Quantitation of 241Pu in low-level waste and environmental samples is of interest because 241Pu is a precursor of other transuranium nuclides that have longer half-lives, greater environmental mobility, and greater toxicity. In addition, 241Pu is the most difficult of the plutonium isotopes to detect and measure. As a result, radioactivity standards of 241Pu are of great interest for many environmental monitoring and radioecological studies. A new solution standard of 241Pu was developed and disseminated as Standard Reference Material 4340B in 2007, during this standardization an unexpected and inexplicable discrepancy (7.7 % difference) between CIEMAT/NIST 3H-standard efficiency tracing method (CNET) and triple-to-double-coincidence ratio (TDCR) counting method was uncovered. In an effort to further study and resolve this measurement discrepancy, an international measurement comparison amongst a few select national metrology laboratories was planned and conducted.
The 241Pu international comparison was hosted by NPL. This discrepancy between CNET and TDCR was found in the preliminary determinations of the NPL solution but further investigation during this calibration showed some of the reasons for the discrepancy. The use of a Polya-based model, which resolves the problems of operating below the single photoelectron threshold (1.8 %); plastic LS vials which reduces the losses due to internal reflection of UV photons at the glass-air interface of the LS vial (0.98 %); manual discriminator threshold settings, which ensures that the setting of the discriminator level match the experimental detection efficiency to the theoretical detection efficiency; a new beta spectrum shape that was determined with a cryogenic metallic magnetic calorimeter at the Laboratoire Nationale Henri Becquerel (LNHB) (3.2 %), and proper impurity corrections in which contributions from alpha emitting impurities and 241Am are subtracted from the raw data (2.3 %), led to incremental increases in the calculated activity and caused the difference between CNET and TDCR to change from -7.7 % to +0.7 %.
The 241Pu primary standardization for the international comparison was based on relative LS rate measurements with a 30+ years aged solution, whose activity in turn had been determined by 4pa(LS)-g(NaI) live-timed anticoincidence (LTAC) measurements of 241Am to follow the in-growth from 241Pu (LTAC/LS). The result was confirmed by measurements on the NPL solution by three other methods: (i) CNET; (ii) TDCR method; (iii) LTAC (LTAC/241Am) measurements of 241Am to follow the in-growth from 241Pu directly on the NPL solution. The expanded (k = 2) uncertainties for the three confirmatory methods were: (i) 3.8 %; (ii) 2.6 %; and (iii) 3.1 %, respectively. All of the confirmatory measurements agreed with the NPL reported value within their respective measurement uncertainties. The 241Pu content of the 30+ aged solutions, by 241Am ingrowth, has been followed by various determinations. Every determination was within 1 % of the previous result irrespective of measurement method, i.e., 4pα(LS) - g(NaI) coincidence and anti-coincidence counting of 241Am daughter ingrowth, LS following the ingrowth of 241Am, LTAC, and CNET.
As part of the international comparison, impurities of the solution were determined and reported. Both 241Pu solutions, (NIST SRM 4340B and NPL solution) were analyzed for 239&240Pu, 238Pu, 242Pu impurities, and the 241Am daughter by alpha spectrometry using an isotope dilution method with 243Am and 242Pu as tracers. The separation and purifications were performed using TEVA and TRU extraction chromatographic resin (Eichrom). The relative massic activity impurity levels were less than 0.1% that of the 241Pu. High-resolution photon spectrometry with HPGe (Be window) calibrated detector in defined geometry was used for the determination of gamma impurities in the NPL solution. None were detected.