Amy M. Gaffney

An intercomparison of the radio-chronometric ages of four distinct plutonium-certified reference materials varying in chemical form, isotopic composition, and period of production are presented. The cross-comparison of the different ²³⁴U/²³⁸Pu, ²³⁵U/²³⁹Pu, ²³⁶U/²⁴⁰Pu, and ²⁴¹Am/²⁴¹Pu model purification ages obtained at four independent analytical facilities covering a range of laboratory environments from bulk sample processing to clean facilities dedicated to nuclear forensic investigation of environmental samples enables a true assessment of the state-of-practice in “age dating capabilities” for nuclear materials. The analytical techniques evaluated used modern mass spectrometer instrumentation including thermal ionization mass spectrometers and inductively coupled plasma mass spectrometers for isotopic abundance measurements. Both multicollector and single collector instruments were utilized to generate the data presented here. Consensus values established in this study make it possible to use these isotopic standards as quality control standards for radio-chronometry applications. Results highlight the need for plutonium isotopic standards that are certified for ²³⁴U/²³⁸Pu, ²³⁵U/²³⁹Pu, ²³⁶U/²⁴⁰Pu, and ²⁴¹Am/²⁴¹Pu model purification ages as well as other multigenerational radio-chronometers such as ²³⁷Np/²⁴¹Pu. Due to the capabilities of modern analytical instrumentation, analytical laboratories that focus on trace level analyses can obtain model ages with marginally larger uncertainties than laboratories that handle bulk samples. When isotope ratio measurement techniques like thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry with comparable precision are utilized, model purification ages with similar uncertainties are obtained.

Nature 541, 77–80 (2017); doi:10.1038/nature20136 In this Letter, on page 78, the sentence beginning “At this ‘null zone’20” should have cited ref. 18, instead of ref. 20. The original Letter has been corrected online.