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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.
Wiki mentions that alpha decay is well modelled as a quantum tunnelling event, see also Geiger-Nuttall law.
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
Video 2. Quantum tunnelling and the Alpha particle Paradox by Physics Explained (2022) Source.