Alpha particle

α - particles

Alpha rays or α - rays is ionizing radiation that occurs during the radioactive alpha decay. A radioactive nuclide which emits this radiation is referred to as alpha emitters. It is a particle. The decaying nucleus emits a helium-4 nucleus, which is in this case called alpha particles.

Since the alpha particle is composed of two protons and two neutrons, the mass number of parent nucleus takes the alpha decay to four units and the atomic number by two units from.

The symbol for an alpha particle is the small Greek letter alpha: α. The chemical symbol is. Thus, the particle is described as a doubly ionized helium atom, so as the divalent cation. That's why one of the alpha radiation also for ion radiation. The exit velocity of the core is from 10,000 km / s and 20,000 km / s

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 light emitted by a given nuclide alpha particles, unlike, for example during beta decay, only certain values ​​of the kinetic energy, ie, their energy spectrum is a line spectrum. This spectrum is characteristic of the respective radionuclide. Its measurement can therefore be used to determine this nuclide.

• 3.1 Isotope battery
• 3.2 Smoke detectors

• 5.1 final helium

Formation

The decay is free kinetic energy which, according to the mass that is lost as a mass defect by nuclear decay. The kinetic energy is distributed to the daughter nucleus and the alpha particle in the inverse ratio of the two masses (see kinematics ( particle impingement ) ). The alpha particles will be attracted by the strong interaction of the core and electrically repelled due to the same name charges. However, nuclear power has only a short range, while the electrostatic repulsion is long range. The result is that the potential barrier is a kind of the so-called Coulombwall.

Since the height of Coulombwalls exceeds the energy of the alpha particle, it would not be possible according to the laws of classical mechanics that the alpha particle overcomes the Coulombwall. The classical alpha would be stably bound to the core, therefore, is referred to this condition as metastable. With a certain probability, which determines the half-life of decay, it leaves the mother nucleus, however, still by means of the quantum tunneling effect. This allows a particle to penetrate a finite time and finally high energy barrier with a certain probability, even if his energy for this classic is not sufficient.

Identifies the parent X and Y the daughter nuclide, the energy released, and as usual mass numbers written above and ordinal numbers down, thus generally applies to the alpha decay:

A specific example is:

After ejection of the nucleus remains in some cases in an excited state. The necessary energy can be part of and thus is not as kinetic energy available; occurs in the spectrum of alpha radiation on an additional line with a correspondingly lower energy. The subsequent transition of the nucleus from the excited to the ground state is connected with the emission of gamma radiation.

Since the alpha decay, the atomic number decreases by two units, but not the number of electrons changes in the electron shell first, is the resulting daughter atom initially an excess of electrons before. Due to the recoil of decay and interaction with the surrounding matter, the daughter atom is initially lose more electrons until after braking a charge balance with other atoms / ions.

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

The alpha decay is calculated to according to the empirical Weizsäcker mass formula of the liquid-drop model for all nuclei from mass number 165 is a positive energy release, because the so- calculated sum of the masses of the alpha particle and the daughter nucleus is smaller than the mass of the parent nucleus. Nevertheless, in many heavy nuclei an alpha decay has never been observed so far. However, some nuclides previously considered stable in recent decades been " unmasked " as extremely long-lived alpha emitters, such as 149Sm, 152Gd and 174Hf. Was detected with half-lives of a few trillion years was only in the 2000s, then at 180W and 209Bi alpha decay.

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

Interaction with matter

Penetration depth, range

Because of their electric charge and relatively large mass of 4 u alpha particles have a very low penetration depth in matter.

The range of the alpha particles depends on their energy and is in air at atmospheric pressure is about 10 cm (at 10 MeV). At low air pressure, the range of the alpha particles is greater because the number of collision partners (molecules ) on the alpha release their kinetic energy decreases with the air pressure.

Add water or organic material, the penetration depth of a 5 MeV alpha particle is 40 microns. A somewhat stronger sheet of paper or a few centimeters of air thus generally range from already to completely shield alpha particles. This comes from the fact that the ionization density of alpha particles - that is, the number ions, which generates the particles per unit length of its travel - much higher than about beta or gamma radiation. In a cloud chamber therefore see from the train tracks, compared with those of beta-rays of similar energy, shorter and thicker generated by alpha radiation.

Biological effect

Alpha radiation, which acts externally on the human body itself is relatively dangerous because the alpha particles penetrate only mainly in the upper dead layers of the skin due to their low penetration depth. A in the body through inhalation or ingestion of stored alpha emitters, however, is very harmful, because in this case the dead skin layers, but living cells are damaged. In particular, the accumulation of a decaying with alpha radiation nuclide in an organ leads to a high burden of this body, as doing a high dose of radiation their damaging effect on small space and to important body cells exerts ( radiation sickness ).

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

In the Radonbalneologie a healing effect low -dose alpha radiation through the radon content of some spas is assumed (eg, Bad Gastein ).

Applications

Nuclear battery

Alpha emitters of heavy elements (mainly Transuranium Elements ) with a high density and a relatively short half-life can be heated by its own alpha decay to red heat. This is possible because almost all high-energy alpha particles produced in their decay are stopped by their heavy atoms still inside her and give up their kinetic energy as heat to it. If they also produce little gamma radiation and its half-life (usually several years or decades ) long enough, the heat given off can be used in radionuclide for obtaining electrical energy.

Smoke Detector

In addition, alpha emitters are used in ionisation. You see the smoke by measuring the conductivity of the ionized by alpha rays air as smoke particles decrease the conductivity.

History of Research

Alpha radiation was the first form of radioactivity detected. Antoine Henri Becquerel discovered in 1896 by the darkness of light-tight packaged photographic plates by uranium salts. Further research of Marie Curie and Pierre Curie have resulted in the isolation of uranium decay products of radium and polonium and the evidence that these are also alpha emitters. The three researchers were awarded the Nobel Prize in Physics for these services in 1903.

Ernest Rutherford showed 1898, the distinctness of different types of radioactive radiation by their different penetration and coined it the names α - and β - radiation. 1899 demonstrated Stefan Meyer, Egon Schweidler and Friedrich Giesel distinctness by various deflection in the magnetic field.

By observing the spectral lines at gas discharge Rutherford 1908 could prove the identity of the alpha particles as helium nuclei.

1911 Rutherford used alpha rays for his scattering experiments that led to the formation of Rutherford's atomic model.

Kasimir Fajans and 1913 presented Frederick Soddy on radioactive displacement laws that determine the nuclide formed during alpha decay.

With alpha rays, which met on nitrogen nuclei, Rutherford was able to observe for the first time in 1919 an artificial transmutation: there was oxygen in the nuclear reaction 14N ( α, p) 17O or written detail,

1928 was George Gamow quantum mechanical explanation of the alpha decay by the tunnel effect, see also Gamow factor.

" Alpha rays " for reasons other than radioactive sources

The term typically refers to alpha particles in physics each fully ionized helium - 4 nucleus, even if it is not from a radioactive decay. For example, about 12 % of all particles of the galactic cosmic radiation such alpha particles. This is not surprising, since one of the most common elements is helium. However, this part of the cosmic radiation never reaches the ground.

Alpha particles can also be produced artificially out of helium gas in an ion source. If they are accelerated in a particle accelerator whose beam is accordingly sometimes called Alpha jet.

Final helium

Have degraded alpha particles after many interactions with matter most of their energy, they are so slow that they could combine with negatively charged particles. All helium compounds decompose, however, only the combination of two electrons to the helium atom is chemically stable. The helium atom is very small and light.

In the Earth's interior emitted alpha radiation is helium, which diffuses relatively easily by minerals. In columns it migrates because of its low density up, it backs up to natural gas in bubbles to concentrations in the percent range, so that individual sources of natural gas are also profitably used for helium extraction.

Present in the atmosphere of helium increases due to the low mass upward helium at high altitudes even the predominant gas. Due to thermal motion, it escapes the influence of Earth's gravity into space.