Records of Cosmogenic Isotope Production Rates

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Records of Cosmogenic Isotope Production Rates

• Lizz León

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(No Transcript)
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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/)

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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

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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 gt20GeV
• (Stone et al., 1997)

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Slow (Thermal) Neutrons

• Low energy
• What a neutron probe measure - geophysics!

Nucleus
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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

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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)

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Desilets Zreda, 2002 cont...Spatio-temporal
distribution of cosmic-ray nucleon fluxes

• Neutron intensity with atmospheric depth and RC

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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 Sierras
• 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)

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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)

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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

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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!

         
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The Production of Cosmogenic Isotopes Thanks
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