What is radioactivity.
Both stable and unstable isotopes are found in nature. The nuclei of some nuclides are unstable, the number of neutrons in them exceeds the number of protons. The nuclei of such unstable isotopes have the ability to spontaneously transform into other nuclei or transition from the excited state to the ground state. This process is called radioactive decay. It can be accompanied by the emission of alpha particles, beta particles, neutrons or
gamma-ray radiation. Nuclides (isotopes) capable of radioactive decay are called radionuclides (radioisotopes).
The phenomenon of spontaneous decay of nuclei with the emission of gamma quanta or particles and gamma quanta is called radioactivity.
Radioactive isotopes (radionuclides) are characterized by the amount of activity, the type of radiation (α-, β-, y-), the energy of the emitted particles and gamma radiation, as well as the half-life.
Activity of a radioactive substance.
The most important characteristic of a radionuclide, among other properties, is its radioactivity, that is, the number of decays per unit of time (the number of nuclei that decay in 1 second).
The unit of activity of a radioactive substance is Becquerel (Bq).
1 Becquerel = 1 decay per second.
The value of radioactivity expressed in Becquerels can be very large. For example, the body of an adult contains approximately 4000 Becquerels of the natural (natural) radioactive substance potassium-40.
Until now, an off—system unit of activity of a radioactive substance is still used – Curie (Ci). 1 Ci = 3.7 * 1010 Bq.
The half-life of a radioactive substance
The half-life (T1/2) is a measure of the rate of radioactive decay of a substance – the time it takes for the radioactivity of a substance to decrease by half, or the time it takes for half of the nuclei in the substance to decay.
Figure 1.8. Half-life of radionuclides.
After a time equal to one half-life of the radionuclide, its activity will decrease by half of the original value, after two half-lives – by 4 times, and so on. The calculation shows that after a time equal to ten half-lives of the radionuclide, its activity will decrease by about a thousand times.
The half—lives of various radioactive isotopes (radionuclides) have values from fractions of a second to billions of years. Radioactive isotopes with half-lives of less than a day or months are called short-lived, and more than a few months — long-lived.
TYPES OF IONIZING RADIATION.
All radiation is accompanied by the release of energy. When, for example, a human body tissue is exposed to radiation, part of the energy will be transferred to the atoms that make up this tissue.
Radiation that carries a sufficient amount of energy is capable of removing electrons from atoms. This process is called ionization, and radiation capable of removing an electron from an atom is called ionizing (unlike, for example, electromagnetic radiation from the sun, which is not).
Unstable nuclides tend to move to a stable state. They they can release their excess energy during the decay process. Decay means that a radioactive nuclide emits ionizing radiation in the form of particles or electromagnetic waves (gamma quanta).
In everyday life, ionizing radiation is mistakenly called radioactive radiation. The correct expression is ionizing radiation. We will consider the processes of alpha, beta and gamma radiation. All of them occur during the decay of atomic nuclei of radioactive isotope elements.
In Figure 1.9, an unstable nucleus is in the process of emitting its excess energy due to the emission of a particle that is a helium nucleus and consists of two protons and two neutrons. This particle is called the alpha particle and is denoted by the Greek symbol α.
Alpha particles are positively charged helium nuclei with high energy.
Figure 1.9. Alpha radiation.
Ionization of a substance by an alpha particle
In Figure 1.10, the alpha particle passes close to the atom. When an alpha particle passes in close proximity to an electron, it attracts it and can tear it out of its normal orbit. The atom loses an electron and is thus transformed into a positively charged ion. This is how alpha particles usually ionize matter.
Figure 1.10. Ionization of a substance by an alpha particle.
Ionization of an atom requires approximately 30-35 eV (electron volts) of energy. Thus, an alpha particle having, for example, 5,000,000 eV of energy at the beginning of its movement can become a source of creation of more than 100,000 ions before it goes into a state of rest.
The mass of alpha particles is about 7,000 times the mass of an electron. The large mass of alpha particles determines the straightness of passage through the electron shells of atoms during ionization of matter.
The alpha particle loses a small part of its original energy on each electron that it tears off from the atoms of matter, passing through it. The kinetic energy of the alpha particle and its velocity are continuously decreasing.
When all kinetic energy is used up, the α-particle comes to rest. At this moment, it captures two electrons and, having transformed into a helium atom, loses its ability to ionize matter.
Figure 1.11 shows an example of the emission of a beta particle, which is indicated by the symbol β. Beta radiation is the process of emitting electrons directly from the nucleus of an atom. An electron in the nucleus is created when a neutron decays into a proton and an electron. The proton remains in the nucleus, while the electron is emitted as beta radiation.
Figure 1.11. Beta radiation.
Ionization of a substance by a beta particle.
Figure 1.12 shows a possible course of events when an electron (β-particle) ejected from the nucleus of a radionuclide knocks out one of the orbital electrons of a stable chemical element. These two electrons have the same electric charge and mass. Therefore, when the electrons meet, they will repel each other, changing their original directions of movement.
Figure 1.12. Ionization of a substance by a beta particle.
When an atom loses an electron, it turns into a positively charged ion.
Gamma radiation is indicated by the symbol – y. Gamma radiation does not consist of particles like alpha and beta radiation. It, as well as the light of the Sun, is an electromagnetic wave (Fig. 1.13). Gamma radiation is electromagnetic (photonic) radiation consisting of gamma quanta and emitted during the transition of nuclei from the excited state to the ground state during nuclear reactions or particle annihilation.
This radiation has a high penetrating power due to the fact that it has a much shorter wavelength than light and radio waves. The energy of gamma radiation can reach large values, and the propagation speed of gamma quanta is equal to the speed of light. As a rule, gamma radiation accompanies alpha and beta radiation, since there are practically no atoms in naturethat emit only gamma quanta.
Gamma radiation is similar to X-ray radiation, but differs from X-ray radiation by the nature of origin, the length of the electromagnetic wave and frequency.
Figure 1.13. Emission of gamma radiation by an atom.
Ionization of matter by gamma radiation.
Gamma radiation passing through a substance has the ability to ionize this substance, transferring its energy to the electrons of the atoms that make it up. The radiation energy is gradually decreasing.
Since gamma radiation has no electric charge, its ability to ionize atoms of matter is less than that of alpha and beta radiation. The effect of gamma radiation on a substance leading to the separation of an electron from the electron shell of an atom is shown in Figure 1.14.
Figure 1.14. Ionization of matter by gamma radiation.
FSUE “St. Petersburg Research Institute of Radiation Hygiene named after Professor P.V. Ramzaev”, 2011
Shutov V.N., Kaduka M.V., Kravtsova O.S., Prakhomenko V.I., Samoylenko V.M. Protection from radiation. Popular science manual. – St. Petersburg, 2011.-88s.
Artist Melnikova T.Yu.