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

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

We propose to measure the cosmic ray intensity incident on the Earth as a function of time over the past 600 million years. That history is imbedded in salt deposits. Since cosmic ray muons interacting with salt deposits convert 23Na into 22Ne, the concentration of 22Ne in a given salt deposit is a measure of the integrated muon flux since the formation of that deposit. Salt deposits have well known ages based on fossils in adjacent strata and on their own internal clock, the ratio of 40Ar/40K: 40K decays into 40Ar with a half life of 1.28 billion years. In addition to its intrinsic interest, the cosmic ray chronology of the Earth also will permit us to look for variation of cosmic ray intensity over time due to nearby supernova and see if there is a temporal relationship between these events and mass extinctions on Earth. It is generally assumed that high energy proton cosmic ray primaries are generated in the outbound shock waves of supernovae. These shock waves accelerate protons for ~106 years. The protons then proceed in spirals with Larmor radii ~ 300 x E(eV)/B cm, where E is the energy of the proton, and B (in TGauss) is the interstellar Galactic magnetic field. Every ~107 years (t = 1/Nosc=107 yrs), these protons collide with interstellar matter. Since these collisions are primarily with other protons, the incident proton loses about half its energy per collision. After several collisions the energy of the initial proton is sufficiently reduced so that it is no longer a significant producer of high energy muons in the Earth's atmosphere. Thus, the effective duration of the high energy cosmic ray signal from a given supernova is 2 -3 x 107 years. Nearby supernova, which occur every 108 years or so, will result in increased cosmic ray intensity for a period of 2-3 x 107 years. Since salt deposit ages range to 600 million years and beyond, they contain the records of several nearby supernova. Indeed, we may even be living in such a high cosmic ray flux period. Comparing the present cosmic ray flux with the average over the memory of various salt deposits will answer that question. The extraction of neon and argon from salt solutions is identical to the extraction of argon from perchloroethylene, as in the Homestake chlorine solar neutrino detector, and the extraction of xenon from the sodium iodide detector we operated at LAMPF. For a constant cosmic ray flux, the ratio of 22Ne/40Ar depends only on the depth of the deposit (and on the relevant cosmic ray muon cross sections and 40K concentration and life time). The depth history of each measured salt deposit will be determined by a combination of vitrinite reflection, illite crystalinity and fission track thermal-chronology together with the geological history of the region.