Methods and apparatus for calibrating radioactive sources are described. An array of scintillation detectors forms a receptacle within which a sample or sample container can be retained by a holder. The scintillation detectors are coupled via light transducers such as photomultiplier tubes (PMTs) to independent electronic counters. Coincidence processing of time-tagged events yields a correlated event rate. One or more corrections can be applied as needed, for background counts, deadtime, or random coincidences. Voltage tuning of PMTs yields improved reproducibility. Accuracy of 1% has been demonstrated over a range of 10 kBq-3 MBq.