An efficient method for γ-spectrometric determination of radium-226,228 via manganese fibers

Radium isotope activities are typically at such low levels in natural waters, especially in seawater, that their measurement requires preconcentration from very large samples. Many investigators accomplish this by passing large volumes ( 20 to 150 L) of seawater or groundwater through a cartridge loaded with MnO2-impregnated acrylic fiber. We describe here a new approach for converting "Mn fiber" into a form suitable for measurement of long-lived Ra-226 and Ra-228 by gamma-spectrometry. The fiber is packed into custom-made crucibles manufactured from stainless-steel sheets. The fiber is then ashed at 550 degreesC resulting in a sample mass reduction of approximately 90%. The crucibles are subsequently folded to enclose the ash, pressed using a hydraulic press, and sealed on top with silicon caulking to prevent radon escape. This procedure has an advantage over similar techniques because there is no sample transfer needed, and it is very time efficient. Radium-228 is determined by counting photopeaks at 338 and 911 keV from its daughter Ac-228 ( T-1/2 = 6.1 h). Radium-226 is determined either by its photopeak at 186 keV ( clear from interferences from U-235 as U is not extracted on Mn fiber from oxidizing waters) or by counting the more abundant photopeaks from the granddaughters Bi-214 and Pb-214 after a 3-week in-growth period. A recent International Atomic Energy Agency intercomparison study for radium determination in water as well as comparison to the traditional radon emanation method showed that this technique provided good results and is appropriate for radium measurements in natural waters.