"Proof of concept" and first application of a novel Deuteron-Deuteron (D-D) fusion neutron generator for 40Ar/39Ar sample irradiation

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The 40Ar/39Ar dating method is broadly applied to determine the age of Earth and planetary materials. It requires neutron activation of 39K to produce 39Ar, which is conventionally conducted in a fission reactor. 40Ar (from decay of 40K) and 39Ar (as a proxy for K content) are subsequently analyzed in a noble-gas mass spectrometer with their ratio defining the age of the sample material. The spectral character of neutron energy in a fission reactor causes two major drawbacks: (1) Activation of Ca, Cl, and K produces Ar isotopes, interfering with the radiogenic and irradiation-produced Ar, which requires corrections; this increases the uncertainty of dates; (2) High-energy neutrons transfer kinetic energy to 39Ar, displacing it in the sample material (recoil effect); this limits the reliably dateable grain size to tens of micrometer. A novel high-flux Deuteron-Deuteron fusion neutron generator built at the Berkeley Geochronology Center provides means to irradiate samples with quasi-monoenergetic (2.45 MeV) neutrons at flux rates of 1011 n s-1 cm-². My project aims to provide the "proof of concept", quantify capabilities of the novel instrumentation, and conduct first applications. After initial tests with metallic fluence monitors, I will irradiate synthetic Ca-, Cl-, and K-bearing phases and analyze the activated Ar isotopes. This will quantify the anticipated reduction of the interfering Ar isotopes. I will irradiate geometrically characterized biotite and plagioclase and subsequently quantify the grain-size dependent loss of 39Ar during these irradiations to quantify recoil length scales. Numerical modeling suggests a reduction by an order of magnitude, enabling reliable dating of fine-grained material (~7 mikrom). If successful, the new instrumentation will provide means to reliably date momentous material. For example, glass shards from difficult-to-date tuffs from hominid sites can improve our understanding of the timing of human evolution, and precise dating of fine grained Ca-Al-rich inclusions (CAIs) from meteorite samples may refine our comprehension of the early Solar System's history.