Cosmogenic nuclide production within the atmosphere and long period comets

Abstract

The Earth is constantly bombarded by cosmic rays. These high energy particles collide with target nuclei, producing a shower of secondary particles. These secondaries contribute significantly to the radiation background at sea level and in the atmosphere, as well as producing rare cosmogenic nuclides. This contribution is variable over long time scales as astrophysical events change the cosmic ray flux incident on the Earth. Our work re-examines a previously proposed climate effect of increased cosmic ray flux due to galactic location. Although our work does not support this effect, cosmic ray secondaries remain a threat to terrestrial biota. We calculate the cosmogenic neutron flux within the atmosphere as a function of primary spectrum. This work is pivotal in determining the radiation dose due to any arbitrary astrophysical event where the primary spectrum is known. Additionally, this work can be used to determine the cosmogenic nuclide production from such an event. These neutrons are the fundamental source of cosmogenic nuclides within our atmosphere and extraterrestrial matter. We explore the idea that excursions in 14C and 10Be abundances in the atmosphere may arise from direct deposition by long-period comet impacts, and those in 26Al from any bolide. We find that the amount of nuclide mass on large long-period comets entering the Earth's atmosphere may be sufficient for creating anomalies in the records of 14C and 10Be from past impacts. In particular, the estimated mass of the proposed Younger Dryas comet is consistent with its having deposited sufficient isotopes to account for recorded nuclide increases at that time. The 26Al/10Be ratio is much larger in extraterrestrial objects than in the atmosphere, and so, we note that measuring this ratio in ice cores is a suitable further test for the Younger Dryas impact hypothesis. This portion of our work may be used to find possible impact events in the geologic record as well as determination of a large bolide impact rate.