Records of Cosmogenic Isotope Production Rates Lizz León

Some General Facts High-energy cosmic rays shower the Earth's surface, penetrating meters into rock and producing long-lived radionuclides Such as Cl-36, Al-26 and Be-10 Production rates of cosmogenic isotopes are almost unimaginably small A few atoms per gram of rock per year, down to levels of a few thousand atoms per gram Build-up of cosmogenic isotopes gives us a way to age rocks and rock surfaces, and to calculate erosion or soil accumulation rates Scaling Factors are calculated to determine cosmic ray exposure ages - assume a uniform relationship between altitude and atm. pressure (http://depts.washington.edu/cosmolab/)

Types of Cosmogenic Isotopes Atmospheric  Rain, or just in the atmosphere {36Cl, 14C, others} Secondary fast neutrons In-Situ  Minerals, few meters from the surface {36Cl, 14C, 10Be, 3He, others} Thermal neutrons Muons Hydrogen Deuterium Tritium

The Scoop on Muons Cosmic-ray muons originate mostly in the uppermost ~100 g/cm2 of the atm. They have been decayed from ± K± mesons, after primary interactions, before meeting other atm. nuclei Why do they penetrate the surface? Weakly interacting particles and energetic! At points of high rigidity cutoff (RC), solar modulation effects are smaller for muons of higher energies >20GeV (Stone et al., 1997)

Slow (Thermal) Neutrons Low energy What a neutron probe measure - geophysics! Nucleus

General Affecting Factors on Production Rates Elevation effects AS [ Altitude depth ] & [ Pressure ] = [ Production Rates exponentially ] Mainly due to muons - high energy progenitor Main Asteroid Belt Production rates are 1000x greater than on earth Some fall onto earth as meteorites Magnetic Field Latitude RC~ 5 Km

Case Study: What do production rates depend on? Spatio-temporal distribution of cosmic-ray nucleon fluxes Nucleon Attenuation Length Solar modulation - high latitudes Rigidity cutoff (RC) Changes over time because of the changing geomagnetic pole intensity (Desilets & Zreda, 2002)

Neutron intensity with atmospheric depth and RC Desilets & Zreda, 2002 cont...Spatio-temporal distribution of cosmic-ray nucleon fluxes Neutron intensity with atmospheric depth and RC

Case Study: In-situ 36Cl in K-feldspar Releasing Cl-rich fluid from inclusions in samples of crushed K-spar What? K-spar & Biotite Where? Ice-scoured bedrock in the Sierra’s Why? High 36Cl prod. rates to date (agree with a range of latitudes, altitudes, and exposure ages) Compared to? Scotland & Antarctic samples Antarctic prod. rates were 35% higher - WHY? Not attributed to meteoric 36Cl 2 options 1.) differing on the 104 & 106 time scales 2.) current altitude scaling factor underestimating for Antarctic atm. (Evans, J. M. et al., 1997)

Case Study: In-situ 36Cl in Calcite by Muons Profile from limestone of 20 m depth How is 36Cl in Calcite? 1. Negative muon capture by Ca 2. Capture by 35Cl of secondary neutrons produced in muon capture and muon-induced photodisintegration reactions Traditionally, many cases only use production values solely due to spallation in estimating erosion rates - 40% error! (Stone, J. O. H. et al., 1997) Neutron capture

Stone, J. O. H. et al., 1997 Cont… More on 36Cl Major source of 36Cl in calcite in the first meter of the crust is due to Spallation of Ca 35Cl captures thermalised secondary neutrons (after spallation) close to the surface to produce 36Cl

Stone, J. O. H. et al., 1997 Cont… 36Cl in Calcite by Muons Constitutes for nearly half of the cosmic ray flux at ground level Small contributing of cosmogenic isotope production a the surface Major source of production at depths below a few meters Less steep production gradient than the gradient for spallation - less responsive to erosion!          

The Production of Cosmogenic Isotopes Thanks You!!