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David Argento
(some aspects of) cosmogenic nuclide production
David Argento
Chart of the Nuclides
http://atom.kaeri.re.kr/
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David Argento
Primary Cosmic Rays
• 90% protons, ~9% helium nuclei, ~1% electrons and heavier nuclei
• Wide spectrum of energies from 10MeV to 1015MeV!
• Origins: extra solar!– Most are from supernovae, being accelerated on the shockwave– The ultra-energetic CRs come from extra-galactic sources,
possibly accelerated by galactic magnetic fields.
David Argento
Heliosphere
David Argento
Primary CR Flux
David Argento
Interactions
• Ionization – cosmic rays strike/scatter electrons attached to atoms, creating ion-electron pairs
• Scattering – strikes nucleus, but glancing, and does not overcome the Coulomb Barrier
• Transmitted energy often re-emitted as X-rays and gamma rays
• Spallation – strikes nucleus, overcomes the Coulomb Barrier, causes nuclear fragmentation
David Argento
Spallation
• Nuclear fragmentation
• Protons & neutrons ejected by (i) knock-on, (ii) ‘evaporation’ from excited nucleus
• Pions ejected if E > 140 MeV
• Breakup results in a new atom, with a different number and configuration of nucleons
David Argento
The chronology of a spallation event
• Nucleus is struck by an energetic particle
• Some or no nucleons ejected immediately from kinetic energy transfer (knock-on)
• Nucleus briefly remains in an excited state
• Nucleus ejects more nucleons to reduce energy and find more stable configuration (evaporation)
David Argento
David Argento
Equivalence of energy and mass
• Einstein’s equation:
E = mc2
• Binding Energy of nuclear force can be calculated from the difference in nuclide mass
David Argento
Mass – energy units
• Standard mass units: kg
• Particle physics: amu
• Standard Energy: J
• Particle Physics: eV & MeV
David Argento
For Example: the alpha particle
• Atomic mass of proton (mp): 938.3 MeV• neutron (mn): 939.6 MeV• He-4: 4.0026032 amu = 3728 MeV
• Subtract 2 electrons – mass of alpha: 3727 MeV• Find total mass of free nucleons: 2*mn + 2*mp = 3756 MeV• Find the difference in mass of the helium nucleus, and the
free nucleons: Δm = 28.03 MeV• Find the binding energy per nucleon: Δm /4 = 7.075
MeV/nucleon
David Argento
Binding energy per nucleon
David Argento
Chart of the Nuclides
http://atom.kaeri.re.kr/
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N
David Argento
Secondary Cosmic Rays
• Product of spallations
• Primarily knock-on and evaporation neutrons
• Multiplicity
David Argento
David Argento
Cosmogenic Nuclide production in the atmosphere: meteoric
Two primary modes of production:
– Slow neutron capture: 14N(n,p)14C to produce radiocarbon
• T1/2 = 5730 years • Atmospheric concentration massively altered by
nuclear bomb testing in the 1950s & 1960s
– Spallation: 7Be, 10Be, 36Cl and others produced mainly from N, O, and Ar
David Argento
Cosmogenic Nuclide production in the lithosphere:
in-situ
Four primary modes of nuclide production
– Spallation: 3He, 7Be, 10Be, 14C, 21Ne, 26Al, 32Si, 36Cl, 38Ar, 41Ca, 53Mn, 93Zr, 129I, others
– Slow neutron capture: 36Cl, 41Ca– Fast Muons: All of the above (10Be, 26Al, 36Cl have
been experimentally calibrated)– Slow Muon capture: All of the above (10Be, 26Al, 36Cl
have been experimentally calibrated)
David Argento
Cross sections
• Nuclear radius ~ 10-12 cm (1 fm)
• Actually …
• R ~ a0 A1/3 where a0 ~ 1.35 fm
• Hence … ‘geometric’ cross-section of the nucleus:
~ 5.7 x 10-26 A2/3 cm2 ~ 0.057 A2/3 b
1 barn = 10-24 cm2
14N (neutron absorption x-section) … mainly 14N(n,p)14C
David Argento
What does all this get us?
• Primary Cosmic Rays• Interact with upper atmosphere:
– Ionization, scattering, and spallation
• Secondary cosmic Rays formed, and cascade through atmosphere
• Small portion of original flux makes it to the surface of the earth
• In-Situ spallation gives us
David Argento
Production!
• 10Be: ~5 atoms/g quartz/year
• 26Al: ~30 atoms/g quartz/year
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