Radium-226 is one of the main analytes of concern for NORM Testing
Radium-226 is one of the main analytes of concern for NORM Testing and there are multiple ways of testing for this isotope. Here is a brief description of four different methods to help you determine which one is the right approach for your project.
Radium-226 by Gamma Spectroscopy using Pb and Bi ingrowth:
This method involves sealing the sample in a steel can and allowing 21 days for the ingrowth of the Pb-214 and Bi-214 progeny. The analysis entails quantification of the Pb-214 and Bi-214 progeny, assuming secular equilibrium, and reporting the average of the Pb and Bi activities as Ra-226.
This method is for soils only. It is reasonably inexpensive and gets detection limits of 0.5 – 1.0 pCi/g. Sample volumes of approximately 250 – 300 grams are required (dry weight). Due to the required ingrowth period, the standard turnaround time (TAT) is between 30 – 45 days. The ingrowth can be shortened to get a faster TAT, but in doing so the data will have the potential to be biased low.
Radium-226 by Gamma Spectroscopy using the 186 keV line:
This is a direct measurement technique for the analysis of Ra-226 using the 186.1 keV gamma sumpeak which includes contributions from the 185.7 keV gamma peak of U-235.
This is reasonably inexpensive and gets detection limits of 0.5 – 2.5 pCi/g or 175 – 250 pCi/l. Sample volumes of approximately 125 grams or 550 grams are required for solids, 1L for waters, and 125 mls for waste liquids. Note that waste liquids are reported on mass basis. The larger solid sample volume can get you a lower detection limit. Since there is no ingrowth period to wait for, fast TATs are possible. This approach will always have the potential for a high bias as it will include contributions from U-235 to the Ra-226 result.
Ra-226 by Radon Emanation (903.1):
This method was developed for waters, but has been modified for soils. In a water analysis, the radium is co-precipitated as Ba(Ra)SO4 to chemically separated the radium out and then ingrow. The resulting Rn-222 progeny is transferred as a gas and analyzed by alpha scintillation. In a soil analysis, the sample is digested and the digestate is transferred directly to the apparatus, without chemical separation. The resulting Rn-222 progeny is then analyzed the same as the water.
This method has no known interferences for Ra-226 and gets detection limits of 0.5 – 1.0 pCi/g or pCi/L. This method can be more expensive. The required ingrowth period is 4 to 21 days, depending on the detection limit needed. Fast TATs (fastest is 7 days) are possible without qualifying the data, since the degree of ingrowth can be precisely calculated. Soil matrices can be problematic, since a thorough dissolution of the sample is needed for accurate quantification.
Ra-226 by GFP (Methods 903.0 or 9315):This method is designed to provide Total Alpha-Emitting Radium results for water samples only. It is meant to be a screening method for Ra-226. Radium is co-precipitated as Ba(Ra)SO4 to separate the radium isotopes. All alpha emitting radium isotopes and their progeny are analyzed by gas flow proportional counting (GFPC) counting and reported as a single analyte, “Total Alpha Emitting Radium”. It is a reliable method for water samples, but not suited to soil sample analyses. Soil samples are not amenable to this method, as the chemical separation is subject to significant interferences commonly found in most soils.
This method is reasonably fast, inexpensive, and gets detection limits of 0.2 – 1.0 pCi/L. As an indication of Ra-226, the method is biased high by the presence of Ra-223 and Ra-224 (Ra-228 is a beta emitter). This high bias may be reduced, but not eliminated with a 14 – 45 day decay period.
A Note on Ra-228
The methods listed above are all specific to Radium-226. It is also common to need Radium -228 in addition to the Ra-226. Ra-228 is a beta emitter where as the Ra-226 is an alpha emitter. Ra-228 can be analyzed for at the same time as the Ra-226 when using either gamma method. If you are using the 903.1, 903.0, or 9315 methods, you will need to have the Ra-228 analyzed separately using either gamma, 904.0 or 9320.