The probability of alpha decay occurring is highest for nuclides with high mass numbers.

For example, radium-224, which has 224 nucleons (mass number / protons and neutrons), an atomic number of 88 (number of protons) and accordingly contains (224 - 88 =) 136 neutrons in the atomic nucleus. During alpha decay, an alpha particle consisting of two neutrons and two protons is emitted from the atomic nucleus of the parent nuclide..

The alpha particle is therefore more often described as a helium nucleus, since it has the same nuclear charge numbers as a helium atom. Only the outer electrons are missing, which means that the alpha particle has an electric-positive charge in addition to its kinetic energy.


The kinetic energy carried by the alpha particle is always a sharp energy line and is given in the unit [MeV].

In the case of radium-224, an emitted alpha particle carries an energy of 5.788 MeV (94.73 % decay probability). The daughter nuclide is a radon-220 (86 protons + 134 neutrons).

Due to their relatively large mass and their electrical charge, alpha particles have the property of being directly ionising. This means that the range of alpha particles is small in relation to other types of radiation, since alpha particles can interact more easily with other atoms precisely because of their size, mass and electrical charge.


The range of alpha particles depends on:

  • the energy of the particle
  • the density of the interacting material

In air, the range of alpha particles is about 1 cm per MeV.

In water, the range is reduced again by a factor of 1 : 1000.

When alpha particles interact with matter, they always give off their entire energy.

Finally, the alpha particle is slowed down until it is slow enough to take electrons from foreign atoms so that it completes itself to a helium atom (with a complete electron shell).

Radiation protection

These properties make alpha particles and thus alpha emitters dangerous for humans, animals and the environment.

External contact (e.g. via the skin) is nevertheless not considered serious, since the alpha particles are "intercepted" in the uppermost layers of the skin.

However, the situation is different in the case of incorporation (i.e. penetration into the organism) of alpha emitters, which usually occurs through unrecognised contamination of surfaces. The absorption of an alpha emitter can cause immense damage within the body, directly to cells and the cell nucleus.

Berthold SCINT technology reliably measures contamination at the workplace and contamination of personnel by alpha emitters and can thus optimally contribute to reducing the risk of carry-over and incorporation and ultimately radiation exposure at your workplace.

LB147 Frisker shows how to use

Sources and References:

Source 1: Grupen, Claus (Springer) (2008), Grundkurs Strahlenschutz, 4. Auflage, Heidelberg.

Source 2: Knoll, Glenn F. (Wiley) (2010), Radiation Detection and Measurement, 4. Auflage, Hoboken (US).

Source 3: (aufg. 01.04.2021)