Specification and Preparation of Pure Plutonium Metal Standards for Nondestructive Assay

To calibrate neutron coincidence and neutron multiplicity counters for passive assay of plutonium, certain detector parameters must be determined. When using small plutonium metal samples, biases can be introduced from non-zero multiplication and impurities. This paper describes preparation of small, pure plutonium metal standards with well-known geometries to enable accurate multiplication corrections and with acceptable low levels of impurities. To minimize multiplication, these standards are designed as thin foils of diameter 2 cm, with varying thicknesses, and masses 1.4, 3.6, and 7.2 g of plutonium. These standards will significantly improve characterization and calibration of neutron coincidence and multiplicity counters. They can also be equally useful for gamma-ray spectrometry and calorimetry. Five sets will be made, four for other United States Department of Energy plutonium facilities, and one set to remain at Los Alamos. We also describe other nondestructive assay standards that are planned for the next few years.