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 (

4. 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

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. 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

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. 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

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!!