Answer to Question #10372 Submitted to "Ask the Experts"

I am considering carrying out a study on the determination of uranium and its daughters in the radiological risk assessments for freshwater ecosystems in the Niger Delta Region of Nigeria. What is the best experimental method for this and what is the conversion process from activity in Bq L^-1 to dose in mSv h^-1?

There are numerous analytical methods that apply to the determination of uranium and progeny from uranium decay. As to which methods are best, this is somewhat subjective and depends, at least in part, on the levels of radioactivity in the samples and the sensitivity requirements that you impose.

The simplest analytical methods involve the use of gamma spectrometry, usually using a high purity germanium detector and an associated multichannel analyzing system. Samples with measurable activity may be placed in an appropriate container, such as a Marinelli beaker, in order to measure gamma radiation from particular progeny (e.g., ²¹⁴Pb and ²¹⁴Bi from the ²³⁸U decay series). Such analyses, however, require making some assumptions about the extent of equilibrium that exists among the progeny and precursors in order to assess, for instance, the activity of the parent of the chain, such as ²³⁸U. If gamma analysis is not suitable because of lack of equilibrium, insufficient activity, etc., more sensitive methods that involve physical/chemical separations of radionuclides of concern may be necessary.

Isolation of uranium and preparation by precipitation and alpha counting in a gas flow proportional counting system or electrodeposition and counting with a solid state alpha spectrometry system are examples of methods recommended for uranium analysis. You can get an idea of methods that have been used and recommended, along with indications of their sensitivities and usual sample sizes by referring to this online document by the U.S. Department of Energy that summarizes some U.S. Environmental Protection Agency (EPA)-approved procedures.

The conversion of activity concentration to dose (rate) depends on how the exposure occurs, and the particular physical and chemical characteristics of the radionuclides. If you are referring to internal dose resulting from drinking contaminated water, you may want to refer to publications of the International Commission on Radiological Protection (ICRP). For example ICRP Publication 72, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 5 Compilation of Ingestion and Inhalation Dose Coefficients, in Table A.1, provides committed effective dose per unit intake (Sv Bq^-1). For a particular volume of water taken in the activity intake of a specific radionuclide would be the activity concentration (e.g., Bq L^-1) multiplied by the volume ingested; when this is multiplied by the dose conversion factor for the aged individual of interest you would have an estimate of the effective committed dose (Sv) from that intake. You can also find committed effective dose data, taken from International Atomic Energy Agency Safety Series document No. 115 on the  Radiation Dose Assessment Resource site. 

If you are concerned with external dose from immersion in water or exposure to contaminated soil, you can also find relative online data. One source is Federal Guidance Report No. 12, External Exposure to Radionuclides in Air, Water, and Soil, EPA-402-R-93-081, 1993. (Be aware that in this report effective doses are based on the earlier set of tissue weighting factors that ICRP had used. This is discussed in the report.) Another useful tool is the Radiological Toolbox developed by Oak Ridge National Laboratory for the U.S. Nuclear Regulatory Commission. To evaluate external dose from contaminated air, water, or soil, first specify the radionuclide(s), Select Dose Coefficients on the left and Public External Coefficients (FGR 12) at the bottom, and click the radio button to pick the external exposure pathway, which includes submersion in air or water, exposure to a surface ground source, or exposure to volume distributed activity in contaminated soil layers (1 cm, 5 cm, 15 cm, or infinite thickness). Clicking "Display" shows the individual organ doses and the effective doses, based on both ICRP 26 recommendations and ICRP 60 recommendations. Data and methods used to obtain the values here are essentially the same as those used for EPA FGR 12.

For your specific needs you will likely have to do more research, but I hope the above will get you started in the right direction.

George Chabot, PhD