David Argento

(some aspects of) cosmogenic nuclide production

David Argento

Chart of the Nuclides

http://atom.kaeri.re.kr/

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N

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.

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Heliosphere

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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

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Mass – energy units

• Standard mass units: kg

• Particle physics: amu

• Standard Energy: J

• Particle Physics: eV & MeV

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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/

Z

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