Records of Cosmogenic Isotope Production Rates

1. Records of Cosmogenic Isotope Production Rates Lizz León

3. 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/)

4. Hydrogen Deuterium Tritium 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

5. 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)

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

7. 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

8. 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)

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

10. 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)

11. Neutron capture 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)

12. 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

13. 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!

14. The Production of Cosmogenic Isotopes Thanks You!!