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

该实验的目的是测量过去6亿年中入射到地球上的宇宙射线强度随时间的变化。 这将是第一次测量宇宙射线强度超过地球存在的这一大部分。 高能宇宙线的初级波很可能是由超新星的向外冲击波产生的。 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，这一测量将确定在过去6亿年中，太阳系附近何时发生过超新星。地球的宇宙射线历史埋藏在盐矿床中。 宇宙射线μ子与盐沉积物相互作用，将23 Na转化为22 Ne。 给定盐存款中的22 Ne的浓度测量自其形成以来入射到该存款上的积分μ子通量。 盐沉积物是在明确的时间内由内海的蒸发形成的。 一个特定盐存款的形成时间可以由邻近地层中的化石年龄来确定。 此外，由于盐矿床具有自己的内部时钟，40 Ar/40 K：40 K的比例衰变为半衰期为12.8亿年的40 Ar，因此盐存款的年龄可以通过盐中40 Ar与40 K的比例确定。用稀有原子探测技术对埋藏在不同年代和深度的盐矿床中的~（22）Ne和~（40）Ar进行了提取和测量。 通过测量这些浓度，将重建过去6亿年中入射到地球上的宇宙射线的强度，有人认为，附近的超新星可能引发一系列事件，导致主要生物物种的大规模灭绝。提出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层，因此允许来自太阳的紫外线辐射到达地球表面。 这种紫外线辐射然后杀死占优势的生物物种。 虽然SALT实验无法验证附近超新星可以引发生物物种大灭绝的假设，但它可以确定宇宙射线强度增加与大灭绝发生之间是否存在时间上的对应关系。 这项工作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。 它为我们生存的基本问题提供了关键的输入数据--附近灾难性的天文事件是否与同样灾难性的陆地事件相吻合--加速了物种进化、大规模灭绝、重大的气候变化？ 这项研究影响了多个科学领域以及整个社会。 研究生将参与这一研究项目的各个方面，此外，建立这一宇宙射线年表可能允许确定其他各种盐层的年代，如墨西哥湾地区和其他地方的盐丘。 由于这些盐丘与石油矿床密切相关，它们从地下深处出现到现在的浅部的时间可以为进一步勘探相关的石油矿床提供重要信息。

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.