What are alpha particles 2

Alpha radiation


charge2 e
(+3,204 · 10−19 C)
Dimensions4.001 506 179 127 (62) u
6,644 656 20(33) · 10−27 kg
7294,299 5365(31) · me
3727.379 109 (93) MeV /c2
Spinparity 0+
Isospin 0 (z component 0)
average lifespan stable

Alpha radiation or α radiation is ionizing radiation that occurs during radioactive decay, the Alpha decay, occurs. A radioactive nuclide that emits this radiation is called Alpha emitters designated. It is a particle radiation. The decaying atomic nucleus sends out a helium-4 atomic nucleus, which in this case Alpha particles is called.

Emission of an alpha particle (red protons, blue neutrons)

Since the alpha particle consists of two protons and two neutrons, the mass number of the parent nucleus decreases by four units and the atomic number by two units when the alpha decays.

The symbol for an alpha particle is the small Greek letter alpha: α. The chemical symbol is $ \ textstyle {{} ^ 4_2 \ mathrm {He} ^ {2+}} $. The particle is thus described as a doubly ionized helium atom, i.e. a divalent cation. That is why alpha radiation also counts as ion radiation. The exit speed from the core is between 10,000 km / s and 20,000 km / s.

The name comes from the division of ionizing rays from radioactive decay into alpha rays, beta rays and gamma rays with their increasing ability to penetrate matter.

The alpha particles emitted by a given nuclide have, in contrast to, for example, beta decay, only very specific values ​​of the kinetic energy, i.e. that is, its energy spectrum is a line spectrum. This spectrum is characteristic of the respective radionuclide. Its measurement can therefore be used to determine this nuclide.

Alpha spectrum of the plutonium isotopes 242Pooh, 239Pu /240Pooh and 238Pooh The tailing of each peak on its low-energy (left) side is caused by decelerating collisions of the α-particles in the sample.


Coulombwall. Model potential for an alpha particle, which is composed of the short-range core potential approximated by a potential well and the long-range Coulomb potential.

The emitted alpha particle has a kinetic energy which, according to $ E = mc ^ 2 $, corresponds to the mass that is lost as a mass defect due to the nuclear decay. It is attracted to the core by the strong interaction and electrically repelled due to charges of the same name. However, nuclear power has only a short range, while electrostatic repulsion is long-range. This means that the potential represents a kind of barrier, the so-called Coulomb wall. Since the height of the Coulomb wall exceeds the energy of the alpha particle, according to the laws of classical mechanics it would not be possible for the alpha particle to overcome the Coulomb wall. Classically, the alpha particle would be stably bound to the core, which is why this state is called metastable. With a certain probability, which determines the half-life of the decay, it leaves the mother nucleus anyway by means of the quantum mechanical tunnel effect. This allows a particle to penetrate a finitely long and finitely high energy barrier with a certain probability, even if its energy is classically insufficient.

If X denotes the mother nuclide and Y the daughter nuclide, $ \ Delta E $ the released energy, and if mass numbers $ A $ are written above and ordinal numbers $ Z $ below, then the following applies to alpha decay in general:

$ {} ^ {A} _ {Z} \ mathrm {X} \ to {} ^ {A-4} _ {Z-2} \ mathrm {Y} + {} ^ {4} _ {2} \ mathrm {He} + \ Delta E $.

A concrete example is:

$ {} ^ {146} _ {62} \ mathrm {Sm} \ to {} ^ {142} _ {60} \ mathrm {Nd} + {} ^ {4} _ {2} \ mathrm {He} + 2 {,} 45 \, \ mathrm {MeV} $.

After ejection, the atomic nucleus remains in an excited state in some cases. The energy required for this is part of $ \ Delta E $ and is therefore not available to the alpha particle as kinetic energy; An additional line then appears in the spectrum of the alpha radiation with a correspondingly lower energy. The subsequent transition of the nucleus from the excited to the ground state is associated with the emission of gamma radiation.

Since the atomic number decreases by two units during alpha decay, but the number of electrons in the electron shell does not initially change, the daughter atom that is created initially has an excess of electrons. Due to the recoil of the decay and interaction with the surrounding matter, the daughter atom will initially lose further electrons until, after the deceleration, a charge equalization with other atoms / ions takes place.

Typical alpha emitters occurring in nature are uranium and thorium as well as their decay products radium and radon. The kinetic energy of an alpha particle is typically on the order of 2 to 5 MeV. Alpha particles from artificially generated nuclides can, however, have energies of over 10 MeV. The alpha energies and half-lives of the individual nuclides can be looked up in the list of isotopes.

According to the empirical Weizsäcker mass formula of the droplet model, the alpha decay results in a positive energy release for all nuclei from mass number 165, because the sum of the masses of the alpha particle and the daughter nucleus calculated in this way is smaller than the mass of the mother nucleus. However, alpha decay has never been observed in many heavy nuclei. However, in the last few decades some nuclides that were previously considered stable have been "exposed" as extremely long-lived alpha emitters, for example 149Sm, 152Gd and 174Hf. Only in the 2000s could 180W.[1] and 209Bi[2] Alpha decay with half-lives of a few trillion years can be demonstrated.

The observed relationship between the half-life and the energy of the emitted alpha particles is described by the Geiger-Nuttall rule.

Interaction with matter

Alpha radiation is the easiest ionizing radiation to shield.

Penetration depth, reach

Due to their electrical charge and relatively large mass of 4 u, alpha particles have only a very small depth of penetration into matter.

The range of the alpha particles depends on their energy and is around 10 cm in air at normal pressure (at 10 MeV). When the air pressure is low, the range of the alpha particles is greater, as the number of collision partners (molecules) to which the alpha particles give up their kinetic energy decreases with the air pressure.

In water or organic material, the penetration depth of a 5 MeV alpha particle is 40 μm. A somewhat thicker sheet of paper or a few centimeters of air are generally sufficient to completely shield alpha particles. This is due to the fact that the ionization density of alpha particles - i.e. H. the number of ions that the particle generates per unit length of its path - is much higher than, for example, for beta or gamma radiation. In a cloud chamber, the traces generated by alpha radiation therefore look shorter and thicker compared to those of beta rays of similar energy.

Open alpha spectrometer with specimen and detector (above)

Biological effect

Alpha radiation, which acts on the human body from the outside, is itself relatively harmless, as the alpha particles mainly only penetrate into the upper, dead skin layers due to their shallow penetration depth. An alpha emitter stored in the organism through inhalation or ingestion with food, on the other hand, is very harmful, since in this case it is not the dead skin layers but living cells that are damaged. In particular, the accumulation of a nuclide that decomposes with alpha radiation in an organ leads to a high level of stress on this organ, since a high dose of radiation exerts its damaging effect on a small area and on important body cells (radiation sickness).

The radiation weighting factor for alpha radiation is fixed at 20, while it is 1 for beta and gamma radiation. For the same energy input, 20 times the harmful effect is assumed for alpha radiation. This factor is not a physical measured variable, but a set standard for the purpose of simplified handling in radiation protection. It is used to convert the absorbed dose in Gray into the dose equivalent in Sievert (outdated units: Rad or Rem).

In radon balneology, low-dose alpha radiation is assumed to have a healing effect due to the radon content of some therapeutic baths (e.g. Badgastein).


Isotope battery

A plutonium pellet (238Pu) glows by its own decay

Alpha emitters of heavy elements (mainly transuranic elements) with a high density and a relatively short half-life can heat up to red heat due to their own alpha decay. This is possible because almost all the high-energy alpha particles produced during their decay are still held in their interior by their heavy atoms and give them their kinetic energy as heat. If they also generate little gamma radiation and their half-life (usually a few years to decades) is long enough, the heat given off can be used in radionuclide batteries to generate electrical energy.

smoke detector

Alpha emitters are also used in ionization smoke detectors. You can recognize the smoke by measuring the conductivity of the air ionized by alpha rays, as smoke particles reduce the conductivity.

Research history

Alpha radiation was the first form of radioactivity to be detected. Antoine Henri Becquerel discovered it in 1896 through the blackening of light-tight packaged photo plates with uranium salts. Further research by Marie Curie and Pierre Curie led, among other things, to the isolation of the uranium decay products radium and polonium and the proof that these are also alpha emitters. The three researchers received the Nobel Prize in Physics for this achievement in 1903.

In 1899 Ernest Rutherford showed that different types of radioactive radiation could be distinguished by their different penetration capabilities and coined the terms α, β and γ radiation. Also in 1899, Stefan Meyer, Egon Schweidler and Friedrich Giesel demonstrated the differentiation through different deflections in the magnetic field.

By observing the spectral lines during gas discharge, Rutherford was able to prove the identity of the alpha particles as helium nuclei in 1908.

In 1911, Rutherford used alpha rays for his scattering experiments, which led to the establishment of Rutherford's atomic model.

In 1913, Kasimir Fajans and Frederick Soddy set up the radioactive displacement theorems that determine the nuclide produced during alpha decay.

With alpha rays hitting nitrogen atomic nuclei, Rutherford was able to observe an artificial element conversion for the first time in 1919: oxygen was created in the nuclear reaction14N (α, p)17O or, more fully written,

$ ^ {14} _ {7} \ mathrm {N} + {} ^ {4} _ {2} \ alpha \ to {} ^ {17} _ {8} \ mathrm {O} + {} ^ {1 } _ {1} \ mathrm {p} $.

In 1928 George Gamow found the quantum mechanical explanation of the alpha decay through the tunnel effect, see also Gamow factor.

"Alpha rays" from sources other than radioactive

With the term alphaparticles In physics, one usually designates every completely ionized helium-4 nucleus, even if it does not come from a radioactive decay. The galactic cosmic rays and the solar wind consist of five to ten percent of such alpha particles. This is not surprising as helium is one of the most common elements. However, this part of the cosmic rays never reaches the ground.

Alpha particles can also be artificially created from helium gas in an ion source. If they are accelerated in a particle accelerator, its beam sometimes becomes alpha accordinglybeam called.

End product helium

If alpha particles have broken down most of their energy after many interactions with matter, they are so slow that they could combine with negatively charged particles. However, all helium compounds decay; the combination with two electrons to form the helium atom is chemically stable. The helium atom is very small and light.

Alpha radiation emitted in the earth's interior forms helium, which diffuses relatively easily through minerals. Due to its low density, it migrates upwards in crevices; it accumulates in natural gas bubbles to concentrations in the percentage range, so that individual natural gas sources can also be used profitably for the production of helium.

The helium in the atmosphere rises due to its low mass, and helium even becomes the predominant gas at high altitudes. Through heat movement, it escapes the influence of earth's gravity into space.


  1. ↑ Cristina Cozzini et al., Detection of the natural α decay of tungsten, Physical Review C (2004), preprint
  2. ↑ Pierre de Marcillac et al., Experimental detection of alpha particles from the radioactive decay of natural bismuth, Nature 422, 876-878 (April 24, 2003), Table of Results


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