Answer to Question #12800 Submitted to "Ask the Experts"

Category: Radiation Basics

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

Q

I am trying to work out dose rates for 226Ra at 1 meter. I have been advised that 226Ra gives off a dose rate of 223 microsieverts (mSv) h-1 at a distance of 1 m. I cannot find any references to support this. Furthermore, as I am dealing with soil, I am unsure if this includes the daughters of 226Ra, more specifically 214Pb, 219Pb, and 214Bi.

Also, the references I have found pertaining to 226Ra gamma ray constants seem to be in contradiction when I convert them into Australian measurements.

A

It is interesting that despite being one of the earliest discovered and most explored radionuclides, 226Ra is still the source of a variety of questions and concerns. Your question is a valid one.

I believe one of the most commonly used gamma radiation dose-related constants that is still in use is an exposure rate constant that I recall first seeing in Jaeger et al. (1968). On page 29 the value given is 8.25 R h-1 cm2 mCi-1. The value is based on the assumption that all of the radioactive progeny of radium decay are in secular equilibrium with the radium. Converting this to SI units, assuming an equivalence of 1 R to about 1.0 × 104 µSv for a small volume of tissue at the dose point, we may estimate an equivalent dose rate constant of (8.25 R h-1 cm2 mCi-1)(1 × 104 µSv R-1)(1 mCi/3.7 × 107 Bq)(1 m2/104 cm2) = 2.23 × 10-7 µSv h-1 Bq-1 m2. You do not say how much activity applies to the value that you cite so I cannot comment on its applicability. The significant digits are the same, however, and the same values would apply if the quantity of activity intended by your value was 109 Bq (1 GBq).

Since you state that you are dealing with soil, there is an available reference that might be useful. It is US EPA Federal Guidance Report (FGR) 12 that provides external dose data for exposure to radionuclides in air and a second set of data that provides external dose data for exposure to radiation from radionuclides distributed either on the ground surface or distributed to varying depths in the soil. The given doses are to both individual tissues and effective doses to individuals. If you use the values for 226Ra you will have to add in the effects of the progeny, most notably 214Pb and 214Bi, which are listed separately.

I hope this is helpful to you.

George Chabot, PhD

Reference

Jaeger RG, Blizard EP, Chilton AB, Grotenhuis M, Hönig A, Jaeger ThA, Eisenlohr HH, eds. Engineering compendium on radiation shielding: Shielding Fundamentals and Methods Vol 1. New York: Springer-Verlag; 1968.

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