Calibrations are performed using the manganous sulfate bath technique in which the emission rate of the source to be calibrated is compared to the emission rate of NBS–1, the national standard Ra–Be photo-neutron source. Neutron source calibrations typically have a relative expanded uncertainty of about 2.5%, depending on the details of the source encapsulation.
The principal customers for this service are commercial vendors of radioisotopic neutron sources, manufacturers of instruments and devices that monitor radiation exposure and dose, secondary calibration laboratories that service the aforementioned radiation monitoring instrumentation, nuclear electric generating stations, nuclear fuel fabrication facilities, US Department of Defense establishments, and government and private research and development laboratories. In addition to its external customers, the neutron source calibration facility also provides important contributions to other neutron irradiation and calibration services provided by NIST, as well as to intramural research programs in neutron metrology and fundamental neutron physics.
In 2012 the Mn bath was renovated. All of the plumbing was replaced, a flow meter was installed, several additional valves were incorporated in the design allowing for better characterization of the flow, the remote manipulator was upgraded, and a new high purity germanium detector was installed in order to compliment the sodium iodide detectors already present. We are in the process of installing a closed loop chilled water circulator in order to reduce temperature fluctuations of the bath caused by variations in temperature of the local city water.
In a typical year, 8—10 external vendor neutron sources are calibrated. In support of these measurements, our own standard neutron sources (NBS–1, BIPM) and the background rates are measured several times throughout the year. NIST provides a neutron emission rate calibration service in support of US nuclear programs.