Cosmogenic Chlorine-36 Production in Calcite by Muons
Cosmogenic Chlorine-36 Production in Calcite by Muons
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DOI：
10.1016/s0016-7037(97)00369-4
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发表时间：
1998-02
影响因子：
5
通讯作者：
J. Stone;J. Evans;L. Fifield;G. Allan;R. Cresswell
中科院分区：
文献类型：
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作者：
J. Stone;J. Evans;L. Fifield;G. Allan;R. Cresswell
在几米以下的深度，方解石中36 Cl的产生几乎完全由宇宙射线μ子引发。主要的反应是：（1）Ca; 40 Ca（μ−，α）36 Cl直接俘获负μ介子;（2）35 Cl俘获μ介子俘获和μ介子诱导的光致衰变反应中产生的次级中子。我们从20 m（5360 g cm−2）深度的石灰岩剖面上测定了方解石中μ子俘获引起的36 Cl和中子产生率。纯方解石中Ca俘获μ子的36 Cl产额为0.012 ± 0.002个原子/停止的负μ子。因此，在海平面和高纬度地区，方解石中钙的μ子俘获产生36 Cl的表面速率为2.1 ± 0.4 atom g−1a− 1，约为钙的11%。如果假设34%的负μ子被方解石中的Ca原子俘获，那么μ子俘获后40 Ca的α产额为0.043 ± 0.008，略低于最近μ子辐照实验的结果（0.062 ± 0.020），但在现有理论预测的极值（0.0033-0.15）之内。纯方解石中μ介子俘获后的平均中子产额为0.44 ± 0.15次级中子/停止的负μ介子，与现有的理论预测很好地吻合。在测定年轻地貌表面的年代时，必须考虑到μ子产生的宇宙成因同位素，特别是那些仅由几米覆盖岩石挖掘而产生的地貌表面。注意μ介子产生的同位素对精确确定地表侵蚀速率也至关重要。由于与宇宙线强子相比，μ子的渗透深度很深，μ子产生的36 Cl的积累对侵蚀的敏感性低于散生36 Cl。虽然在表面产生的μ子只有一小部分的生产spiritus，μ子产生的部分36 Cl在快速侵蚀的石灰岩表面可以接近50%。在这种情况下，使用传统的模型估计的侵蚀率，其中属性生产单独的孢子，将在误差高达40%。剥落和μ子产生的36 Cl侵蚀的敏感性的差异，提出了深侵蚀面的定年方法，检查计算侵蚀速率时的稳态假设，并解开多阶段暴露和侵蚀的历史。
At depths below a few metres,36Cl production in calcite is initiated almost entirely by cosmic ray muons. The principal reactions are (1) direct negative muon capture by Ca;40Ca(μ−,α)36Cl, and (2) capture by35Cl of secondary neutrons produced in muon capture and muon-induced photodisintegration reactions. We have determined rates for36Cl and neutron production due to muon capture in calcite from a 20 m (5360 g cm−2) depth profile in limestone. The36Cl yield from muon capture by Ca in pure calcite is 0.012 ± 0.002 atom per stopped negative muon. The surface production rate of36Cl by muon capture on Ca in calcite is, therefore, 2.1 ± 0.4 atom g−1a−1at sea level and high latitude, approximately 11% of the production rate by Ca spallation. If it is assumed that 34% of the negative muons are captured by the Ca atom in calcite, the α-yield from40Ca following muon capture is 0.043 ± 0.008, somewhat lower than the result of a recent muon irradiation experiment (0.062 ± 0.020), but well within the extremes of existing theoretical predictions (0.0033–0.15). The average neutron yield following muon capture in pure calcite is 0.44 ± 0.15 secondary neutrons per stopped negative muon, in good agreement with existing theoretical predictions. Cosmogenic isotope production by muons must be taken into account when dating young geomorphic surfaces, especially those created by excavation of only a few metres of overlying rock. Attention to isotope production by muons is also crucial to determining surface erosion rates accurately. Due to the deep penetration of muons compared to cosmic ray hadrons, the accumulation of muon-produced36Cl is less sensitive to erosion than that of spallogenic36Cl. Although production by muons at the surface is only a small fraction of production by spallation, the fraction of muon-produced36Cl in rapidly eroding limestone surfaces can approach 50%. In such cases, erosion rates estimated using conventional models which attribute production solely to spallation will be in error by up to 40%. The difference in sensitivity to erosion of spallogenic and muon-produced36Cl suggests methods for dating deeply eroded surfaces, checking the assumption of steady-state when calculating erosion rates, and unravelling multi-stage exposure and erosion histories.