Alpha particle
Alpha particle
Alpha particles have a net spin of zero. When produced in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV and a velocity in the vicinity of 4% of the speed of light. They are a highly ionizing form of particle radiation, with low penetration depth (stopped by a few centimetres of air, or by the skin).
Alpha particle
However, so-called long-range alpha particles from ternary fission are three times as energetic and penetrate three times as far. The helium nuclei that form 10–12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and thus may be highly penetrating and able to traverse the human body and also many metres of dense solid shielding, depending on their energy. To a lesser extent, this is also true of very high-energy helium nuclei produced by particle accelerators.
Energy and absorption
!Example selection of radioactive nuclides with main emitted alpha particle energies plotted against their atomic number. Each nuclide has a distinct [alpha spectrum.](//upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Alphadecayenergiesexample.svg/500px-Alphadecayenergiesexample.svg.png)
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