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

用于 40Ar/39Ar 样品辐照的新型氘核-氘核 (D-D) 聚变中子发生器的“概念验证”和首次应用

• 批准号: 287320649
• 负责人: Dr. Daniel Rutte
• 金额: --
• 依托单位: Department of Earth and Planetary Science
• 依托单位国家: 德国
• 项目类别: Research Fellowships
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2017-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/287320649?language=en
• 关键词: Proof; concept; first; application; novel

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.

40 Ar/39 Ar定年法被广泛应用于确定地球和行星物质的年龄。它需要39 K的中子活化来产生39 Ar，这通常在裂变反应堆中进行。40 Ar（来自40 K的衰变）和39 Ar（作为K含量的代表）随后在稀有气体质谱仪中进行分析，它们的比例定义了样品材料的年龄。裂变反应堆中中子能量的光谱特性导致两个主要缺点：（1）Ca、Cl和K的活化产生Ar同位素，干扰放射性和辐照产生的Ar，这需要校正;这增加了日期的不确定性;（2）高能中子将动能转移到39 Ar，使其在样品材料中移位（反冲效应）;这将可可靠定年的晶粒尺寸限制在数十微米。伯克利地质年代学中心建造的一个新型高通量氘-氘聚变中子发生器提供了用准单能（2.45 MeV）中子以1011 ns-1 cm-2的通量率照射样品的方法。我的项目旨在提供“概念验证”，量化新仪器的能力，并进行首次应用。在用金属通量监测器进行初步测试后，我将照射合成的Ca-、Cl-和K-轴承相，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。我将照射几何特征的黑云母和斜长石，随后量化的粒度依赖的损失39 Ar在这些照射，以量化反冲长度尺度。数值模拟表明，减少了一个数量级，使可靠的测年细粒材料（约7 mikrom）。如果成功的话，新仪器将提供可靠的手段来确定重要材料的日期。例如，来自原始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夹杂物（CAIs）的精确年代测定可以改善我们对早期太阳系历史的理解。