Answer to Question #388 Submitted to "Ask the Experts"

Yes, you can use dose conversion factors to back calculate an estimated soil radioactivity from radiation measurement over the soil surface.

For example, one can use the factors in Federal Guidance Report No.12, External Exposure to Radionuclides in Air, Water, and Soil, EPA 402-R-93-081, September 1993. If we use Table III.7 for soil contaminated to an infinite depth and select the values for ²²⁶Ra and its decay products, we find for the effective dose equivalent:

Multiplying this by 5.76 x 10¹² converts this to 3.45 x 10^-4 µSv h^-1 per Bq kg^-1. (Assume a soil density of 1.6 g/cm³ or 1.6 x 10³ kg/m³; 106 µSv/Sv x 3.6 x 10³ s/h x 1.6 x 10³ kg/m³ = 5.76 x 10¹²) Then the soil ²²⁶Ra concentration (infinite depth) is: C_Ra, Bq kg^-1 = Effective Dose Equivalent Rate, µSv h^-1 ÷ 3.45 x 10^-4 µSv h^-1 per Bq kg^-1. FGR-12 also gives conversion factors for soil contamination to finite depths of 1, 5, and 15 cm (Tables III.4, III.5, and III.6); the conversion factors decrease with decreasing depth. At low concentrations of soil Ra, you should correct for the cosmic ray response of the detector (and possibly the ⁴⁰K contribution to the ambient radiation); the importance of these corrections diminishes as the soil Ra concentration increases.

The uncertainty in this calculation also will be affected by the distance above the ground surface represented by the detector location and how the electret system was calibrated—for example, was it calibrated for the dose equivalent (µSv) for a small mass of tissue free in air or for the effective dose equivalent (µSv) in a human? The conversion coefficients quoted above were calculated for the effective dose equivalent (the risk-weighted sum of doses to the various tissues of the body) based on a mathematical adult human phantom standing upon the soil at the air-ground interface. The reasonableness of the factor developed above from FGR-12 information can be evaluated by comparison with factors taken from other sources:

1. The National Council on Radiation Protection and Measurements in NCRP Report No. 94, Exposure of the Population in the United States and Canada from Natural Background Radiation, (1987), Table 5.1, pg 69, gives the absorbed dose in air at one meter height for a soil concentration of 1 Bq/kg of ²³⁸U and daughters as 4 µGy/y. Ignoring the few percent difference between the conversion factor for ²³⁸U and daughters and that for ²²⁶Ra and daughters, and assuming the air to tissue conversion is about 0.7 Sv (tissue)/Gy (air), and considering 8766 hrs/yr, this corresponds to approximately 3.2 x 10^-4 µSv h-1 per Bq kg-1.
    
2. One of our experts, Dr. Keith Schaiger, reports that from many measurements over uranium mill tailings piles he developed the empirical factor: Exposure rate, µR/hr = 2.5 x Ra concentration, pCi/g. Assuming an approximate conversion of 0.006 µSv (tissue)/µR (air) and the factor 0.027 pCi/g per Bq/kg, this corresponds to approximately 4.0 x 10^-4 µSv h^-1 per Bq kg^-1.
    

The value of 3.45 x 10^-4 µSv h^-1 per Bq kg^-1 as suggested above seems reasonable in that it is within the range of other reported factors.

Charles E. Roessler, CHP, PhD