Nuclear energy is released by the redistribution of the constituent particles of the atomic nucleus — protons and neutrons — in contrast to chemical energy released due to the rearrangement of atoms with the redistribution of electrons between them. Protons and neutrons tightly packed in the nucleus are a relatively stable system due to the forces of attraction between protons, neutrons and between protons and neutrons. These attractive forces act only at such distances as intra-nuclear ones, and many times exceed the electric repulsion forces of positively charged protons.
The stability or instability of nuclei, i.e. the probability of redistribution of nuclear particles or a nuclear reaction, depends on the ratio of neutrons and protons in the nucleus. The nuclei of various atoms are usually characterized by the atomic number Z, equal to the number of protons in the nucleus (the number of single positive charges), and the mass number A, equal to the total number of protons and neutrons. The number of neutrons in an atomic nucleus is equal to the difference A—Z.
Atoms having the same atomic number 7 but different mass numbers A are called isotopes.
Consequently, the isotopes differ from each other only in the number of neutrons A— in the nuclei. In total, about 280 stable and about 50 unstable isotopes are found in nature. In addition, more than 700 unstable isotopes for all currently known elements have been artificially obtained using various nuclear reactions.
To denote any particular isotope X, the mass number A is indicated simultaneously with the name or symbol of the element: X-A or XA.
Thus, an isotope of uranium with a mass number of 238 can be written as U-238 or U238. Sometimes the atomic number Z:XAZ is also indicated in the record. For example, U23892.
For the stability of the nucleus, it is necessary that the ratio of the number of neutrons to the number of protons (i.e. (A—Z) / Z) in this nucleus lies within certain limits. With the growth of the mass number, this ratio (in the stability region up to Z = 81) gradually increases from 1 to 1.56. For U23892, it is equal to (238-92) / 92 = 1.59. There is an upper and lower stability bound for the ratio of the number of neutrons to the number of protons in the nucleus. The upper bound is determined by the ability of neutrons to overcome the electric repulsion of protons, which increases rapidly with increasing atomic number (as Z2* A -1/3). The lower bound arises due to the fact that the increasing number of protons in turn leads to instability due to electrostatic repulsion.
Thus, natural elements with large mass numbers, such as polonium (200-218), thorium (223-235), radium (213-230) and uranium (227-240), have only unstable or radioactive isotopes. These substances are characterized by spontaneous transformations, which are usually called radioactive decay. Almost all elements with an atomic number Z less than that of thallium (Z=81) occur in nature as stable isotopes.
Unstable nuclei become stable after one or a series of successive radioactive transformations. At the same time , the core emits
an electrically charged particle: a-particle (helium nucleus) or B-particle (electron, positron). After the emission of charged particles, most of the daughter nuclei are in an excited state for some time (~ 10-15 seconds) and release excess energy (excitation energy) in the form of y-radiation. If the nuclei contain more neutrons than is required for stability, then in these nuclei there is a spontaneous transformation of neutrons into protons with the emission of negative B-particles (electrons), while the atomic number of the daughter element becomes one higher than that of the original (parent), and the mass number does not change. If the nuclei contain an excess of protons, there is a spontaneous transformation of protons into neutrons with the emission of positive B-particles (positrons).
In this case, the atomic number of the child element becomes one less than that of the parent, and the mass number does not change.
The other two types of nuclear transformations — a-decay and K-capture — are characterized by: the first is the emission of an a—particle (a helium nucleus consisting of two protons and
two neutrons), the second is the capture of an orbital electron (the process reverse to the emission of a positron nucleus).
The nuclear energy released during radioactive decay is measured by the energy of the emitted particles. It reaches several million electron volts (Mev).
The potential energy resources of each nucleus are determined by the binding energy of particles in the nucleus equivalent to the mass defect: E = mc2, where E is the energy equivalent to mass m, and c is the speed of light. The ratio of the binding energy of the nucleus to the mass number represents the average value of the binding energy per nucleon (neutron or proton) in a given nucleus.
With the exception of a small number of light nuclei (up to oxygen), the value of the average binding energy per one nucleon remains within wide limits almost constant
and close to 8 Mev. Therefore, the total binding energy for these nuclei is approximately directly proportional to the mass number A, i.e. The total number of protons and neutrons in the nucleus. The release of even a tenth of this energy in each act of the decay of nuclei such as uranium (A = 238) is a very impressive value (~ 200 Mev).
Uranium, as one of the unstable natural elements, attracted attention by the fact that, in addition to the usual radioactive decay with the emission of a and b particles and y quanta, the nuclei of some of its isotopes decay by fission.
From the book:
PROTECTION FROM RADIOACTIVE FALLOUT edited by A.I.Burnazyan
R.V., Petrov, V.N. Pravetsky, Yu.S. Stepanov, M.I. Shalnov
State Publishing House of Medical Literature Moscow -1963