Cosmogenic Isotopes Produced In Situ in Terrestrial Rocks: Quantifying the Effect of Altitude and Depth on the Production Rates

虽然在理论和实验工作的基础上已经了解了原地产生的宇宙成因同位素的一般系统学，但需要几个领域的新的定量信息来完善宇宙成因定年方法。最近对实验生产率和古地磁强度影响的研究表明，在一个地点和时间确定的生产率不能在不引入系统误差的情况下转移到其他地点或时间，估计系统误差约为20%。这些不确定性有许多具体的原因，都围绕着地球上宇宙射线强度分布的基本问题。为了提高我们对中子分布的理解，我们开始了一个中子监测计划，根据这个计划，我们正在测量中子强度和宇宙成因同位素产生率作为纬度和海拔的函数。目前资助的一年期项目将确定快中子和宇宙成因36 Cl（由快中子和热中子产生）和3 He（由快中子产生）在空气中的衰减长度。这些测量是八分之三，是必要的高度和深度依赖宇宙生成率的充分表征。拟议项目将提供其余五项：热中子在空气中的衰减长度、快中子和热中子以及快中子和热中子产生的宇宙成因核素在岩石中的衰减长度。这个建议的目的是确定从宇宙射线中子和宇宙成因核素在岩石中积累的测量得到的衰减长度之间的关系。这一目标将通过以下措施来实现。(1)从海平面到4000米（莫纳克亚山顶部），热中子强度随海拔的变化。为了避免影响热中子的空气-地面边界，将在地面以上300米处使用连接在小型压力气球上的裸露中子探测器进行测量。(2)岩石中0-100 cm处快中子和热中子强度随深度的变化。这些测量将在岩石中钻出的水平通道孔中进行，并用玄武岩板屏蔽仪器顶部。(3)浅（100厘米）垂直岩芯中的宇宙成因14 C、36 Cl和3 He浓度。将对两个岩芯进行研究，每个岩芯15个样品;取样将集中在顶部20厘米处。三个具体目标是：确定快中子产生的宇宙成因同位素的衰减长度;定义热中子产生的同位素的深度函数;确定快中子产生的同位素是否受空气-地面边界的影响。同位素和中子数据将进行比较，以确定中子强度和宇宙成因生产率之间的关系。这一评估对于发展计算缺乏同位素数据的地点的可靠宇宙成因生产率的能力至关重要。这项工作的结果将被用来构建一个改进的海拔定标公式宇宙成因的生产率，这将是一个重要的贡献，以改善所有的宇宙成因测年方法。

ZredaEAR-0126209Although the general systematics of in-situ produced cosmogenic isotopes has been understood based on theoretical and experimental work, new quantitative information in several areas is needed to refine cosmogenic dating methods. Recent research on experimental production rates and effects of paleomagnetic intensity has shown that the production rates determined at one location and time cannot be transferred to other locations or times without introducing systematic errors that are estimated to be on the order of twenty percent. There are many specific reasons for these uncertainties, all revolving around a fundamental general problem of the distribution of the cosmic-ray intensity on earth. To improve our understanding of the distribution of neutrons, we started a neutron monitoring program, under which we are measuring the neutron intensity and cosmogenic isotope production rates as a function of latitude and elevation. The currently-funded one-year project will determine the attenuation lengths in the air for fast neutrons and for cosmogenic 36Cl (produced by fast and thermal neutrons) and 3He (produced by fast neutrons). These measurements are three out of eight that are necessary for a full characterization of altitude and depth dependence of cosmogenic production rates. The proposed project will provide the remaining five: the attenuation length in the air for thermal neutrons, and attenuation lengths in the rock for fast and thermal neutrons and for cosmogenic nuclides produced by fast and thermal neutrons. The goal of this proposal is to determine the relationships between the attenuation lengths obtained from measurements of cosmic-ray neutrons and of cosmogenic nuclides accumulated in rocks. This goal will be achieved by conducting the following measurements. (1) The thermal neutron intensity as a function of elevation, from sea level to 4000 m (top of Mauna Kea). To avoid the air-ground boundary, which affects thermal neutrons, the measurements will be conducted 300 m above the ground using bare neutron detector attached to a small pressure balloon. (2) The fast and thermal neutron intensity as a function of depth, from 0-100 cm in the rock. These measurements will be conducted in horizontal access holes drilled into the rock and by shielding the top of the instrument by slabs of basalt. (3) Cosmogenic 14C, 36Cl and 3He concentrations in shallow (100 cm) vertical cores. Two cores will be studied, with 15 samples per core; sampling will concentrate in the top 20 cm. Three specific objectives are: to determine the attenuation length for cosmogenic isotopes produced by fast neutrons; to define the depth function for thermal-neutron-produced isotopes; and to determine whether fast-neutron-produced isotopes are affected by the air-ground boundary. Isotopic and neutron data will be compared in order to define the relationship between the neutron intensity and cosmogenic production rates. This assessment is critical for developing the ability to calculate reliable cosmogenic production rates for locations where isotopic data are lacking. The results of this work will be used to construct an improved altitudinal scaling formulation for cosmogenic production rates, which will be an important contribution to the improvement of all cosmogenic dating methods.