[en] Recently, the use of short-lived radionuclides in medical, biological, and pharmaceutical research and clinical applications has increased. Activity measurement of radionuclides that are not commonly used are often required. Even though calibration setting numbers (factors) are given for ∼200 radionuclides for some dose calibrators, one may not find the calibration setting number for the radionuclide of interest. The calibration setting number can be calculated if one has data on gamma-ray (and beta-ray and electron capture when applicable) intensity, x-ray fluorescence yield associated with decay of nuclides of interest, and the detector sensitivity of the calibrator to radiations. When one tries to measure the production yield of radionuclides that are not commonly used, the first question that may have to be solved is the accuracy of the activity measurement. The accuracy of an activity measurement can be affected by the following: 1) calibration accuracies of calibrator and spectrometer, 2) accuracy of calibration setting numbers, 3) sample configuration and size, 4) presence of radioactive impurities, 4) presence of radioactive impurities, 5) presence of radioactive daughters, 6) accuracy of time measurement (for short-lived nuclides), 7) accuracy of decay data, 8) effects from background radiations, especially for low activity, 9) instrument adjustment, such as zero, and 10) other experimental factors, such as fractionation, saturation of instruments, decay correction, etc