Physics Division Research Highlights

A Shorter 146Sm Half-Life and Its Implications for the History of the Early Solar System

Accelerator Mass Spectrometry (AMS) at ATLAS was used to re-measure the half-life of alpha-decaying 146Sm. The new value, 68 My, is much shorter than the adopted one (103 My), leading to a shortening of the 146Sm-142Nd chronology in the Early Solar System. The determination was made relative to the precisely known half-life (107 Gy) of natural 147Sm by measuring both the 146Sm/147Sm alpha activity and atom ratios. AMS measures the atom ratio using the high ion energy from ATLAS and a gas-filled Enge spectrograph and separates 146Sm ions from 146Nd isobaric impurities, a potential background in conventional mass spectrometry.

Samarium AMS spectra

Fig.1: Alpha-activity (left) and AMS spectra (right) showing the clean separation of 146Sm from 146Nd.

146Sm, a p-process nuclide now extinct, was extant in the Early Solar System and is widely used to study its time evolution over the first few hundred million years. Recent measurements in terrestrial and lunar rocks and Martian meteorites provide timing information for the differentiation of the silicate portion from the mantle of planetary bodies owing to a delicate difference in geochemical properties between samarium and its a daughter neodymium. The shorter 146Sm half-life results in an estimated earlier differentiation that is consistent, in the case of a recently dated lunar rock, with other chronometers.

Fig. 2: New value of 146Sm half-life (68 My) relative to adopted previous value.


N. Kinoshita et al., Science 335, 1614 (2012)


Previous Highlight - Next Highlight