Answer to Question #6219 Submitted to "Ask the Experts"

Q

I want to find a conversion of μSv/hour to ppm (parts per million) for uranium and thorium. We currently measure the latter on-site but have been given guidelines by the EPA (Environmental Protection Agency) in μSv/hour to adhere to. I realize that there is not an exact conversion factor or formula, but some sort of guideline would be appreciated.

A

Thank you for your question pertaining to a conversion factor for uranium and thorium concentration to radiation level in Sv per hour. You recognize in your question that there is not an exact conversion factor or formula. This is certainly the case given the number of variables that impact the relationship between radionuclide concentration to radiation level, such as geometry and depth of the radioactivity, soil density, and moisture, presence of soil overburden, and receptor location (for example, 1 m above ground) to mention a few.

Two computer codes might be useful in establishing this relationship: RESRAD and MicroShield. RESRAD is a computer model designed to estimate radiation doses from residual radioactivity in soils. MicroShield(r) is a photon/gamma-ray shielding and dose-assessment program. While both of these software modeling tools can help establish a conversion factor for uranium and thorium concentration to radiation level in Sv per hour, it would be better to try RESRAD first since that software tool is free.

Perhaps even more useful is the External Radiation Dose Calculator provided by the WISE Uranium Project. This dose calculator can determine the radiation dose from a shielded gamma source. The source can be a point source or a cylindrical volume source with an evenly distributed concentration of radionuclides. The receptor location can be varied in two dimensions.

Eric W. Abelquist, CHP