Table 2.

Certified massic activities of ²³⁴U, ²³⁵U, and ²³⁸U in the natural uranium solution standard SRM 4321d. The uncertainties on these certified values are expanded uncertainties, U = k u_c. The quantity u_c is the combined standard uncertainty that was calculated according to NIST and JCGM guidelines [5, 6]. To obtain U, the combined standard uncertainty u_c was multiplied by a coverage factor of k = 2, which was chosen to obtain an approximate 95 % level of confidence. The uncertainty components that comprise U are given in Table 3.

Uranium isotope	Massic activity of isotope in SRM 4321d	Relative expanded uncertainty, U (k = 2)
234U	220.16 Bq g−1	0.52 %
235U	10.540 Bq g−1	0.31 %
238U	228.93 Bq g−1	0.25 %