Standardization of ²⁴⁰Pu (SRM 4338b)

Plutonium-240 decays 100 % by alpha transitions to uranium-236 with a half-life of 6561 years. It is a manufactured isotope and can be found in irradiated uranium fuel or spent uranium fuel. Along with ²³⁹Pu, ²⁴⁰Pu is a byproduct of nuclear reactor operations and nuclear bomb explosions. The certified massic activity for ²⁴⁰Pu was obtained by 4pα liquid scintillation (LS) counting using three different LS measurement systems (with widely varying operating systems). For the determination two different LS cocktails compositions (with 5 counting sources each) were measured 5 times on each of the three instruments. The combined standard uncertainty (k = 2) on the standardization is 1.1 %. This solution is gravimetrically linked to a ²⁴⁰Pu stock solution obtained from Oak Ridge National Laboratory in February 1980 and was used to prepare the previous NIST ²⁴⁰Pu standard SRM 4338a, which was certified using NIST “0.1 π”α defined-solid-angle counter with scintillation detector and a reference time of 1 May 1996.  A remeasurement of SRM 4338a at the time of this work in 2019 agreed with the decay-corrected 1996 certified value to within 0.26 %.