How does an anti-neutron work



 

Beta radiation or β radiation is a type of ionizing radiation that occurs during radioactive decay, the Beta decay, occurs. A radioactive isotope that emits beta radiation is called Beta emitters designated.

Beta radiation is a particle radiation consisting of electrons with the more common β-Radiation or positrons at the β+-Radiation.

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.

Beta decay

Creation of beta radiation

The Beta decay is a radioactive type of decay of an atomic nucleus. As a result of the decay process, an energy-rich beta particle - electron or positron - leaves the nucleus. At the same time, an antineutrino or neutrino is created. The beta decay is differentiated according to the type of particles emitted. The emitted electron is Beta-minus decay), if the positron is emitted Beta plus decay+).

Nuclides with an excess of neutrons decay via the β--Process. A neutron in the nucleus transforms into a proton and sends out an electron and an electron antineutrino. Both the electron and the anti-neutrino leave the atomic nucleus because both are leptons and are not subject to the strong interaction. Since there is one neutron less but one more proton in the nucleus after the decay process, the mass number remainsA. unchanged while the atomic numberZ increased by 1. So the element goes into its successor in the periodic table.

Write mass numbers as usual A. above and atomic numbers Z at the bottom of the symbols, the decay of the neutron can be described by the following formula:

.

If X denotes the mother nuclide and Y the daughter nuclide, then applies to β--Decay in general:

.

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

.

The β+-Decay occurs with proton-rich nuclides. Here a proton of the nucleus is converted into a neutron. This creates an electron neutrino together with a positron. As with the β-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 proton is converted into a neutron by:

.

The general β+- Describe decay by:

.

The most common naturally occurring β+-Highlight is 40K. Here is the formula:

.


A competitive process to the β+-Decay is the so-called Electron capture. 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. This process occurs especially when the conversion energy released is small.

history

In the early days of nuclear physics, the observation of beta electrons temporarily led to the false conclusion that electrons were part of the atomic nucleus. However, the two emitted particles are only generated at the time of the nuclear transformation. The weak interaction mediates the conversion of one of the quarks present in the neutron or proton into another quark, creating the released particles.

That negative beta rays are the same as electrons, i.e. the particles that make up the atomic shell, is proven by the fact that β--Particles when meeting with shell electrons are obviously subject to the Pauli principle. If they were a different type of particle, they could be captured on all quantum-mechanically possible "orbits" (orbitals) independently of the electrons present; the absorption of β--Radiation in matter would then have to be orders of magnitude stronger than observed. Correspondingly, the annihilation of β proves+-Radiation in matter that it is positrons, the antiparticles of electrons.

Decay of the free neutron

In contrast to this nucleus-bound decay of the neutron, there is also the beta decay of a free neutron, the half-life of which is not exactly known. It is around 615 seconds, i.e. around 10 minutes. The reason for the inaccuracy of the half-life is its difficult measurement: Free neutrons can be obtained with neutron sources, nuclear reactions or by nuclear fission. However, they are captured by matter in a very short time before decay takes place. For scientific calculations, however, the lifetime of free neutrons is an elementary constant that had a significant influence on the development of the cosmos. In fact, in an early phase of the universe, free neutrons made up an important part of matter. In this way, the formation of the light elements in particular (and their isotope distribution) could be better understood if the decay constant of the neutron were known exactly. In addition, one expects a better understanding of the weak interaction that is responsible for beta decay. Various working groups around the world are working on measuring the decay time of the free neutron more precisely. This traps neutrons in a three-dimensional magnetic trap. The interaction of the neutron with the magnetic forces of the cage takes place via the weak magnetic dipole of the neutron. This requires a particularly sophisticated design of the field in the cage. The neutrons that enter the trap from a research reactor are slowed down and trapped by superfluid helium in the chamber. The high-energy electron from the decay serves as evidence in the chamber. It ionizes several helium atoms on its trajectory, which emit a measurable light signal via molecular processes (excimers).

Interaction with matter

When beta particles penetrate a material, the highest energy transfer to the material and the highest ionization takes place in a thin layer that corresponds to the penetration depth of the particles.

Biological effect

When the human body is exposed to beta rays, only layers of the skin are damaged. However, there can be intense burns and the resulting long-term effects such as skin cancer. If the eyes are exposed, the lens can become cloudy.

Are beta emitters absorbed into the body (incorporated), high levels of radiation are the result in the vicinity of the radiator. Thyroid cancer as a consequence of radioactive iodine-131 (131I) that collects in the thyroid gland. There are also fears in the literature that strontium-90 (90Sr) can lead to bone cancer and leukemia because strontium, like calcium, builds up in the bones.

Radiation protection

nuclide energy air Plexiglass Glass
3H 19 keV 8 cm - -
14C. 156 keV 65 cm - -
35S. 167 keV 70 cm - -
131I. 600 keV 250 cm 2.6 mm -
32P. 1.710 MeV 710 cm 7.2 mm 4 mm

Beta rays can be shielded well with an absorber a few millimeters thick (e.g. aluminum sheet). However, part of the energy of the beta particles is converted into X-ray bremsstrahlung. In order to reduce this process, the shielding material should have atoms that are as light as possible, i.e. have a low atomic number. Behind it, a heavy metal can serve as a second absorber, which shields the bremsstrahlung.

For β-emitters, a material-dependent maximum range define, because β-particles give their energy (like alpha particles) in many single collisions to atomic electrons; the radiation is therefore not attenuated exponentially like gamma radiation. The selection of shielding materials results from this knowledge. The ranges in air, Plexiglas and glass are calculated in the table on the right for some of the β-emitters that are widely used in research. Safe shielding can be achieved with a plexiglass shield of 1 cm.

Applications

In radiation therapy, beta emitters (e.g. Sr-90, Ru-106) are used in brachytherapy.

The β+Spotlights 18F, 11C, 13N and 15O are used as tracers in positron emission tomography. The radiation resulting from pair annihilation is evaluated.

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

Research history

In 1903, Ernest Rutherford and Frederick Soddy developed a hypothesis according to which the radioactivity, discovered by Antoine Henri Becquerel in 1896, is linked to the conversion of elements. The beta decay was identified as the source of the beta radiation. Based on this, Kasimir Fajans and Soddy formulated the so-called in 1913 radioactive displacement theorems, with which the natural decay series are explained by successive alpha and beta decays.

In 1911 Lise Meitner and Otto Hahn showed that the energies of the emitted electrons are distributed over a continuous spectrum. However, since the energy released during the decay is constant, a discrete spectrum, as is also observed during alpha decay, was expected. In order to explain this apparent non-conservation of energy (and a violation of the conservation of momentum and angular momentum), Wolfgang Pauli suggested in a letter in 1930 that a neutral, extremely light elementary particle should participate in the decay process, which he named "Neutron". Enrico Fermi changed this name to neutrino in 1931, as a diminutive of the much heavier neutron discovered almost at the same time. The first experimental detection of the neutrino was only possible in 1956 by Clyde L. Cowan and Frederick Reines in one of the first large nuclear reactors.

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

In 1956, an experiment carried out by Chien-Shiung Wu succeeded in demonstrating the parity violation of beta decay postulated shortly before by Tsung-Dao Lee and Chen Ning Yang.

Artificial electron beams

Occasionally, free electrons that are artificially generated (e.g. by a hot cathode) and brought to high energy in a particle accelerator are also inexactly referred to as beta radiation. This is also indicated by the name of the electron accelerator type Betatron.

literature

  • Werner Stolz, Radioactivity. Basics - Measurement - Applications, Teubner, 5th edition 2005, ISBN 3-519-53022-8
Nuclear physics
  • Theo Mayer-Kuckuk, Nuclear physics, Teubner, 6th edition 1994, ISBN 3-519-03223-6
  • Klaus Bethge, Nuclear physics, Springer 1996, ISBN 3-540-61236-X
  • Jean-Louis Basdevant, James Rich, Michael Spiro, Fundamentals in Nuclear Physics: From Nuclear Structure to Cosmology, Springer 2005, ISBN 0-387-01672-4
Research history
  • Milorad Mlađenović, The History of Early Nuclear Physics (1896-1931), World Scientific 1992, ISBN 981-02-0807-3
Radiation protection
  • Hanno Krieger, Basics of radiation physics and radiation protection, Teubner 2004, ISBN 3-519-00487-9
  • Claus groups, Basic course in radiation protection. Practical knowledge for handling radioactive substances, Springer 2003, ISBN 3-540-00827-6
  • James E Martin, Physics for Radiation Protection, Wiley 2006, ISBN 0-471-35373-6
medicine
  • Günter Goretzki, Medical radiology. Physical-technical basics, Urban & Fischer 2004, ISBN 3-437-47200-3
  • Thomas Herrmann, Michael Baumann, Wolfgang Dörr, Clinical radiation biology - in a nutshell, Urban & Fischer February 2006, ISBN 3-437-23960-0
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