Calibrations: Gamma-ray Spectrometry System

The NIST high resolution gamma-ray spectrometry facility is composed of six High Purity Germanium (HPGe) detectors and a silicon-drifted detector. The HPGe detectors are used for the calibration of gamma-ray emitting sources in an energy range from 30 keV to 4.5 MeV (the range depends on detector and geometry), while the silicon-drifted detector is used for sources emitting low-energy gamma-rays between 5 keV and 140 keV. One of the HPGe spectrometers is equipped with a Compton suppression system that allows the measurement of low activity sources with photons that may be present in the Compton energy region of a high activity radionuclide that may also be present in a source. These systems require constant updates and maintenance of the detectors and associated electronics. Efficiency measurements are routinely updated and checked for different source geometries including point sources, Marinelli beakers, gas spheres and glass ampoules. Monte Carlo models were developed using different computer codes to enhance the measurement capabilities.

These detectors are used for calibrating gamma-ray emitting radioactive sources and for measuring the impurities potentially present in different types of sources. The impurity measurements are used to support the development of primary standards at NIST as well as for calibration of radioactive sources using, among other techniques, ionization chamber measurements. Activity calibration measurements are performed for radioactive sources that require spectrometry measurements and cannot be measured by other methods. This system also is used to measure radionuclide half-lives and absolute photon emission intensities to improve currently available nuclear decay data.