Calibrations: Neutron Source Strength

Summary

The neutron source calibration facility operated by NIST is a world-class calibration laboratory providing neutron source calibration services for radioisotopic sources with neutron emission rates ranging from 5 x 10⁵ s^-1 to 1 x 10¹⁰ s^-1.

Description

Photograph of the Mn Bath. The blue sphere holds the manganous sulfate solution. A source enters the bath through the re-entrant port at the top of the sphere. The robotic arm at the left is used to manipulate sources remotely.

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 (k = 2) 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. In 2016, a closed loop chilled water circulator was installed to reduce temperature fluctuations of the bath caused by variations in temperature of the local water supply.  In 2020, a parallel data acquisition system featuring a smart waveform digitizer was installed.  Eventually, the new system will replace the older analogue-based system.  In the meantime, parallel operation is allowing us to vet the new system.

In a typical year, 6 to 8 external vendor neutron sources are calibrated. In support of these measurements, our own standard neutron sources (NBS–1, BIPM-1) 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.

A major effort is currently underway to recalibrate NBS-1 using the Alpha-Gamma method.