Alpha Particles


Alpha particles or \ (\ alpha \)-particles is a type of ionizing radiation that occurs during alpha decay, a radioactive decay. Alpha particles are helium-4 nuclei, consisting of two protons and two neutrons.

\( \alpha \)-Zerfall

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During alpha decay a nucleus emits an alpha particle (helium-4 nucleus). This decreases the mass number of the nucleus by 4, and the atomic number by 2. The alpha decay can be described in general for a nucleus \( \mathrm{X} \) decaying into the nucleus \( \mathrm{Y} \) and the released energy \( \Delta E \):

$$ {}^{\mathrm{A}}_{\mathrm{Z}} \mathrm {X} \to {}^{\mathrm{A}-4}_{\mathrm{Z}-2} \mathrm {Y}^{2-} + {}^{4}_{2} \mathrm {He}^{2+} + \Delta E $$
\( \mathrm{A} \) = Mass number, \( \mathrm{Z} \) = Atomic number

An example for alpha decay is Samarium-146:

$$ {}^{146}_{62} \mathrm {Sm} \to {}^{142}_{60} \mathrm {Nd}^{2-} + \underset{ \mathrm{alpha particle} }{\underbrace{ {}^{4}_{2} \mathrm {He}^{2+} }} + \underset{ \mathrm{\Delta E} }{\underbrace{ 2{,}45\, \mathrm{MeV} }} $$

Electron balancing

Since during the alpha decay the atomic number decreases by two units, but the number of electrons does not change in the electron shell, is the resulting nuclide initially has an excess of electrons. After reaching the steady state, it then has two electrons less than the parent nuclide because of charge balancing with other atoms / ions.

Typical \( \alpha \)-emitter

In nature, alpha emitters uranium and thorium, and their decay products radium and radon can be found.



The energy of the alpha particle is typically in the magnitude of 2-5 \( MeV \). Alpha particles from artificial nuclides can have energies of about 10 \( MeV \).

Interaction with matter

Alpha particles unlike other ionizing radiations do not penetrate very deep into matter since they are electrically charged and have a relatively large mass of 4 \( u \). Therefore, alpha particles can already be shielded by a piece of paper or some centimeters of air. (The range of alpha particles with an energy of 10 \( MeV \) in air is about 10 \( cm \)).

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Biological effect on humans

Outside the body

Alpha particles penetrate only in the upper, dead layers of the skin. It is therefore relatively harmless, since no living cells are damaged.

Inside the body

If an alpha emitter has passed through ingestion or through inhalation into the body, living cells are damaged near to the radiator which is a major threat to the organism.

Certain radioactive isotopes are stored in organs. This leads to high levels of radiation to a small area that can damage the body.