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

40Ar/39Ar定年法被广泛应用于确定地球和行星材料的年龄。它需要39K的中子激活来产生39Ar，这通常是在裂变反应堆中进行的。40Ar（来自40K的衰变）和39Ar（作为K含量的代表）随后在稀有气体质谱仪中进行分析，它们的比值定义了样品材料的年龄。裂变反应堆中中子能量的光谱特征导致了两个主要缺陷：(1)Ca、Cl和K的活化产生Ar同位素，干扰了放射成因和辐照产生的Ar，这需要修正；这增加了日期的不确定性；(2)高能中子将动能传递给39Ar，使其在样品材料中置换（反冲效应）；这就限制了可靠测定的晶粒尺寸为几十微米。在伯克利地质年代学中心建造的新型高通量氘核-氘核聚变中子发生器提供了以1011 n s-1 cm-²的通量速率照射样品的准单能（2.45 MeV）中子的手段。我的项目旨在提供“概念验证”，量化新仪器的功能，并进行首次应用。在用金属通量监测仪进行初步测试后，我将照射合成的含Ca、Cl和k相，并分析活化的Ar同位素。这将量化干扰Ar同位素的预期减少量。我将辐照具有几何特征的黑云母和斜长石，并随后量化这些辐照过程中39Ar的粒度损失，以量化反冲长度尺度。数值模拟表明，该数值降低了一个数量级，从而可以可靠地测定细粒材料（~7微米）的年代。如果成功的话，新的仪器将提供可靠地测定重要材料日期的手段。例如，来自古人类遗址的难以确定年代的凝灰岩的玻璃碎片可以提高我们对人类进化时间的理解，而来自陨石样本的细粒富钙铝包裹体（CAIs）的精确定年可能会改善我们对早期太阳系历史的理解。

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.