The reference sources are unmoderated 252Cf and 252Cf moderated with a D2O sphere (as shown in the photograph). The spontaneous fission neutron spectrum of unmoderated 252Cf has been extensively studied and is known well enough to have achieved "benchmark" status. The moderated spectrum, with an abundance of low and intermediate energy neutrons, is more characteristic of reactor working environments and is often preferred for that reason. Some personnel dosimeters are sensitive to neutrons scattered off the person wearing the dosimeter, so that a frequent exposure configuration has dosimeters adjacent to an acrylic phantom. In all exposure geometries, corrections are made for air scatter and room return. NIST periodically compares its standard fields to those of other national standards laboratories.
NIST is also able to provide calibrations to 241AmBe reference fields as a special test.