Quantitative determination of ¹⁴⁶Sm/¹⁴⁷Sm ratios by accelerator mass spectrometry with and ECR ion source and linear acceleration for ¹⁴⁶Sm half-life measurement

The alpha-decaying ¹⁴⁶Sm nuclide is used for chronology of the Solar System and silicate mantle differentiation in planets. We performed a new determination of ¹⁴⁶Sm half-life by measuring ¹⁴⁶Sm/¹⁴⁷Sm alpha activity and atom ratios in ¹⁴⁷Sm activated via (g,n), (n,2n) and (p,2n) reactions and obtained a value (68 Myr), smaller than that adopted so far (103 Myr), with important geochemical implications*. The experiment required determination of ¹⁴⁶Sm/¹⁴⁷Sm ratios by high-energy (6 MeV/u) accelerator mass spectrometry to discriminate ¹⁴⁶Sm from isobaric ¹⁴⁶Nd contaminant. Activated Sm targets were dissolved, chemically purified and reconverted to metallic Sm. Sputter cathodes, made by pressing the Sm metal into high-purity Al holders, were used to feed the Argonne ECR ion source. ¹⁴⁶Sm²²⁺, ¹⁴⁷Sm²²⁺ ions were alternately injected and accelerated with the ATLAS linac by proper scaling of ion source and accelerator components. A tightly-fitted quartz cylindrical liner was inserted in the ECR plasma chamber to reduce contamination from the walls. ¹⁴⁶Sm ions were eventually counted in a gas-filled magnet and ¹⁴⁷Sm ions either measured as charge current or counted after proper attenuation. * N. Kinoshita et al., Science 334, 1614 (2012) Supported by U.S. DOE, Office of Nuclear Physics, contract No. DE-AC02- 06CH11357, NSF JINA Grant Nr. PHY0822648 and Science Research Prog. of Japan Society for the Promotion of Science (20740161)