Answer to Question #10403 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

Is the terrestrial gamma dose rate calculated using activity concentrations of 238U, 232Th, and 40K the same as the terrestrial gamma dose rate measured by a dosemeter installed at 1 m above the ground after subtracting the cosmic influence? I would also like to know if the moisture saturation in soil can affect the terrestrial  gamma dose rate at 1 meter above the ground.

A

If the geographic region of concern is not contaminated with other radionuclides as a result of operations that have been conducted by humans, it is reasonable to assume that the terrestrial gamma dose rate one meter above the ground surface is the result primarily of gamma radiation from the gamma-emitting radionuclides that make up the decay chains parented by 238U and 232Th, and from 40K. A much smaller contribution comes from gamma emitters in the actinium series, headed by 235U, and smaller contributions still from other single, long-lived radionuclides in the earth.

One significant uncertainty in calculating expected gamma dose rates from the natural decay series has to do with the degree of equilibrium that exists among the decay progeny and the parent radionuclides. Any physical and/or chemical processes that might have changed the concentrations of particular radionuclides in the earth could lead to disturbances among the equilibria, and these can affect the relative importances of the contributions of some gamma emitters to the dose rates and lead to miscalculations if the disequilibria are not known and accounted for. In cases when one is quite sure that physical/chemical processes have not affected equilibrium, the assumption that each of the decay progeny achieves the ultimate same activity concentration, modified by appropriate decay branching fractions, as that of the parent of the decay chain is legitimate.

The measured dose rate will expectedly agree with the calculated dose rate at the one-meter elevation if the assumed parent radionuclide concentrations are correct, the equilibrium assumptions that went into the dose calculations are correct, and the dose quantity being calculated is the same as that being measured. The latter point is sometimes overlooked, and differences between the dose-related quantity that has been calculated and the quantity that has been measured can have a noticeable impact. For example, the calculation of dose rate may involve the effective dose rate as is true for estimations made by the Environmental Protection Agency in Federal Guidance Report No. 12, External Exposure to Radionuclides in Air, Water, and Soil, EPA-402-R-93-081, 1993. An instrument used for measurement may simply measure the exposure rate, air kerma rate, or dose equivalent rate at a point with no consideration of the effective dose to an individual. Depending on the photon energies being considered there may be a significant difference between the calculated and measured quantities. The instrument may have been calibrated to estimate the ambient dose equivalent at 1 cm, a quantity recommended by the International Commission on Radiation Units and Measurements (ICRU) to simulate effective dose, but even for this case significant variations between the ambient dose equivalent and effective dose may exist, being most noticeable at photon energies below about 100 keV. If a personal dosimeter was mounted on a suitable phantom and used for the one-meter measurement it presumably would have been calibrated to read the personal dose equivalent (dose equivalent evaluated at 1 cm depth). The personal dose equivalent is used to simulate the effective dose, but variations of 10s of percent between the two are not unusual and differences exceeding 200 percent may prevail at low photon energies (around 25 keV), with the personal dose equivalent exceeding the effective dose.

The one-meter gamma dose rate from the terrestrial gamma radiation is affected by the composition of the earth in the region of concern and by the amount of moisture in the soil and, in the winter time, by the amount of moisture in the forms of ice and snow in the soil and on the surface of the soil. This has been discussed in the answer to a related question, No. 10406, that appears at this link on the HPS Ask the Experts web site.   

I hope this adequately addresses your questions.

George Chabot, PhD

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