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Introduction

What is gamma radiation?

Gamma radiation is electromagnetic radiation with the highest energy quanta, emitted by atomic nuclei during radioactive decay. Unstable atomic nuclei can be converted by alpha or beta transformation. During the alpha transformation the alpha particle (helium nucleus consisting of two protons and two neutrons) is emitted by nucleus. As a result, the remaining nucleus has an atomic number lower by 2, and a smaller mass number by 4. The transformation of the beta is based on emissions from the atomic nucleus electron named also beta particles. Of course, the nucleus contains only protons and neutrons, and electrons occurs through the transformation of one of the neutron into a proton, an electron and another particle with zero mass called antineutrino. Proton remains in the nucleus, increasing atomic number by 1, and an electron and an antineutrino fly out of the nucleus. Nucleus arising from the conversion of alpha and beta is usually in the excited state, which means that their energy is greater than the smallest possible energy, which is the energy of the ground state. The nucleus in a short time diposes of the excess energy by emitting gamma quanta. So gamma rays is always accompanied by alpha or beta radiation.

For measurements we are using 137Cs radioactive source. This nucleus transformed by beta conversion emitt a gamma quantum of energy 0.66 MeV.

 MeV (megaelektronowolt) is a million times more energy than the energy acquired by an electron accelerated by a potential difference of 1V.

beta conversion emitt a gamma quantum of energy 0.66 MeV.

 

The interaction of gamma radiation with matter.

What happens to the beam of gamma rays as they pass through the matter?

Ionization is to tear off electrons from atoms or molecules by passing a charged particle in the resort. On the trace the particles remain pairs of ions: negative - electrons and positive - atoms without an electron.

 

Becouse of electrical neutrality of gamma radiation is not posiible to ionize of the the medium, however, gamma can interact with matter in the various ways. These are the photoelectric effect, Compton effect and the creation of electron-positron pairs.

 

The photoelectric effect is the interaction of gamma photon with the atomic shell of atom by which the photon is absorbed. As a result an electron is ejected. Photon that caused the photoelectric effect, is removed from the beam.

 

The Compton scattering of a photon at the free electron is similar to the collision of billiard balls. An electron captures some part of the foton energy, electron begins to move, and photon with lowered energy, changes their direction. In a matter of course, there is usually no free electrons, but the electrons in valence band are weakly bonded to, and can be considered as quasi-free (nearly free), and they just take part in the Compton scattering. Photon that caused the Compton phenomenon, although not disappear, but change the direction of movement and also are removed from the beam.

 

 

Rest energy of the particle is E = mc2, where m is the mass of the particle, and c is the speed of light

 

 

The effect of the creation of electron-positron pairs is the most mysterious phenomena described here. Foton simply disappears, turning into a pair of particles: an electron and a positron antielectron called. This can happen only in the vicinity of the nucleus, which receives part of the energy and momentum. Not every photon can cause the creation of electron-positron pairs. His energy must be sufficient to create 2 particles. Resting energy of the electron and the positron is the same and equals to 0.511 MeV. The minimum (threshold) energy of the photon, which can lead to creation of the pair is therefore 1,022 MeV. Gamma quanta emitted by 137Cs with an energy of 0.66 MeV is not able to cause the creation of electron-positron pairs.

We already know that the beam of gamma rays passing through matter is weakened, because the loss of the photons that produce such phenomena as the photoelectric and Compton. How can you describe quantitatively weakening of the beam? The phenomenon of gamma quanta interaction with matter is a random phenomenon. The matter of probability is the case whether the photon passes through a layer of absorbent material, or cause one of the events in the result, which will be removed from the beam. The number of the loss photons from the beam ΔN, after moving Dx absorbent layer is thus directly proportional to the total number of photons incident on this layer, N, and the layer thickness Dx.

 

(1)

ΔN (x) = - μΔxN

 

μ is the coefficient of proportionality, called the coefficient of beam attenuation and is characteristic to the material. The minus sign means that the photons are removed from the beam. Mathematical transformation (integration) of the above formula leads to the following equation:

 

(2)

 N (x) = N0E-μx

 

 where: N0 is the number of photons incident on a layer of absorbent, N (x) - the is the number of photons tremaining in the beam after passing through the absorbent having a thickness of x (Fig. 1).

 

 

The number of photons passing through a matter will thus fall exponentially with increasing thickness of the absorbent x (Figure 2)

 

 

Determination of gamma-radiation attenuation factor

 

Attenuation coefficient of a material can be determined from the formula (2), if we could measure the number of gamma rays after passing through the absorbent having a thickness x, N (x) and the number of photons without an absorbent N0. Unfortunately this is not possible. We remember that gamma radiation is accompanied by beta radiation. If you execute measurement without absorbent, you will get the total number of gamma rays and beta particles. However, even the thinnest plate fully absorbs beta radiation. By measurements with absorbents, so we get information only about the number of the gamma rays.

 

For the receiving the graph of Fig. 2 in more convenient form let we do logarithm of both sides of equation (2). Using the properties of logarithms, we get:

 

(3)

LNN = lnN0-μx

This equation presents a linear function y = ax + b where: y = LNN, a = -μ, b = lnN0 (Fig. 3).

y=lnN, a=−μ, b=lnN0 (Rys. 3).

rys3n.pngRys weakness. 3. The right attenuation of a gamma beam of semi-logarithmic scale, the formula (3)

When setting the slope of the line, we obtain a wanted beam attenuation factor for the material.