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The calibration of alpha-particle sources at the National Bureau of Standards (NBS) began about 1950 with the measurement of 2π alpha-particle-emission rates using a hemispherical gas-flow proportional counter. Today activities are also determined using defined-geometry counters with scintillation detectors and 4πα-γ coincidence counters. As the measurement techniques have evolved, the overall uncertainty associated with the determination of the activity of an alpha-particle source has decreased by about an order of magnitude, from several percent to several tenths of one percent. The factors contributing to the overall uncertainty fall into two general categories: those associated with the detection system (such as uncertainties in geometry, scintillation efficiency, and deadtime) and those associated with the source (such as uncertainties in self-absorption and scattering). These factors are discussed, along with suggestions for minimizing the overall uncertainty.
radiation, environments, alpha particles, detectors, liquid-scintillation counter, proportional counter, radioactivity, scintillation counter, standardization
Research chemist, National Bureau of Standards, Washington, D.C.,