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

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

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.