Background: This study consists of two parts. The first part deals with both qualitative and quantitative analysis of ²²⁶Ra using alpha spectrometry measurement method. In the second part, the percentage of radioactive equilibrium between ²²⁶ Ra and its daughter products were determined by alpha spectrometry and total alpha measurement system after elapsed time of 15 days from precipitation.

Materials and Methods: Twelve ²²⁶Ra samples as barium-radium sulfate in form of micro-precipitate on millipore and Whatman #42 filters were prepared. An alpha spectrometer with surface barrier detector and a total alpha measurement system consists of scintillation crystal assembly ZnS (Ag) were used for counting.

Results: The minimum detection limit of alpha spectrometry and total alpha counting for ²²⁶Ra measurements in samples for counting time equal to 10000 seconds, were found to be 3.7 mBq and 15.8 mBq respectively. Results from total alpha counting showed that radioactive equilibrium between ²²⁶Ra and its daughter products reached to about 92%±3.5, where as, in the case of
alpha spectrometry radioactive equilibrium, it was destroyed due to vacuum during counting the sample. Also in case of alpha spectrometry, the optimum sample to detector distance, was found to be 0.5 centimeter.

Conclusion: From this study it was concluded that micro-precipitation can be used as a proper method for sample preparation and alpha spectrometry due to its lower detection limit to measure low concenteration of ²²⁴Ra and ²²⁶Ra in these precipitates, prepared from different samples. Besides it is not time consuming and sources can be measured immediately after sample preparation. Iran . J. Radiat. Res., 2004; 2 (1): 41-44