PHYSICAL AND ENERGY FOUNDATIONS OF RADIATION TECHNOLOGY FOR SOLIDS WITH AN IONIC TYPE OF CHEMICAL BOND

A. A. VOROB'EV

In a certain approximation a crystal may be regarded as one large molecule. Irradiation of molecules or of a crystal may be accompanied by their excitation or ionization. These acts change many properties of the crystal.

Bombardment of a crystal by heavy particles of high energy may be accompanied by the displacement of ions from lattice sites into interstices and by the formation of vacancies and interstitial ions. Such structural disturbances are accompanied by changes in some properties of the crystal and by the acquisition of new properties that are absent, or only weakly expressed, in a crystal with an undisturbed lattice. Electrons liberated by radiation during irradiation of a crystal may occupy the places of negative ions in lattice sites. In such a lattice, in which anion vacancies are occupied by electrons, the simplest color centers are formed; new physical properties appear in the crystals. The formation of an ionic vacancy is a property associated with the ionic structure of the crystal. The ionization of the crystal lattice and the localization of an electron at a vacant site in the lattice, on the one hand, are connected with the ionic structure of the lattice and with its properties determined by the ionic composition, and on the other hand with electrons located in atomic vacancies. From this point of view one may say that radiation processes in an ionic lattice are connected with electronic and ionic phenomena in the lattice of alkali-halide crystals. The study of excitation, ionization of a crystal, formation of vacancies in the lattice, and their interrelation are central problems of the radiation physics of solids.

Vacant sites in a crystal lattice are also formed during evaporation of ions from a crystal heated to a high temperature. Upon rapid cooling of a crystal heated to a high temperature, it is possible to preserve in it, at low temperatures, a vacancy concentration exceeding the equilibrium concentration for the given temperature.

Annealing can reduce the concentration of vacancies in a crystal. The concentration of vacancies in a crystal can be increased by its plastic deformation.

The concentration of vacancies in a crystal can be changed by means of radiation, thermal, or mechanical treatment of the crystal. By changing the vacancy concentration in crystals through the indicated external actions, many of their properties are changed. In this sense, the change

the properties of crystals under irradiation, along with mechanical and thermal treatment, can serve technological purposes.

The development of the physical and energy foundations of radiation technology for solids is of scientific interest. Clarifying possible applications of radiation technology and its technical and economic advantages is important for the practice of materials science and the radiation technology of materials and products.

The mechanical and thermal properties of crystals, determined by ionic phenomena leading to the formation of vacancies, are associated with electronic phenomena in alkali-halide crystals.

For the reasons indicated, radiation processes in alkali-halide crystals, associated with electronic and hole processes, are considered by us in connection with ionic processes and the properties of alkali-halide crystals.

In this way it is possible to establish correlations and relationships of practical interest between the properties of irradiated crystals and the normal physical properties of crystals. This approach does not always make it possible to explain the physics of the phenomena under consideration, but it does make it possible to systematize facts conveniently, to determine regularities useful for practice, and to orient oneself in the properties of irradiated bodies.

With such a consideration of the problems of radiation treatment of solids, common scientific foundations emerge with mechanical and thermal methods of treatment.

Many properties of bodies are determined by the energy stored in them during thermal or mechanical treatment. In irradiated bodies, latent energy is also stored, determining certain properties of the bodies.

The formation of color centers may be regarded as a process of energy storage. Thus, for example, each \(F\)-center in KCl or KBr crystals stores about 5 eV of energy.

In the theory of the crystal lattice, the normal mechanical, thermal, optical, and electrical characteristics of an ionic crystal are determined through the magnitude of the change in the free energy of the system. With some accuracy, in a linear approximation, the free-energy quantity in the equations is replaced by the energy of the crystal lattice. The free energy of a body increases under external influences, including irradiation. With an increase in free energy due to radiation defects, the structure of the crystal is weakened. The change in free energy has a simple correlation with chemical processes for certain series of compounds. The mechanical, electrical, chemical, and temperature stability of crystals increases with increasing lattice energy. Previously, the selection of irradiation objects was made without taking into account the commonality and differences in the properties of cations and anions and the periodicity in the change of their parameters. In our studies, under identical measurement conditions, during irradiation of ionic compounds of one group of D. I. Mendeleev’s table, changes in several physical and chemical properties were studied, and a correlation was established with the lattice energy, free volume, and other parameters of the system.

Irradiated crystals have greater solubility and a larger linear coefficient of thermal expansion than unirradiated ones, which indicates a weakening of their structure.

The weakening of a body under irradiation is determined by the absorbed dose of energy, which increases with increasing effective atomic number of the compound, i.e., on passing to crystals with lower lattice energy. Therefore, when calculated per unit of exposure or absorbed dose, compounds with lower lattice energy are weakened during irradiation relatively more strongly than compounds with higher energy.

lattice. Consequently, the properties of compounds with lower cohesive energy change more strongly, and, according to the definition, in a radiation field they are less radiation-resistant than compounds with higher cohesive energy. Compounds with lower lattice energy will give a larger radiation-chemical yield of radiolysis products. Radiation technology should be built on the basis of data on the properties of bodies in the irradiation field, and not on residual changes in properties after irradiation.

Radiation changes after irradiation are commensurate with the stored energy, and not with the exposure energy or the absorbed energy.

The stored energy under irradiation has the same order of magnitude as the energy obtained during quenching, and is two to three orders of magnitude smaller than the energy stored by a body when impurities are introduced and a solid solution is formed.

Thus, chemical processes whose completeness correlates with the change in free energy in compounds with an ionic type of bond, after the action of irradiation, will change their course in the same way as after thermal treatment.

The hypothesis of a strong influence of the consequences of irradiation on the completeness and rate of occurrence of processes determined by the change in free energy, and of a significant change in the properties of bodies with an ionic type of bond, is not confirmed by the results of calculations of the change in free energy for compounds with small chemical-bond forces.

Compounds with lower lattice energy absorb more radiation energy, but, owing to more rapid annealing, retain fewer radiation changes than compounds with higher lattice energy. According to the definition, the former compounds have greater radiation resistance. These compounds have lower resistance under other types of action.

The indicated contradiction is a consequence of the fact that, in determining radiation resistance, one proceeds from residual changes after irradiation, whereas resistance under mechanical testing, temperature testing, etc., is determined during the action of the load.

Comparison of the resistance of a material under different external actions upon it must be made under identical conditions, i.e., either during the action of the external factor, or after the end of its action, or after the preliminary action of some fraction of the destructive magnitude.

With such a comparison, compounds with lower lattice energy will prove less resistant to the action of radiation, mechanical load, high temperature or an electric field, and chemical reagents. In crystals with higher lattice energy, the residual changes after the action of radiation, mechanical or thermal treatment will prove greater than in compounds with lower lattice energy.

The resistance of ionic structures with Coulomb bond forces during their radiation destruction increases with increasing lattice energy.

Many physical properties of crystals and the processes in them are not directly related to lattice energy; for example, the scattering of photons and electrons. The magnitude of the cross section of the scattering process increases with the radius of the ions. With increasing ionic radius the lattice energy decreases. As a result, with decreasing lattice energy one observes an increase in radiation absorption, a decrease in thermal conductivity and sound velocity. The indicated correlation makes it possible to extend to these processes and to the corresponding characteristics the possibility of classifying the properties of ionic structures by means of lattice energy.

In order to obtain the required product, the body is processed. Its shape, dimensions, material structure, and surface condition are changed.

Mechanical processing to give a body the required shape and dimensions is associated with separating from the body a part of its volume. The greater the strength of the bond between the particles of the body, the greater its strength, and the more energy must be expended on processing. Knowing the magnitude of the lattice energy, one can calculate the energy expenditures for processing the body and compare machinability. The required shape can be imparted to a body by pressing. The greater the lattice energy, the greater the work of extruding the material through an opening. The energy expenditures for performing this operation are also related to the magnitude of the lattice energy.

Thermal processing of a material, associated with the formation of solid solutions and diffusion, also depends on the energy of the bond between particles. The greater the bonding energy, the greater the heat of diffusion.

By means of irradiation it is possible to change the dimensions of a body, as well as its mechanical, optical, thermal, and other properties. The degree of change in properties depends on the chemical composition of the body, its structure, and the irradiation conditions.

Theoretically, the interaction of radiation with matter is limited to considering the scattering of radiation by the electrons and nuclei of isolated atoms, without taking into account their interactions and the presence of the electronic structure of a solid. At high densities of bodies and with a large thickness of the absorber layer, it is necessary to take into account processes in the body itself, and its change during the passage of radiation, which affect the absorption process.

Under irradiation, changes in the surface and bulk properties of bodies are possible, and these phenomena can be used for processing bodies.

Bombardment of the surface of a body by electrons, ions, protons, neutrons, or fission fragments, depending on the intensity of the beam, is accompanied by sputtering or melting of the surface. The effectiveness of surface destruction increases as the lattice energy decreases, or in accordance with the surface energy of the body, which changes in sympathy with it. Crystals of alkali-halide salts, when irradiated in a nuclear reactor with a dose of \(10^{21}\ \frac{\mathrm{n}}{\mathrm{cm}^{2}}\), split into separate pieces. Under the action of fast neutrons with an energy of 1 MeV at a temperature of 100°C, specimens of sintered beryllium oxide disintegrate after an irradiation dose equal to

\[ 2 \cdot 10^{20}\ \frac{\mathrm{n}}{\mathrm{cm}^{2}} . \]

The disintegration of sintered beryllium oxide probably occurs as a result of a significantly greater increase in the parameter \(C\) than in the parameter \(A\).

Specimens of BeO irradiated with fast neutrons with energies greater than 1 MeV at doses of \(1.5 \cdot 10^{21}\ \mathrm{n/cm^{2}}\), at a temperature of 110°C, turned into powder.

Fragmentation of BeO specimens was observed under irradiation with doses up to \(2.6 \cdot 10^{21}\ \mathrm{n/cm^{2}}\) at temperatures from 800 to 950°C. Specimens irradiated at intermediate doses and intermediate temperatures remained undestroyed.

In the experiments, cylindrical specimens pressed at high temperatures and cold-pressed specimens synthesized at 1650°C were used.

After bombardment by light heavy particles, the surface—

the surface layer of the body is strengthened. The depth of the layer and the degree of strengthening depend on the dose and irradiation conditions, and increase with increasing lattice energy. Under known conditions, bombardment of the surface by high-energy particles etches the surface and reveals the necessary structural details.

Of considerable importance is the explanation, advanced by E. L. Andronikashvili, of the physical nature of strengthening and weakening under irradiation. \(F\)-centers reduce hardness and weaken the crystal, whereas \(V\)-centers increase hardness and strengthen the crystal. The microscopic effect of strengthening or weakening of a crystal depends on the accumulation of \(F\)- or \(V\)-centers. Under different irradiation conditions the accumulation of \(F\)- and \(V\)-centers proceeds differently. Accordingly, the mechanical properties of crystals change in different ways.

Penetrating electromagnetic radiation causes reversible changes in the surface and bulk properties of bodies, changes in dimensions, their mechanical destruction, and chemical transformations—radiolysis.

In the radiation field, bonds in a body are weakened. Its processing, associated with mechanical destruction, evaporation, or dissolution, is facilitated.

An increase in the concentration of vacancies in a body after irradiation as a result of the disintegration of their aggregates will facilitate the course of certain chemical reactions.

The absorbed radiation energy may, to a considerable extent, be expended on irreversible processes (including luminescence), chemical and thermal processes, and partly on the formation of defects in which energy is stored. The stored energy is associated with reversible processes and radiation defects that are annealed. The change in the properties of bodies under irradiation is determined by the magnitude of the absorbed energy, which varies in a complex way with the composition of the object and the irradiation conditions. The absorbed dose increases with increasing effective atomic number, and varies differently with the thickness of the absorber and the radiation energy.

In view of the above, calculating the absorbed energy from absorption coefficients without taking irradiation conditions into account can lead to erroneous results.

It is necessary to scientifically standardize the conditions for determining the energy absorbed by a specimen.

The radiation-chemical resistance of ionic compounds increases with decreasing ionic radii and free volume and with increasing lattice energy. Radiolysis is determined by the magnitude of the absorbed energy, which increases with increasing effective atomic number or with the simultaneous decrease in lattice energy that occurs with it.

The expected radiation-chemical changes should be evaluated by the magnitude of the energy absorbed during irradiation.

The magnitude of the stored energy is determined by the prior treatment of the specimen and the conditions of its irradiation. The magnitude of the stored energy increases with increasing lattice energy and depends in a complex way on the absorbed energy.

Changes in the dimensions of a body after irradiation are small, and irradiation can be used in the precision machining of a body. The increase in vacancy concentration after irradiation, which causes changes in the properties of bodies, occurs without a significant rise in temperature, in contrast to what occurs during quenching.

Radiation technology for solids with an ionic type of bond has known advantages over mechanical and thermal treatment, which determines the practical interest in its development.