Gamma ray

Characteristic

The gamma rays are more penetrating than the radiations alpha and the beta, but are less issuant Ion. They are of comparable nature that the x-rays but are of different origin. The gamma rays are produced by nuclear transitions while x-rays are produced by electronic transitions in general caused by the collision from a Atome with an electron at high-speed. As it is possible for certain electronic transitions to be energy than nuclear transitions, there exists a certain overlapping between x-rays of high energy and the gamma rays of weak energy.

The shielding against the gamma rays requires great quantities of Matière. For example a shielding which reduces by 50% the intensity of the gamma rays requires 1 cm of Plomb, 6 cm of Béton or 9 cm of ground.

Danger

The gamma rays coming from radioactive fallout would be probably the greatest danger in the case of a nuclear Guerre. If the gamma rays are ionizing than the rays alpha or beta, they ask for thicknesses of shielding much greater to be protected some. They produce damage similar to those produced by the X-rays or the Ultraviolets, such as Brûlure S, Cancer S and genetic changes.

Interaction with the matter

While passing by the matter, the gamma rays are absorbed in an exponential way:

\displaystyle

I (d) = I_0 e^ {- \ driven D} Here, μ = N σ is the coefficient absorption , measured in cm^-1, N the number of atoms by cm³ in the matter, σ the cross Section of absorption in cm² and D the thickness of material in cm.

The gamma rays interact with the matter via three principal mechanisms:

Photoelectric effect

In the photoelectric Effect, a Photon gamma interacts with the matter by transferring integrality from its energy to a electron from orbital which is then ejected Atome on which it was dependant. The kinetic energy of this photo - electron is equal to the energy of the photon gamma minus the energy binding of the electron. The photoelectric effect is supposed to be the principal mechanism of transfer of energy of x-rays and the gamma rays of energy lower than 50 keV, but is much less important with higher energies.

Compton diffusion

In the case of the Diffusion Compton, the photon gamma has an energy than sufficient more to tear off an electron of orbital; remaining energy is re-emitted in the form of a new photon gamma of less energy and from which the direction of emission is different from the incidental direction of the photon gamma of origin. The effectiveness of the Compton diffusion decreases with the increase in the energy of the photons; it is thought that it is the principal mechanism of absorption of the gamma rays in the range of energy between 100 keV and 10 MeV, which is that which includes the greatest part of radiations gamma coming from a nuclear Explosion. The Compton diffusion is relatively independent of the Atomic number of the matter absorbing the photons gamma.

Production of pairs

While interacting with the Force of Coulomb in the vicinity of an atomic nucleus, the energy of the photon incidental gamma can spontaneously be converted into Masse in the shape of a pair electron - Positron. The production of such a pair requires an energy higher than the Rest mass of the particles which compose it, that is to say 1,022 MeV : surplus energy is transferred in the form of kinetic energy to the pair formed like with the core from the atom. The produced electron, which is often called secondary electron , is highly ionizing. As for the positron, very also ionizing, it has very a short duration of life in the matière : 10^-8 second, because as soon as it is about stopped, it combines with another électron ; the total mass of these two particles is then converted into two photons gamma of 0,511 MeV each one.

The electrons (positrons) produced by these three processes, produce much Ionization S, which slow down them until the end of their course, where the positrons are destroyed.

Simple: Gamma ray Zh-min-nan: Gamma siā-soàⁿ

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