Ciro Santilli
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Alpha decay | 🗖 nosplit | ↑ parent "Radioactive decay" | 152, 4, 152

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Most of the helium in the Earth's atmosphere comes from alpha decay, since helium is lighter than air and naturally escapes out out of the atmosphere.
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Wiki mentions that alpha decay is well modelled as a quantum tunnelling event, see also Geiger-Nuttall law.
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As a result of that law, alpha particles have relatively little energy variation around 5 MeV or a speed of about 5% of the speed of light for any element, because the energy is inversely exponentially proportional to half-life. This is because:
  • if the energy is much larger, decay is very fast and we don't have time to study the isotope
  • if the energy is much smaller, decay is very rare and we don't have enough events to observe at all
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