Transcript Document
Records of Cosmogenic Isotope
Production Rates
Lizz León
It's all About the Secondary Rays!
Interaction with matter
Cosmogenic isotopes
Atm
In-Situ
GW dating
Land form dating
Slow (thermal) neutrons
Water content
Erosion etc...
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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
Hydrogen
Deuterium
Tritium
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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 >20GeV
(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
Typ e
A ttenuatio n (g /cm2 ) R ig id ity cuto ff
Nucleo n
1 2 8 -1 4 2
0 .5 & 1 4 .9
Thermal
1 3 4 -1 5 5
s ame
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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 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)
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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
Neutron capture
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!
Erosion
Percent abundance Muon Spallation (micrometers/a)
X
0
10
X
0
X
2
20
X
0.4
X
10
33
X
3
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The Production of Cosmogenic Isotopes
Thanks You!!
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