Answer to Question #10966 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I possess an RS-230 Radiation Solutions Inc. handheld gamma ray spectrometer. The dose can be displayed in three ways, exposure rate (µR h-1), absorbed dose rate (nGy h-1), and "X" (µSv h-1).
What is the correct name for "X" when measured in µSv h-1?
The dose rate according to International Atomic Energy Agency (IAEA) technical document relates this to absorbed dose in Gy per unit time.
Controlled measurements made by me, using the same measuring object/bedrock and shifting the dose units in the display of the instrument, reveal that the exposure rate (µR h-1) is equal to the "dose rate" (µSv h-1) multiplied by a factor of 100. Translation of exposure to absorbed dose is based on an IAEA conversion, 1 µR h-1 = 8.69 nGy h-1.
This question may seem ridiculous, but I have a discussion with my tutor on this issue, and he would like to say "effective dose rate." However, for me "effective dose rate" is in relation to equivalent dose—assuming full irradiation and using the factor as described in the IAEA technical document.
The unit microsieverts per hour, µSv h-1, may apply to the quantity equivalent dose to a specific mass of tissue, the equivalent dose to a particular tissue, possibly weighted by a tissue weighting factor, or to the effective dose. Therefore, one cannot tell from the dimensions alone which quantity is being referenced. The latter quantity, effective dose, is very difficult to interpret from a measurement made with a portable instrument without having considerable information about the characteristics of the radiation field and of the individual(s) exposed in that radiation field. This is the case because effective dose, as I expect you know, is evaluated by determining the equivalent dose to each significantly irradiated tissue, multiplying each such respective value by the appropriate tissue weighting factor, and summing up all these products to obtain the effective dose. This evaluation requires knowing the direction of the radiation field(s) incident on the body, the types and energies of the incident radiations, the fluence rate uniformity over the body of the exposed individual, the orientation of the exposed individual in the radiation field, and other details I will not discuss here.
The equivalent dose to any tissue target is obtained simply by multiplying the absorbed dose to that tissue by the radiation weighting factor (previously used quality factor), which accounts for differences among types of radiation in producing biological response. For gamma rays, x rays, and beta radiation, the radiation weighting factor is taken as 1.0 so that tissue absorbed dose and equivalent dose have the same numerical magnitude, just different units, the absorbed dose having dimensions of gray, Gy, and equivalent dose having dimensions of sievert, Sv.
The exercise you performed using the same source and switching the meter to provide readouts in different units substantiates the likelihood that the instrument is not actually determining the effective dose, rather it is doing what many other instruments appear to do as well—that is, assuming that an exposure rate of 1 µR h-1, which applies only to air as the irradiated medium and to gamma rays or x rays as the radiation, results in an absorbed dose rate in soft tissue of 1 x 10-2 µGy h-1 or 10 nGy h-1 to use the units you cited. This approximation comes about from the fact that an exposure of 1 µR in air, over a rather wide range of photon energies, can be shown to yield an absorbed dose in air of about 8.76 x 10-3 µGy or 8.76 nGy, based on an International Commission on Radiation Units and Measurements (ICRU) conversion factor (your value would be 8.69 nGy from a different source). This absorbed dose in air can then be converted to absorbed dose in soft tissue by assuming that secondary charged particle equilibrium exists in the irradiated tissue and multiplying the air absorbed dose by an appropriate value of the ratio of the mass energy absorption coefficient for soft tissue compared to air. This ratio changes slowly over quite a wide photon-energy interval, and a value of about 1.1 is a reasonably representative one to use; multiplying the absorbed dose of 8.76 x 10-3 µGy in air by this factor yields an absorbed dose in soft tissue of about 9.6 x 10-3 µGy, acceptably close to 1 x 10-2 µGy for typical health physics assessments. This is consistent with your finding. Assuming a radiation weighting factor of 1.0 for the photon radiation, we would assume the equivalent dose to the same tissue would have the same magnitude as the absorbed dose to the tissue.
The projection of effective dose from instrument measurements requires specific knowledge, as noted above, or certain assumptions. For instance, one might assume that the radiation is incident normally and uniformly over the anterior portion of the body and use this in conjunction with a known or assumed photon energy distribution and published values of (Monte Carlo) calculated values of fluence, exposure, or surface absorbed dose-to-effective dose conversion factors. I do not have available the IAEA technical document that you refer to, but the factor you cite of 0.7 Sv Gy-1 may be the result of such assumptions. For example, for a planar isotropic field the absorbed dose to effective dose conversion factor over a range of photon energies from about 100 keV to several MeV is reasonably estimated by a value of about 0.7 to 0.8 Sv Gy-1 (based on graphical data from ICRU Report 43, Determination of Dose Equivalent from External Radiation Sources–Part 2, ICRU 1988).
Another method that is sometimes used to estimate effective dose from measured air kerma or absorbed dose is to use the ICRU-defined ambient dose equivalent (the term dose equivalent has been replaced by equivalent dose) at a one-centimeter depth in soft tissue as a conservative estimate of the effective dose. Some instruments are designed to measure the 1 cm dose and, with certain assumptions, this can be used as an estimator of the effective dose. The ambient dose equivalent at 1 cm provides a markedly conservative estimate of the effective dose over all photon energies at least up to about 10 MeV and for all common radiation incidence directions.
The above may be more than you need or desire to answer your questions. It appears to me, for the situations you describe, that the instrument exposure and dose-related outputs provided are representative of values associated with small masses of soft tissue imagined to be at the detector location. The biological dose units (µSv h-1) then seem to refer to equivalent dose and not specifically to effective dose, and I lean towards supporting your point of view. I suppose one could make the argument that for all the energies of interest, a constant tissue absorbed dose-to-effective dose conversion factor of 1 µSv µGy-1 is assumed. Such a factor would be reasonable for photons of energies between about 200 keV and several MeV incident normally and uniformly on the anterior surface of the body of a reference individual. The assumptions would underestimate effective dose for energies between about 50 keV and 200 keV. The anterior-posterior field direction assumption tends to be more conservative than the other likely beam incidence directions.
Hope this is useful.
George Chabot, PhD