'''Alpha decay''' is a type of
radioactive decay in which an
atomic nucleus emits an
alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or 'decays') into an atom with a
mass number 4 less and
atomic number 2 less.
For example:
:
although this is typically written as:
(The second form is preferred because the first form appears electrically unbalanced. Fundamentally, the recoiling nucleus is very quickly stripped of two electrons to neutralize the ionized helium cation.)
An
Alpha Particle is identical to a helium nucleus, and both mass number and atomic number are the same.
Alpha decay is a form of
nuclear fission where the parent atom splits into two daughter products. Alpha decay is fundamentally a
quantum tunneling process. Unlike
beta decay, alpha decay is governed by the
strong nuclear force.
Alpha particles have a typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy, i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05c. Because of their relatively large mass, +2 charge and relatively low velocity, they are very likely to interact with other atoms and lose their energy, so they are effectively absorbed within a few centimeters of air.
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