Abstract: A method and apparatus for absolute activity determination of radionuclides, especially of low maximum energy pure beta-emitters as for instance .sup.3 H, .sup.63 Ni, .sup.14 C, .sup.35 S. The application of the invention can also be extended for activity determinations of other radionuclides, for example of those disintegrating through the electron capture. The method contemplates producing pulses of various multiples of coincidence simultaneously, and correlating the counts of the pulses to obtain a measure of the absolute activity. This is achieved by providing a scintillation head containing at least three photomultipliers optically coupled to the scintillator and the source whose activity is to be measured. The outputs of the photomultipliers are connected via amplifiers to coincidence gates having two and three inputs.