Evaluation of the absolute activity of alpha emitters and of the number of nuclei in thin alpha active layers

The main reasons for the inaccuracy in the absolute alpha activity determined with a 2π geometry counter i.e. self-absorption and backscattering, are studied experimentally and compared to theoretical corrections. For very thin layers (≤10 μg/cm²) the agreement is within 1 %, and for layers up to 50 μg/cm², it is within 2%. For thicker layers, the discrepancies are larger. A method is developed to determined the number of nuclei in a thin layer from a measurement of the α-activity, by comparison of the unknown layer to standards of the same isotopic composition in a low geometry counter. An accuracy of 0.5% is achieved for layer thicknesses up to 1 mg/cm². The method was checked bu an isotope dilution technique and a 2π-geometry counter.