The art of realizing the Becquerel Review Article

The state of the art of direct activity measurements a century after the discovery of radioactivity by Henri Becquerel in 1896 is reviewed. The basic ideas of the most important techniques to realize the unit of activity such as defined-solid-angle counting, 4π counting and 4πβ-γ counting and the problems limiting the attainable measurement accuracies are described. The review also covers the maintenance and dissemination of the unit of activity and the systems demonstrating the international equivalence of activity measurements in standards laboratories.