Defined solid-angle counter with variable geometry

We describe a defined solid-angle counter for the standardization of radioactive sources of alpha-particle emitters. It has been built with the aim of combining good counting efficiencies, low uncertainties and flexibility of operation. The distance between source and detector can be changed in a continuous way with a precision guide and a ball screw from 8 to 19 cm, which correspond to counting efficiencies between 0.023 and 0.004 for small size sources. A linear encoder allows the accurate determination of the source position. Alpha spectra of the sources are measured with an implanted silicon detector with an active area of 2000 mm2. Uncertainties, excluding counting statistics, are below 0.1%.