Many neutron sources required by Homeland Security have a lower neutron emission rate than is appropriate for the NIST calibration facility, a Manganese Sulfate Bath system.
The Manganese Sulfate Bath uses a sphere of neutron-absorbing material which surrounds a neutron source. The induced radioactivity is a measure of the neutron source strength. The lower intensity of the DHS sources provides less manganese activation, resulting in a reduced signal over background. NIST developed a reduced-volume bath so that a greater fraction of the manganese is close to the source and therefore induces higher manganese activity per unit volume. Unfortunately, the smaller bath also has a higher neutron leakage. The fraction of neutron leaking from the sphere depends on the neutron spectrum. NIST uses the new bath only as a means to compare one Californium source against another so that the spectrum remains constant. High-fluence Californium sources calibrated in the existing Manganese Sulfate Bath will be used to calibrate the new bath. The reduced-volume bath has been running continuously for more than six years now. Several test-sources have been calibrated for DHS customers. The bath has so far been completely free of the usual leakage problems of manganese sulfate systems, and the water-vapor-barrier vent has kept the bath volume very nearly constant.
A new reduced-volume manganese bath permits calibration of low-intensity neutron sources as required for DHS applications. (Photograph by: NI&D Group)
Lead Organizational Unit:pml
Maynard (Scott) Dewey