Radioactivity β

The radioactivity beta or emission beta (symbol β) is a type of disintegration Radioactive in which a Particule beta (a electron or a Positron) is emitted. One speaks about disintegration beta less (β-) or beta more (β+) according to whether it is an electron (negatively charged particle) or a positron (positively charged particle) which is emitted.

- -

A Neutron is converted into Proton via the weak nuclear force and a particle β - (an electron) and a anti - Neutrino is emitted:
n ~ \ rightarrow~ p+e^-+ \ naked bar {\} _e

The spectrum of energy (many particles emitted according to their kinetic energy) of the β- (electrons) is continuous because of the division of energy between the three bodies. There is no minimal energy.

The reaction is possible énergétiquement in the only condition that the Atomic nucleus wire is less heavy than the core father.

Example of a reaction β - for the Tritium ( 3 H + ) which is transformed into Hélium -3 ( 3 He 2+ ):

{} ^3 \ hbox {H} ^+ \; \ to \; ^3 \ hbox {He} ^ {2+} \; + \; e^-+ \ naked bar {\} _e

Example of a reaction β - for the radioactive isotope Cobalt -60 ( 60 Co) which is transformed into Nickel -60 ( 60 Ni + ) stable:

{} ^ {60} \ hbox {Co} \; \ to \; ^ {60} \ hbox {Nor} ^+ \; + \; e^-+ \ naked bar {\} _e
One notes in this example that the ion produced nickel escapes orbital crystalline usual, especially if cobalt were in crystalline form, where the atom from nickel will have to rearrange itself by collecting nearby electrons. As the electrons emitted beta move in the crystal, orbital other atoms of the crystal are rearranged along its course, and the electron beta can be finally collected by the crystal itself without being able to escape from it.

As the spectrum of energy of emission is continuous, many beta decays occurring in the middle of a metal cobalt-60 crystal do not escape from it, and one possibly detects outside the crystal only the emitted neutrinos (which are very difficult to collect and detect) or of the electrons very slowed down along their course. But the ion nickel produced by disintegration also will enter in collision with the atoms close to the crystal and will cause a shock wave being propagated in all the crystal (cobalt on the surface of the crystal can be sublimated). On the other hand, close to the surface of the crystal, one will detect half of the emissions of electrons beta.

On the other hand, if the neutrino is emitted with a weak energy, the electron beta and the ion nickel will be propelled at high-speed in quasi-opposite directions, the first easily crossing all the crystal, and the ion strongly striking the close crystalline atoms: the electron is then emitted side of the crystal, and one observes a gas cobalt sublimation on other side of the crystal, sublimation amplified by the temperature. On a source very enriched and young of cobalt-60, many disintegrations take place, and the crystal emits uninterrupted a mixture of gas cobalt-60 (still radioactive), neutrinos and electrons beta of which some have very important energies.

Disintegration β+

A proton is converted into neutron via the weak nuclear force and a particle β+ (a positron) and a neutrino are emitted:

p ~ \ rightarrow~ n+e^++ {\ naked} _e

The spectrum of energy of particles emitted according to their kinetic energy of the β+ (positrons) is continuous with the division of energy between the three bodies. We however notice a minimal speed of the positrons. This one is due to the Coulomb repulsion of this last with the core.

This reaction can take place only if the mass of the core wire added with twice the mass of the electron is lower than that of the core father.

Example of a reaction β+ for the Fluorine which is transformed into Oxygène:

{} ^ {18} \ hbox {F} \; \ to \; ^ {18} \ hbox {O} \; + \; \ mathrm {E} ^++ {\ naked} _e

Existence of the neutrino

the study of the beta decay brought to postulate the existence of the neutrino.

In 1931, Wolfgang Pauli not yet proposed that “missing” energy was carried by another particle, découverte : the neutrino.

Here what the presence of the neutrino makes it possible to explain:

  • the spectrum of energy of emission of the particles beta is continuous. This is explained easily if energy is divided between three bodies.
  • the momentum must be preserved, but because of a system with three bodies, the particle beta does not leave in an opposite way to the core.
  • the neutrino allows to preserve the lepton number: the creation of a Lepton is accompanied by that of one anti-lepton (pairs electron/anti-neutrino electron; anti-electron/neutrino electron).

The problem was analyzed in a way more detailed by Enrico Fermi, but it been necessary to await 1959 for the first experimental observations of neutrinos.

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