Where do alpha and beta particles come from?

Beta radiation

introduction

Beta radiation or \ (\ beta \) - radiation is ionizing radiation that occurs during beta decay, a radioactive decay.

A distinction is made between two types of radiation, the \ (\ beta ^ - \) - radiation, which consists of electrons, and the less common \ (\ beta ^ + \) - radiation, which consists of positrons.

Difference to \ (\ alpha \) rays

The alpha particles emitted in alpha radiation have a certain kinetic energy that depends on the parent nuclide. In contrast to this, beta radiation with completely different energies is released when a nuclide decays. The energy of the beta radiation ranges from zero to a maximum value that is characteristic of the decaying core. The reason for this is that when the beta decays, the energy is split between the beta particle and a neutrino that is also generated. The energy distribution fluctuates so that the beta particles have different energies. The typical maximum energy of beta radiation is of the order of 1 \ (MeV \).

Emergence

Beta decay of atomic nuclei

Nuclides with an excess of neutrons decay via the \ (\ beta ^ - \) process. A neutron of the nucleus is converted into a proton and an electron and an electron antineutrino are emitted. Both electron and antineutrino leave the atomic nucleus. However, the converted proton is subject to the strong interaction and remains in the nucleus.

Since there is one neutron less but one more proton in the nucleus after the decay process, the mass number \ (\ mathrm {A} \) remains unchanged, while the atomic number \ (\ mathrm {Z} \) increases by 1. So the element goes into its successor in the periodic table.

The decay of the neutron can be described by the following formula:

$$ {} ^ {1} _ {0} \ mathrm {n} \ to {} ^ {1} _ {1} \ mathrm {p} + \ mathrm {e} ^ {-} + \ overline {\ nu } _e $$

In general, the following applies to the \ (\ beta ^ - \) decay:

$$ {} ^ \ mathrm {A} _ \ mathrm {Z} \ mathrm {X} \ to {} ^ \ mathrm {A} _ {\ mathrm {Z} +1} \ mathrm {Y} + \ mathrm { e} ^ {-} \ mathrm + \ overline {\ nu} _e $$

E.g. decay of the \ (\ beta ^ - \) - emitter Au-198:

$$ {} ^ {198} _ {\ 79} \ mathrm {Au} \ to {} ^ {198} _ {\ 80} \ mathrm {Hg} + \ mathrm {e} ^ {-} + \ overline { \ nu} _e $$

The \ (\ beta ^ + \) decay occurs in proton-rich nuclides. A proton of the nucleus is converted into a neutron and a positron and an electron neutrino are emitted. As with \ (\ beta ^ - \) - decay, the mass number remains unchanged, but the atomic number is reduced by 1, so the element is transferred to its predecessor in the periodic table.

The decay of the proton can be described by the following formula:

$$ {} ^ {1} _ {1} p \ to {} ^ {1} _ {0} \ mathrm {n} + \ mathrm {e} ^ {+} + \ nu_e $$

In general, the following applies to the \ (\ beta ^ + \) - decay:

$$ {} ^ \ mathrm {A} _ \ mathrm {Z} \ mathrm {X} \ to {} ^ \ mathrm {A} _ {\ mathrm {Z} -1} \ mathrm {Y} + \ mathrm { e} ^ {+} + \ nu_e $$

The decay of a typical \ (\ beta ^ + \) - emitter is K-40:

$$ {} ^ {40} _ {19} \ mathrm {K} \ to {} ^ {40} _ {18} \ mathrm {Ar} + \ mathrm {e} ^ {+} + \ nu_e $$

Electron capture

The so-called electron capture is a process that competes with the \ (\ beta ^ + \) decay. A proton in the nucleus is transformed into a neutron and a neutrino by capturing an electron from a shell of the atomic shell close to the nucleus.

Decay of the free neutron

Not only neutrons that are in atomic nuclei, but also free neutrons can decay. It is converted into a proton, an antineutrino and an electron, which can be detected as beta radiation.

The formula for this decay is:

$$ \ hbox {n} \ to \ hbox {p} + \ hbox {e} ^ - + \ overline {\ nu} _ {\ mathrm {e}} $$

However, since free neutrons generally have a relatively long lifespan of about 885.7 seconds, this does not occur very often. Usually, neutrons that are released are captured much faster by other atomic nuclei.

properties

When beta particles penetrate a material, the energy transfer to the material and the ionization take place in a layer close to the surface, which corresponds to the penetration depth of the particles.

Interaction with matter

In contrast to alpha radiation, beta radiation penetrates paper without any problems. You therefore need at least a thin sheet of aluminum for protection.

Effect on humans

Outside the body

Beta radiation penetrates the human skin layers and damages them. This can lead to intense burns and the resulting long-term effects such as skin cancer. The radiation also damages the eyes and the lens can become cloudy.

Inside the body

If beta emitters enter the body e.g. through ingestion with food or inhalation, living cells in the vicinity of the emitter are damaged. Thyroid cancer caused by radioactive iodine-131 accumulated in the thyroid is well documented.

Protection against \ (\ beta \) rays

To protect yourself from beta rays, you can use absorbers a few millimeters thick (e.g. aluminum sheet). These shield the radiation relatively well, but part of the energy of the beta particles is converted into X-ray bremsstrahlung. Therefore, shielding material with the lightest possible atoms, i.e. low atomic numbers, should be used to shield the beta radiation. Behind it, a heavy metal can serve as a second absorber, which shields the bremsstrahlung.

Material-dependent maximum range for \ (\ beta \) particles:

nuclideenergyairPlexiglassGlass
\ ({} ^ {3} _ {} H \)19 \ (keV \)8 \ (cm \)--
\ ({} ^ {14} _ {} C \)156 \ (keV \)65 \ (cm \)--
\ ({} ^ {35} _ {} S \)167 \ (keV \)70 \ (cm \)--
\ ({} ^ {131} _ {} I \)600 (keV)250 \ (cm \)2.6 \ (mm \)-
\ ({} ^ {32} _ {} P \)1710 \ (keV \)710 (cm)7.1 \ (mm \)4.0 \ (mm \)