Fig. 1.

(a) The ease of applying ²³¹Pa–²³⁵U radiochronometry to a uranium sample depends on its ²³⁵U content. The samples analyzed in this study are depleted uranium, containing ~ 0.2 atom percent ²³⁵U. (b) For a 1 mg sample aliquot, the cumulative ingrowth of ²³¹Pa in depleted uranium samples is insufficient for precise measurements via mass spectrometry, assuming standard instrument operating conditions. (c) Sample aliquot size must be increased to compensate for the lack of ²³¹Pa in depleted uranium samples. The amount needed is 10- to 100-fold greater than the amount required for more enriched samples. (d) The challenge of applying ²³¹Pa–²³⁵U radiochronometry to U–Nb alloys lies in isolating ultra-trace level amounts of Pa from weight-percent abundances of Nb and U.