Description: 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. A more direct calibration is also being developed, based on Cf-252 NUBAR, the average neutron emission per fission in Cf-252. A paper was presented on progress in this NUBAR calibration method at the 13th International Symposium on Reactor Dosimetry (ISRD13), in May 2008, Akersloot, Netherlands. The neutron fluence specification required by ANSI standards requires new sources at least every 20 months, usually sooner. The new calibration facility is required to meet the demand. The reduced-volume bath has been running continuously for more than two 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.