Measurement of the Earth's Cosmic Ray Chronology with the SALT Detector

这项实验的目标是测量过去6亿年来入射到地球上的宇宙射线强度随时间的变化。这将是第一次测量地球上这一大片区域的宇宙线强度。高能宇宙线初级极有可能是由超新星的出射冲击波产生的。太阳系附近的超新星将导致撞击地球的宇宙射线强度大幅增加。因此，这项测量将确定在过去6亿年中，超新星何时出现在太阳系附近。地球的宇宙射线历史隐藏在盐层中。宇宙线介子与盐类沉积物相互作用，将23Na转化为22Ne。给定盐层中~(22)Ne的浓度测量了自该盐层形成以来入射到该盐层上的积分介子通量。盐矿是由内海蒸发在明确的时期形成的。特定盐类矿床的形成时间可以根据邻近地层中化石的年龄来确定。此外，由于盐类沉积物携带有自己的内部时钟，40Ar/40K：40K的比值衰变为40Ar，半衰期为12.8亿年，因此可以通过盐类中40Ar与40K的比值来确定盐矿的年龄。将利用稀有原子探测技术提取和测量埋藏在不同年龄和深度的盐矿中的22Ne和40Ar。根据对这些浓度的测量，将重建过去6亿年入射到地球上的宇宙射线的强度。有人认为，附近的超新星可能会引发一系列事件，导致优势生物物种的实质性灭绝。推测的机制是超新星发出强烈的伽马射线爆发。这些伽马射线破坏了地球的臭氧屏障，因此允许来自太阳的紫外线辐射到达地球表面。这种紫外线辐射会杀死占主导地位的生物物种。虽然SALT实验不能检验附近的超新星可能引发生物物种大规模灭绝的假设，但它可以确定宇宙线强度的增加和大规模灭绝的发生之间是否存在时间上的对应。这一努力涉及三个不同的学科：物理学、天文学和地球科学，并可能对第四个学科--生物学--产生影响。它为我们存在的一个基本问题--附近灾难性的天文事件是否与同样灾难性的陆地事件重合--提供了关键的输入数据--物种加速进化、大规模灭绝、重大气候变化？这项研究影响了科学的多个领域，也影响了整个社会。研究生将参与这项研究项目的所有方面。此外，建立宇宙线年代学可能会使各种其他盐岩形成的年代测定成为可能，例如墨西哥湾地区和其他地方的盐丘。由于这些盐丘与油气藏关系密切，它们从地下深处上升到现在的浅层的时间可以为进一步勘探与之相关的油气藏提供重要的信息。

The goal of this experiment is to measure the intensity of cosmic rays incident on the Earth as a function of time over the past 600 million years. This will be the first measurement of cosmic ray intensity over this large fraction of the Earth's existence. High energy cosmic ray primaries are most likely generated by the outbound shock waves of supernovae. A supernova in the vicinity of the solar system will result in a substantial increase in the intensity of cosmic rays hitting the Earth. Thus, this measurement will determine when, in the past 600 million years, supernovae occurred in the vicinity of the solar system.The cosmic ray history of the Earth is imbedded in salt deposits. Cosmic ray muons interacting with salt deposits convert 23Na into 22Ne. The concentration of 22Ne in a given salt deposit measures the integrated muon flux incident on that deposit since its formation. Salt deposits were formed at well-defined times by the evaporation of inland seas. The formation time of a given salt deposit can be determined by the age of fossils in adjacent strata. In addition, since salt deposits carry their own internal clock, the ratio of 40Ar/40K: 40K decays into 40Ar with a half life of 1.28 billion years, the age of a salt deposit can be determined by the ratio of 40Ar to 40K in the salt. The 22Ne and 40Ar imbedded in salt deposits of various ages and depths will be extracted and measured using the rare atom detection techniques. From the measurements of these concentrations, the intensity of cosmic rays incident on the Earth over the past 600 million years will be reconstructed.It has been suggested that a nearby supernova could initiate a chain of events that lead to a substantial extinction of dominant living species. The suggested mechanism is that the supernova emits an intense burst of gamma rays. These gamma rays destroy the Earth's ozone shield and so permit Ultra Violet radiation from the Sun to reach the Earth's surface. This UV radiation then kills the dominant living species. Although the SALT experiment cannot test the hypothesis that nearby supernova can initiate mass extinction of living species, it can determine whether there is a temporal correspondence between increased cosmic ray intensity and occurrence of mass extinctions. This effort involves three different disciplines: physics, astronomy and earth sciences and may have an impact on a fourth, biology. It provides critical input data to a question basic to our existence - did nearby calamitous astronomical events coincide with equally calamitous terrestrial occurrences - accelerated species evolution, mass extinction, significant climatic changes? This research impacts multiple fields of science as well as society as a whole. Graduate students will be involved in all aspects of this research project.In addition, establishing this cosmic ray chronology may permit the dating of various other salt formations, such as the salt domes in the Gulf of Mexico region and elsewhere. Since these salt domes are closely associated with oil deposits, the time at which they arose from deep underground to their present shallow depth can provide significant information for the further exploration of the associated oil deposits.