Beta particle

Beta radiation or β - radiation is an ionizing radiation for a radioactive decay, the beta decay occurs. A radioactive nuclide that emits beta radiation, is referred to as a beta-emitter.

This particle is in the more common β - radiation from electrons in the rare β radiation contrast of positrons.

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

The emitted particles are in contrast to the alpha radiation is not a specific ( discrete) kinetic energy, but their energies are distributed continuously from zero to a characteristic of the disintegrating core maximum value. This is due to the breakdown of the released decay energy on three particles: the beta particle, a neutrino is also produced and the daughter nucleus. The typical maximum energy of beta radiation is of the order of 1 MeV.

  • 2.1 Neutrino mass
  • 4.1 Biological effects
  • 4.2 Radiation Protection


Beta decay of atomic nuclei

Beta decay is a type of radioactive decay of an atomic nucleus. In the wake of the disintegration process leaves a high-energy beta particles - electron or positron - the core. At the same time creates an antineutrino or neutrino. In the early days of nuclear physics, the observation of beta- electrons temporarily led to the mistaken conclusion that electrons were constituents of the atomic nucleus. However, the two emitted particles are generated only at the time of the nuclear transformation. A W - boson mediates the weak interaction and causes the conversion of the neutron (or proton) existing d - quarks (or u - quarks ) in a u-quark (d- quark ), and thus the conversion of a neutron into a proton ( proton into a neutron). While the converted cottage cheese as part of the converted nucleon retains its role, the antineutrino ( neutrino ) and the electron ( positron) leave the nucleus.

Beta decay is distinguished by the nature of the emitted particles. When an electron is emitted, then it is beta-minus decay ( β - ), with a positron emitted by beta-plus decay ( β ).

The fact that beta-minus radiation actually the same particle are like the electrons of the atomic shell, is reflected in its interaction with matter. The Pauli principle prevents the beta particles are trapped in the already occupied with electronic states of the atom, while this is possible, for example, muons and is observed.

Beta-minus decay ( β - )

Nuclides with an excess of neutrons decay by the β - process. A neutron of the nucleus transforms into a proton, which sends an electron and an electron antineutrino from. Both electron and antineutrino leave the nucleus, since both leptons are and are not subject to the strong interaction. Since a neutron is less, but a proton is more after the disintegration process in the nucleus, the mass number remains unchanged while the atomic number increases by 1. The element is therefore in his successor on the periodic table.

If we write, as usual, mass numbers and atomic numbers above the bottom of the symbols, therefore, the decay of the neutron can be described by the following formula:

Identifies the parent X and Y the daughter nuclide, then for the β - decay in general:

A typical β - emitters is 198Au. Here is the conversion formula notation:

Beta-plus decay ( β )

The β decay occurs in proton-rich nuclides. In this case, a proton of the core is converted to a neutron. This together with a positron ( positron radiation ) produced an electron - neutrino. As with the β - decay, the mass number remains unchanged, but the atomic number decreases by 1, the element is therefore in its predecessor in the periodic system.

The conversion of a proton into a neutron is done by:

With the same notations as above, can the general β decay described by:

The most frequently occurring primordial nuclide which the presence of β ( among other things) decay, potassium -40 ( 40K ). Here is the formula:

A competitive process for β - decay is the so-called electron capture. Here, a proton of the nucleus transforms through capture of an electron from a core- shell near the atomic shell into a neutron and a neutrino. This process always occurs next to the positron emission on, as the only decay channel when the transformation energy is smaller than the rest energy of a positron. Also, the electron proves that shell electrons and beta electrons are the same particle.

Decay of free neutrons

A free neutron is subject to Beta decay. It transforms into a proton, an electron and an antineutrino, which can be detected as beta radiation. The formula of the decay is therefore:

The lifetime for this decay is 880.1 ± 1.1 seconds, which is just under 15 minutes. This corresponds to a half-life of approximately 10 minutes. In a normal environment on Earth (eg in air) each released neutron is captured in a much shorter time by an atomic nucleus practical; therefore plays no appreciable role in this decay here.

Energy spectrum

The energy distribution of beta radiation ( beta spectrum) is not a line spectrum, but continuously, because the energy released in the decay energy to a total of three particles, atomic nucleus, electron / positron and antineutrino / neutrino, distributed (see kinematics ( Teilchenprozesse ) ).

The figure shows schematically a simple electron spectrum. More complex spectra occur when beta transitions overlap at different energy levels of the daughter nucleus itself.

Examples of beta maximum energy:

  • 3H 0.0186 MeV (-)
  • Neutron 0.78 MeV (-)
  • 0.96 MeV 11C ( )
  • 37K 5.1 MeV ( )
  • 20F 5.4 MeV (-)

Alone or in addition to, a beta spectrum, a generated by internal conversion electrons line spectrum occur. This was formerly known as Discreet beta spectrum, although it has nothing to do with the actual beta decay. With preceding beta-minus decay then the continuous electron spectrum with additional sitting thereon lines (peaks) appears.

Neutrino mass

The shape of the spectrum near the maximum electron or positron energy gives information about the unknown mass of the neutrino / antineutrino. For this, the high-energy end of the beta spectrum measured accurately (eg in the experiment KATRIN ). An abrupt as opposed to a continuous decrease in the peak energy shows a nonzero neutrino mass and allowed to determine their value.


Beta radiation is spin polarized longitudinally in their emission direction. This is a basic physically interesting feature of the weak interaction, as it proves the non-conservation of parity. For effects and applications of radiation, however, it practically does not matter.

Interaction with matter

If beta particle penetration into a material to find energy transfer instead of to the material and ionization in a layer near the surface, which corresponds to the penetration depth of the particles.

Is the penetrating particles, a positron ( β particles), it hits very soon on an electron, so its antiparticle. This leads to the annihilation resulting from the (usually) two photons in the gamma range.

Biological effect

The human body is exposed to beta rays, only layers of skin are damaged. There, however, there may be intense burns and resulting late effects such as skin cancer. If the eyes are exposed to radiation, it may lead to lens opacity.

Become a beta emitter absorbed into the body ( incorporated ), high radiation levels in the area of the radiator can be the result. Well documented thyroid cancer as a result of radioactive iodine -131 ( 131I ), which accumulates in the thyroid gland. In the literature, there are also fears that strontium -90 ( 90Sr ) can cause bone cancer and leukemia, as strontium as calcium accumulates in the bones.

Radiation Protection

Beta radiation can be screened well with a few millimeters thick absorber ( for example, aluminum sheet ). However, while some of the energy of the beta particles in X-ray bremsstrahlung is converted. In order to reduce this process, the shielding should have the lightest possible atoms, so its low atomic number. It may be followed then a heavy metal used as a second absorber, which shields the bremsstrahlung.

In β decay is important to note that the β - particles annihilate with electrons (see above ), where photons are free. These have energies of about 511 keV ( corresponding to the mass of the electron), which lies in the area of ​​gamma - radiation. For β - emitters can define a material-dependent maximum range, because β - particles release their energy (such as alpha particles ) in many individual collisions to atomic electrons; the radiation is therefore not exponentially attenuated as gamma radiation. Realizing this, the selection results shielding materials. For some of the spread in the research β - emitters, the ranges in air, plexiglass and glass are calculated in the adjacent table. With a Plexiglas shield of 1 cm can be achieved at the indicated energies a safe shield. For beta emitters with higher energy values ​​corresponding to a thicker shielding must be used.


In radiotherapy beta emitters (e.g., 90Sr, 106Ru ) can be used in brachytherapy.

The β emitters 18F, 11C, 13N and 15O are used in the positron emission tomography as a tracer. Evaluated here is the product created by pair annihilation radiation.

In addition to X-rays and gamma rays and beta rays for radiation sterilization are used.

History of Research

Ernest Rutherford and Frederick Soddy in 1903 developed a hypothesis according to which the radioactivity in 1896, discovered by Antoine Henri Becquerel is associated with the transformation of elements. Beta decay was therefore identified as a source of beta radiation. Assuming formulated in 1913 Kasimir Fajans and Soddy called radioactive displacement laws with which the natural decay chains are explained by successive alpha and beta decays.

1911 showed Lise Meitner and Otto Hahn, that the energies of the emitted electrons are distributed over a continuous spectrum. However, since the energy released in the decay energy is constant, there was a discrete spectrum expected, as it is also observed in alpha decay. This apparent non-conservation of energy ( and also occurring violation of momentum and angular momentum conservation ) to explain struck Wolfgang Pauli in 1930 in a letter to the involvement of a neutral, extremely light elementary particles at the decay process before, which he christened " neutrons ". Enrico Fermi changed this name in 1931 in neutrino, as diminutive of almost simultaneously discovered much heavier neutron. The identity of the beta particles with atomic electrons has been demonstrated in 1948 by Maurice Goldhaber and Gertrude Scharff- Goldhaber. The first experimental evidence of the neutrino succeeded in 1956 by Clyde L. Cowan and Frederick Reines in one of the first large-scale nuclear reactors.

The β decay was discovered in 1934 by Irène and Frédéric Joliot- Curie.

In 1956, it succeeded with a carried out by Chien- Shiung Wu experiment to demonstrate the recently postulated by Tsung- Dao Lee and Chen Ning Yang parity violation of beta decay.

Artificial electron

Occasionally, free electrons that have been artificially (e.g., from a hot cathode ) is generated and accelerated to high energy in a particle accelerator, also known as beta radiation inaccurate. The name of the electron accelerator - type betatron Recalls.