Cosmogenic Production Near the Air-Rock Interface

许多重要的地球科学问题可以通过确定特定岩石样本暴露在地球表面的时间来解决。例如，终端冰碛由冰川融化时沉积在冰川前端的巨石组成。因此，巨石的表面暴露年龄限制了冰川退缩的时间。在过去十五年里，通过高能宇宙射线与岩石中原子在地球表面最上层约 1 米的相互作用产生的稀有同位素的积累，测量年龄的技术发展取得了巨大进展。准确的“宇宙成因测年”需要准确地了解同位素产生的速率和系统。该项目将研究这些系统学中人们特别不了解的部分 - 发生在空气-岩石界面的过程，其中宇宙射线从一种介质（大气）过渡到一种非常不同的介质（岩石）。该界面的核过程将通过对宇宙射线（包括氦 3 和氖 21）产生的几种不同同位素的岩石最上面几厘米进行高空间分辨率分析来研究。 此外，合成矿物靶材将在安第斯山脉高海拔地区受到宇宙射线照射，那里的宇宙射线通量非常高。这些目标将以高空间分辨率的同一组同位素进行检索和分析。合成目标补充了天然岩石的工作，因为它们在化学成分和暴露历史方面允许更加受控的环境。这项研究的最终目标是提高使用宇宙同位素作为确定地球表面年龄的工具的准确性和可靠性。这项研究广泛应用于许多相关过程的调查，例如侵蚀速率、断层运动的时间以及记录第四纪气候变化。

Many important Earth science questions can be addressed by determining the time at which specific rock samples became exposed at the Earth's surface. For example, a terminal moraine consists of boulders deposited at the front of a glacier as it melts; hence the surface exposure age of the boulders constrains the timing of glacial retreat. Over the last fifteen years great progress has been made in the development of techniques to measure such ages via the accumulation of rare isotopes produced by interactions in the uppermost ~ 1 meter of Earth's surface between high energy cosmic rays and the atoms in rocks. Accurate "cosmogenic dating" requires that the rate and systematics of isotope production be accurately known. This project will investigate a particularly poorly understood part of these systematics - the processes occurring at the air-rock interface, where cosmic rays transition from one medium (atmosphere) in to a very different one (rock). Nuclear processes at this interface will be investigated through high spatial resolution analyses in the uppermost few cm of rock of several different isotopes produced by cosmic rays, including helium-3 and neon-21. In addition, synthetic mineral targets will be exposed to cosmic ray irradiation at high elevation in the Andes, where the cosmic ray flux is very high. These targets will be retrieved and analyzed at high spatial resolution for the same set of isotopes. The synthetic targets complement the work on natural rocks because they permit a far more controlled environment in terms of chemical composition and exposure history. The ultimate goal of this research is to increase the accuracy and reliability of using cosmogenic isotopes as tools for determining the age of the Earth's surface. This research has widespread application to investigations of many relevant processes such as rates of erosion, timing of fault movements, and in documenting Quaternary climate change.