbed dose of ~ 0.87 rad in air, and ~ 0.97 rad for soft tissue. It is not valid to measure any other radiation (for example, alpha or beta) in terms of the roentgen. In addition to the above noted spatial distribution concern, one needs to consider the temporal radiation delivery "rate," as it will have a great impact on biological effects. Thus, all these radiological units can be expressed in terms of "per unit time" (for example, rad per hour or "rad/h"). Lastly, when specifying these units, the magnitude may be modified by common prefixes that denote a specific decimal shift. For example, the ten to the minus three submultiple "milli" or "m" can be added to the unit rad, that is, 1 mrad = 0.001 rad. Alternately, one could have three orders of magnitude larger than a rad, and note the shift in value with the multiple "kilo," that is, 1 kilorad = 1,000 rad. There are many common prefixes (for example, mico, milli, kilo, mega, etc.) that may be added to any measurement unit (for example, grams, meter, rad, Gy, etc.). David J. Allard, CHP