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Chemical Processes Under the Influence of Radiation
Chapter 16
Chemical Processes Under the Influence of Radiation Chapter Outline 16.1 Radiochemistry and Radiation Chemistry 16.2 Stages of Radiation Processes 16.2.1 Physical Stage 16.2.2 Physicochemical Stage 16.2.3 Chemical Stage 16.3 Radiolysis 16.3.1 Radiolysis of Gases 16.3.2 Radiolysis of Liquids
247 247 248 248 250 251 251 252
16.4 Radiolysis of Water 16.4.1 Properties of Water and Its Role 16.4.2 Physical Stage of the Water Radiolysis 16.4.3 Reactions of the Water Radiolysis of Physicochemical Stage 16.4.4 Chemical Stage of the Water Radiolysis 16.5 Radiolysis of Solids 16.6 The Yield of the Radiolysis Products References
252 252 252 253 253 254 254 255
16.1 RADIOCHEMISTRY AND RADIATION CHEMISTRY Let us indicate the difference between the two branches of science: radiochemistry and radiation chemistry. “Radiochemistry” is the chemistry of radioactive materials; it involves study of chemical transformations of radioactive substances, dealing with actinides and transuranium elements, development of physicochemical principles of handling radioactive waste from nuclear power engineering, solving radioecology problems, developing methods for manufacturing sources of radioactive emissions, and separation of radioactive isotopes. “Radiation chemistry” deals with study of chemical transformations under the action of ionizing radiation, study of radiation-chemical processes, development of methods for predicting the radiation resistance of various materials, and development of methods for their protection against destruction. Radiation chemistry is also responsible for development of radiation-chemical methods for the synthesis of organic compounds and production of modified materials for medicine, biotechnology, and membrane technology; development of new radiation-resistant materials, radiation modification of polymers; development of radiation-induced nanostructures; production of nanoporous track filters; and many other processes occurring in matter under the action of ionizing radiation. This chapter offers the basics of radiation chemistry. Some applied problems are discussed in some chapters below, which are: Chapters 24, 27, 29 and 30.
16.2 STAGES OF RADIATION PROCESSES The sequence of the processes of ionizing radiation effect on a substance is usually divided into three stages: l l l
physical physicochemical chemical
Radiation. https://doi.org/10.1016/B978-0-444-63979-0.00016-1 Copyright © 2019 Elsevier B.V. All rights reserved.
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248 PART | I Fundamentals
16.2.1 Physical Stage The effect of ionizing radiation on a substance begins with two fundamental processesdthe ionization and excitation of molecules. These processes are described in detail in Chapter 4. The whole variety of phenomena that constitute the subject of such sciences as radiation physics, radiation chemistry, radiation biology, radiation material science, and others, starts from these two processes. The interaction of charged particles with matter, i.e., directly ionizing radiation, is described in detail in Chapter 5. Neutral radiation, i.e., gamma quanta and neutrons interacting with matter, transmits energy to charged particles, and those, in their turn, produce ionization and excitation. Therefore, they are called indirectly ionizing radiation. The features of gamma quanta interaction with matter are described in Chapter 6, and those of neutrons in Chapter 7. One more process has to be also pointed out and that is elastic collisions of particles with the atoms of matter. Usually in the balance of energy losses of charged particles, elastic collisions play just a minor role, which increases only at low particle energies and are only significant for heavy particles. The elastic interactions of incident particles in gases and liquids practically do not change anything in the properties of matter, but under certain conditions they can rather significantly affect a structure of solids. This question is considered in more detail in Chapters 15, 25 and 29. In this chapter we confine ourselves to the two indicated processes, i.e., ionization and excitation. In the process of ionization, electrons separated from atoms can receive energy, which is higher than the ionization energy. The resulting electrons with excess kinetic energy are called delta electrons. For details on delta electrons, see Section 5.1. It is essential to point out for the analysis carried out in this book, that delta electrons can carry the energy of a particle away from its path and produce ionization and excitation at some distance. The physical stage continues as long as the particle moves through the substance to a stop. The typical deceleration time of particles with energies of the order of MeV in matter with a density of water or inbiological tissue is several picoseconds (wx∙1012 s) [1]. Many processes that are traditionally attributed to the physical stage are finished much faster, in about 1015 s. This means that the wave of the physical stage processes runs through the substance with the velocity of the particle. And when at the end of the track the physical stage is still developing, whereas at the beginning of the track the reactions of the following stage already started. By the end of the physical stage, the substance contains molecular ions Mþ, free subthreshold electrons (nonexcitation electrons) e, excited molecules M*, and also so-called superexcited molecules M**, which have an energy exceeding the ionization potential of molecules. It is essential that all the excitations formed by radiation are distributed unevenly in space; they are concentrated within the particle tracks (Section 8.2). Both the statistical nature of the energy distribution in the track and the associated fluctuations influence the effect of radiation on matter.
16.2.2 Physicochemical Stage The next step in converting the energy of ionizing radiation in matter is called the physicochemical. Some physical processes continue, and some reactions begin to occur between the products of radiolysis. This stage takes about 1015 to 1012 s. Electrons formed as a result of ionization have some excess kinetic energy. Such electrons are called hot. Moving in matter, a hot electron loses energy in acts of interaction with molecules of the medium until its energy becomes of the order of thermal energy. This process is called thermalization. The thermalization process is considered in detail in Section 8.4. The average thermalization path in water is several nm [2]. In any substance, electrons polarize surrounding atoms or molecules and, therefore, appear in the field of polarization. When moving in matter (by diffusion or drift), electrons move along with this field, which significantly affects the dynamics of their movement. In radiation chemistry, the process of producing a potential well by an electron as a result of the polarization of surrounding molecules is called solvation, and the electron is called solvated. If the medium in which an electron is located is water, then the solvation of an electron is related to the orientation of the polar molecules of water and then this process is called hydration, and the electron is called hydrated. Sometimes an electron, which is off the field of polarization, is called “dry,” and a hydrated electron is called “wet.” The hydrated electron is denoted by the subscript eaq, and the hydration process is presented by the reaction [3,4] e/eaq .
(16.1) 12
s in water. Mobility m and diffusion The process of hydration of electrons formed by radiation takes about 10 coefficient D of hydrated electrons in water are much lower than in nonpolar liquids: m(eaq) ¼ 2 103 cm2/s V,
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D(eaq) ¼ 4.9 105 cm2/s; for comparison m(c-hexane) ¼ 0.24 cm2/s V, D(c-hexane) ¼ 5.5 103 cm2/s; m(liquid Ar) ¼ 480 cm2/s V, D(liquid Ar) w 70 cm2/s. The hydrated electron is characterized by a wide intense absorption band in the red region of the spectrum and by a narrow single line (singlet) in the EPR spectrum. The maximum optical absorption band of a hydrated electron in water at T ¼ 298 K corresponds to 720 nm (Emax ¼ 1.73 eV) [4]. An important role in radiation chemistry is played by so-called free radicals. Free radicals are relatively stable molecules (or atoms) containing one or two unpaired electrons on the outer electron shell. If a radical has a charge, it is called a radical ion. There are various ways to designate a radical. We mark the radical with a bold dot to the right of the chemical formula. Radicals are formed during the dissociation of excited molecules, e.g., water: H2 O /H% þ OH%;
(16.2)
in the dissociative capture of an electron, for example, by a carbon tetrachloride molecule: CCl4 þ e/CCl3 % þ Cl ;
(16.3)
in some radiation chemical reactions, for example, at the water radiolysis: H2 O/Hþ % þ OH .
(16.4)
A radical with an unpaired electron is a highly reactive particle. If radical is not charged, then its interaction with other molecules is determined by the same potential as for any neutral molecule, and the probability of encounter or, in other words, of the collision cross section for the radical and for the neutral molecule is identical. But when a radical is encountered with a molecule, it reacts immediately. The activation energies of such reactions are close to zero. In the case of the interaction of ordinary molecules, i.e., nonradicals, for a reaction to occur, thousands of collisions may be required. Actually, such a delay in the reaction can be associated with the presence of an energy barrier or in the search of the ways to give up the energy released in the reaction. The radical reacts at once. The next process that must be considered in the physicochemical stage is the capture of the electron. As a result of the capture of electrons by electrophilic molecules, negative ions appear in the substance e þ X/X .
(16.5)
In this reaction, X is an electrophilic molecule, it can be a molecule of the basic substance X ¼ M, and maybe an impurity molecule. The process of electron capture is rather complicated, in which the binding energy of an electron in a negative ion must be released. The capture of electrons is discussed in detail in Chapter 9. Electrons and positive ions, as well as encountered in matter ions of opposite signs, can recombine e þ M þ /M;
(16.6)
X þ M þ /M þ X.
(16.7)
Recombination can generate excited molecules, if the energy released during the recombination process is sufficient to excite the molecule, for example, e þ M þ /M .
(16.8)
The problems of recombination of charges in matter are discussed in detail in Chapter 11. Not only charges, but also radicals recombine CH3 % þ C2 H5 %/C3 H8 .
(16.9)
In many complex organic substances, the excitation of molecules leads to nonradiative transitions with high probability and also to their dissociation. In radiation physics this phenomenon is called fragmentation. Some of the charged particle energy turns into heat, and this leads to an instant increase in temperature in the track region. Generally, the increase in temperature can be very significant. So, the energy released on the tracks of some delta electrons is enough to form microbubbles in the liquid. This is on what a bubble chamber operation is based, i.e., the track device, which has played a very important role in the history of nuclear physics. In a superheated liquid, such seed bubbles grow to visible dimensions and allow to record the traces of charged particles. Bubble chambers operate on liquid propane, freon, hydrogen, and some other liquids.
250 PART | I Fundamentals
In the 1950s, to explain the decrease in the luminescence yield of organic substances when excited by charged particles with a large Linear energy transfer (LET), the Russian physicist M.D. Galanin suggested the idea of temperature quenching due to an increase in the excitation channel temperature [5]. Calculations carried out by M.D. Galanin showed that in a channel with a radius of 3∙103 mm, an alpha particle with an energy of 5.3 MeV in anthracene raises the temperature by not less than 100 K. According to A. Mozumder’s calculations [2], particles with small LET (fast electrons) raise the temperature on the track axis in water by about 30 K, alpha particles by about 400 K, and fission fragments by 104 K. The time during which the temperature on the track axis decreases due to the thermal conductivity by half, is equal to 6 1012 s for particles with a small LET, and 1011 s for an alpha-particle track. At high irradiation doses, an additional increase in the temperature of the substance on irradiation is also possible, if the reactions initiated by the radiation are exothermic. The final temperature depends on the absorbed dose and on the heat capacity of the substance. It is estimated that, with an absorbed dose of 10 kGy, an increase in temperature can be w5 C in water and w10 C in polyvinyl chloride. At a dose of 30 kGy, the increase is w15 and w30 S, respectively. One of the ways to remove excitation in the processes of radiation chemistry may be the emission of photons. Questions of luminescence are considered in detail in Chapter 12. Because the energy of ionizing radiation is emitted in the process of luminescence, this process competes with processes, which lead to defect formation, dissociation of molecules, and other processes of nonradiative energy transfer.
16.2.3 Chemical Stage As most of the chemical reactions discussed here occur in mixtures and solutions, the probability of interaction, i.e., the probability of reaction and, hence, the probability of recombination are determined by the mutual diffusion of the reacting partners. They must first meet to react. The rate constant of such encounters k can be calculated from the Smoluchowski equation k ¼ 4pDAB ðrA þ rB Þ;
(16.10)
where DAB ¼ DA þ DB is the coefficient of relative diffusion, and rA and rB are the radii of components A and B. If one of the interaction components is an electron, its “dimensions” are determined by the wavelength of the de Broglie wave. In the case, when the reaction components are charged, we can assume that the radii of the components are equal to the radius of the capture sphere rc ¼ e2 4pεε0 kT; (16.11) and the diffusion coefficient D is related to the charge mobility m by the Einstein relation D ¼ mkT=e;
(16.12)
then the Smoluchowski formula (16.12) transfers to the LangevineDebye formula k ¼
4peðmþ þ m Þ . 4pεε0
(16.13)
The reacting partners after encounter form a diffusion pair and enter the solvent molecules environment. In radiation chemistry, this environment is often called a “cage.” In this cage, they undergo multiple collisions before the reaction really happens. If the reaction occurs at the first meeting, as, e.g., when radicals are recombined, then this reaction is called diffusion controlled. In the chemical stage, the products of the action of radiation interact with each other and with molecules, not affected by the radiation action. One of the variants of the reaction between molecules or of the recombination process of radicals is called a reaction of disproportionation. In such a reaction, a redistribution of atoms or fragments of molecules occurs CH3 % þ C2 H5 %/C2 H4 þ CH4 .
(16.14)
Another option for a chemical reaction with products of radiation exposure is substitution reactions, when certain functional groups, that make up a chemical compound change to the other groups C2 H5 % þ RH/R% þ C2 H6 .
(16.15)
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16.3 RADIOLYSIS 16.3.1 Radiolysis of Gases In gases at pressures close to atmospheric, the structure of the track differs noticeably from tracks in condensed matter. In the process of thermalization, an electron leaves the parent ion for considerable distance. The uniform irradiation is manifested at significantly lower doses than in condensed bodies. LET significantly affects the radiation-chemical yields. Because of the low density of the medium, the probability of deactivation of intermediate products of radiolysis in collisions with surrounding molecules is small. Therefore, at low pressure the intermediate particles in gases have longer lifetimes than in liquids. As an example, we indicate the main reactions of the radiolysis of carbon dioxide CO2 and the reactions leading to the formation of ozone O3 in atmospheric air. In CO2, ionization by charged particles can lead to the appearance of four types of ions [6] CO2 þ hn/COþ 2 þ e; CO2 þ hn/COþ þ O þ e; CO2 þ hn/Oþ þ CO þ e;
(16.16)
CO2 þ hn/Cþ þ 2O þ e. Excited molecules of CO2 quickly dissociate CO2 þ hn/CO2 /CO þ O.
(16.17)
The formed products of primary interaction enter the radiation-chemical reactions among themselves and with unexcited CO2 molecules. As a result of these reactions, molecules of CO, atomic and molecular oxygen appear. The carbon formed in the reactions is rapidly oxidized, so the final radiative yield of carbon is very small. The process of radiolysis of carbon dioxide is significantly affected by water impurities. Ozone in the atmosphere is formed mainly by the hard ultraviolet radiation of the sun; this process occurs in the stratosphere. Because of thunderstorms, fires, and human industrial activity, ozone as well is formed in the troposphere. The formation of ozone follows a reaction, which, in general, can be written as 3O2 /2O3 .
(16.18)
The reaction of ozone formation is endothermic. The reaction (16.18) requires 2.96 eV (285 kJ mol)dthis energy turns to the formation of two molecules of ozone (1.48 eVd142 kJ/mol for one). Let us, at least partially, show the separate stages of the reaction (16.18): O2 þ hn/Oþ 2 þ e;
(16.19a)
O2 þ hn/O2 ;
(16.19b)
O2 þ e/O 2;
(16.19c)
O2 /O þ O;
(16.19d)
O2 þ O/O3 ;
(16.19e)
þ O 2 þ O2 /O3 þ O;
(16.19f)
þ O 2 þ O2 /O2 þ O2 ;
(16.19g)
þ O 2 þ O2 /O þ O þ O2 .
(16.19h)
and so on. The oxygen atoms released in the last reactions are connected with oxygen molecules (16.19e), resulting in the formation of ozone molecules. Let us note that the reaction (16.19c) is presented by provisional view. In the gas phase most likely occurs a three-particle capture (Chapter 9).
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The ozone molecule is relatively stable, but colliding with particles of aerosols or impurities in the liquid (in water), it decomposes easily with the release of atomic oxygen: P3 /O2 þ O.
(16.20)
Calculations show that the formation of ozone due to natural radioactivity in the near-surface layer of the Earth can reach w 1.8 105 tons/year. Substantially large amounts of ozone are generated in the atmosphere during testing of atomic and hydrogen bombs in the air, the operation of nuclear reactors and other nuclear cycle facilities, and the storage of nuclear waste. Nuclear activity of mankind supplies to the atmosphere about 109 tons of ozone per year.
16.3.2 Radiolysis of Liquids The main feature of the radiolysis of organic liquids is a large variety of reactions leading to the formation of a large number of intermediate radiolysis products. However, the results of research obtained so far show that several transformation channels are crucial, which determine the accumulation of the main products of radiolysis. The identification of these channels and the study of the regularities of the reactions proceeding make it possible to obtain the necessary information for predicting the accumulation of radiolysis products. To obtain detailed information on the processes of radiation-chemical transformations in organic compounds, we refer the reader to the special literature [3,6,7]. Here we note that, as a rule, the bonds between carbon atoms are broken with less probability than the CeH bond, and therefore one of the most important products of radiolysis is molecular hydrogen. Aromatic hydrocarbons have a higher radiation resistance in comparison with alkanes and alkenes. This is due to the fact that the excitation in some particular fragment of an aromatic molecule, due to the presence of a conjugated system p-bonds, is delocalized along the aromatic ring. As a result, rupture of the ring becomes unlikely. In these compounds the processes of excitation, transfer of charge or excitation energy, and other processes of energy dissipation, which do not lead to dissociation of molecules, predominate.
16.4 RADIOLYSIS OF WATER Let us consider the radiolysis of water as an example of radiation-chemical transformations of matter.
16.4.1 Properties of Water and Its Role Water is a special substance on the planet Earth and, probably, in the whole universe. In search of life on other planets, scientists focus on the temperature conditions of each planet. It is assumed that life is possible only in the temperature conditions, where liquid water can exist. And if there are no visible traces of water, as, e.g., on Mars, then traces of the existence of liquid water there in the past are sought. Water is of key importance in the creation and maintenance of life on Earth, for the formation of climate and weather. It is the most important substance for all living beings on the planet Earth. A significant part of a living cell is the cytoplasm, consisting mainly of water. In the intercellular space, lymphatic water also plays a significant role. Water is necessary for the life of all, without exception, unicellular and multicellular living beings on Earth. Water is the most common solvent on our planet, which largely determines the nature of chemical processes. Most of the chemistry, when it became a science, began precisely as the chemistry of aqueous solutions of substances. Water can be used as a moderator and coolant in nuclear reactors. Water is a model object of radiation physics and chemistry; just as a hydrogen atom is a model object of atomic physics, an alkaliehalide crystal is a model object in solid-state physics, etc. So, it is obvious that for mankind, it is extremely important to realize what happens to water when irradiated with ionizing radiation.
16.4.2 Physical Stage of the Water Radiolysis As a result of ionization and excitation of water molecules, molecular ions H2Oþ, free subthreshold electrons e, excited H2O*, and superexcited molecules H2O** are formed. The minimum ionization energy of water is 12.6 eV, and the minimum excitation energy is 7.4 eV. Let us present the list of radiolysis products by the end of the physical stage: H2 Oþ ; e; H2 O ; H2 O .
(16.21)
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It is evident that the system at the end of the physical stage is energetically in nonequilibrium. In the subsequent stages occur some transformations with these radiolysis products, and their number changes.
16.4.3 Reactions of the Water Radiolysis of Physicochemical Stage With the products of radiolysis formed at the end of the physical stage, the following transformations occur: 1. Dissociation of excited and superexcited molecules H2 O /H% þ OH%;
(16.22)
H2 O /H2 þ O%;
(16.23)
H2 O /H2 Oþ þ e;
(16.24)
2. Recombination of a “dry” electron with a molecular ion with the formation of an excited molecule and subsequent dissociation processes H2 Oþ þ e/H2 O /HO% þ H;
(16.25)
3. Thermalization and hydration of electrons e/eaq ;
(16.26)
4. Ionemolecule reaction with the formation of hydroxonium ion H2 Oþ þ H2 O/H3 Oþ þ OH%;
(16.27)
H3 Oþ /ðH3 Oþ Þaq.
(16.28)
5. Hydration of hydroxonium ion
Thus, at the end of the second stage, the assortment of particles in the case of radiolysis of pure water has the following form: hydrated electrons eaq, O, PO and P radicals, hydroxonium ions (H3Oþ)aq, and molecular hydrogen H2: eaq ; H%; OH%; ðH3 Oþ Þaq; H2 .
(16.29)
These particles are inhomogeneously distributed in space; they are concentrated in microregions along the particle track. In the case of gamma irradiation, these microregions are mainly the “spurs.” The distribution of particles in the “spur” is also uneven: the radicals O, H, OH, hydroxonium ions (H3Oþ)aq, and hydrogen H2 molecules are located predominantly in the center of the “spur”, and the hydrated electrons are located in the spherical layer at a distance of about 4 nm from the center [6].
16.4.4 Chemical Stage of the Water Radiolysis At the chemical stage, chemical reactions occur in the products, formed at the previous stage, with each other, which leads to the formation of atoms and molecules of hydrogen, hydrogen peroxide, and hydroxyl ions. These reactions occur, mainly, in the ionization cellsdin spurs, blobs, and short tracks. Simultaneously, diffusion of these and previously formed particles from the spurs to the solution volume occurs, which results in the blurring of the spurs and the leveling of the radiolysis products by volume, i.e., the establishment of a homogeneous distribution of the products. Radiolysis products can interact with each other and with the molecules of the solution. The time for establishing a homogeneous distribution depends on the irradiation intensity; at high intensity it can be of the order of 107 s [6]. The main reactions of the chemical stage are: eaq þ H% þ H2 O/H2 þ OH ;
(16.30)
H% þ H%/H2 ;
(16.31)
eaq þ OH%/OH ;
(16.32)
eaq þ H3 Oþ /H% þ H2 O;
(16.33)
254 PART | I Fundamentals
OH% þ OH%/H2 O2 ;
(16.34)
OH% þ H%/H2 O;
(16.35)
H3 Oþ þ OH /2H2 O.
(16.36)
New molecules, not yet produced in the previous stages, are hydroxyl OH and hydrogen peroxide H2O2. Later, under certain conditions, the OP2 radicals and O2 oxygen molecules can be formed. With a high excitation density during the chemical stage, reverse reactions play an important role when one of the radiolysis products collapses and water molecules reappear [6]. H þ OH/H2 O
(16.37)
H2 þ OH/H þ H2 O.
(16.38)
or
If a sufficient concentration of radicals is created in the volume, then one of the main reactions of radicals is their recombination. In particular, in the radiolysis of water, the ion radicals Oþ and POe are formed. As a result of the recombination of these radicals, a series of highly reactive compounds are created, such as superoxide HO2 and peroxide H2O2. In the case of the biological tissues irradiation, such compounds can interact with organic cell molecules to cause significant damage. We also note that among the products of radiolysis, some are short-lived (hydrated electrons, hydrogen atoms, hydroxyl radicals, etc.), whereas others are stable (hydrogen, oxygen, and hydrogen peroxide).
16.5 RADIOLYSIS OF SOLIDS Radiation-chemical transformations in solids have, as a rule, much smaller yields than in liquids or gases. This is due to the fact that rigid bonds between particles, characteristic of a solid, greatly impede the motion of atoms and molecules in a solid and, therefore, inhibit the chemical reactions that result from the collision of these particles. The formation of crystal defects by radiation is described in Chapters 15 and 25. Chapter 29 describes the staining of glasses and crystals by ionizing radiation, which is associated with the capture of electrons by certain color centers. In complex substances, deep chemical transformations are also possible. The changes in the structure of graphite and the related thermal phenomena are especially important because graphite acts as a moderator in uraniumegraphite nuclear reactors and is exposed to very significant fluxes of neutrons and gamma quanta. The well-known physicist E. Wigner had discovered that graphite, when bombarded by neutrons suffers dislocations in its crystalline structure, causing a buildup of potential energy. This energy, if allowed to accumulate, could escape spontaneously in a powerful rush of heat. Underestimation of changes in stored energy during irradiation of graphite resulted in several accidents of nuclear reactors. The most famous is the nuclear reactor accident for the production of weapons-grade plutonium in Windscale, England, in 1957. The accident led to the release of radioactive substances totaling 550e750 TBq (Section 42.2.3). A significant part of the researches of the effect of radiation on solids is devoted to the radiolysis of polymers and polymer composites [3].
16.6 THE YIELD OF THE RADIOLYSIS PRODUCTS To quantify the yield of radiolysis products in radiochemistry and radiobiology, the quantity G is used, the number of products per 100 eV (or per joule) of absorbed energy. Physicists, as a rule, are interested in the yield of ions and excited molecules. To estimate these values, the parameter wi is used, which is the average energy expended for the formation of a pair of ions, or the parameter wph, which is the average energy expended on the formation of a photon. There is a simple relationship between G and w G ¼ 100=w.
(16.39)
The parameter G was proposed by M. Burton in 1952 at the Leeds conference. At present, it is possible to find the yield measurement in SI unitsdmole/J [7]. A connection between different units is Gð1=100eVÞ ¼ 0; 104Gðmmol=JÞ.
(16.40)
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TABLE 16.1 The Initial Yield of Radiolysis Products in Neutral Water for Gamma Radiation [7] Product
eaq L
O
PO
P
O2
H2O2
H3OD
OH-
HO2
G, 100/eV
3.0
0.6
2.8e2.9
0.0067
0.45
0.75
3.3e3.4
0.5e0.6
0.002
The energy yields of reactions excited by ionizing radiation are generally lower than the energy yields of the same reactions excited by light. The point is that with a photochemical reaction, almost all of the radiation energy is used to excite a molecule to such a state that is capable to react. The action of ionizing radiation is not selective; it leads to ionization and excitation to various levels, and some of them cannot participate in the reactions. Thus, the formation of ozone in the gas phase occurs under the action of ultraviolet radiation with a wavelength of the order of 190 nm, which corresponds to a quantum energy of 6.5 eV. Under the action of such quanta, the O2 molecule is excited to the level 3Su. The quantum yield of ozone is about 2; the value of the energy yield is about 30 molecules per 100 eV. The yield of photochemical reactions of ozone formation is about 10 times higher than the radiation yield of ozone in the gas phase and two times higher than the radiation yield when liquid oxygen is irradiated. The energy yields in most radiative reactions are approximately one to five molecules per 100 eV of absorbed energy, but reactions with yields of 0.1 and 15 molecules per 100 eV also occur. In the summaries of the yields of radiationchemical reactions, reactions with yields of 104e105 molecules per 100 eV can be found. Such large yields are a sign of a chain process. For example, the reaction of chlorination of benzene under the action of gamma radiation at 25 C has a yield of 2.3 105. Yields of radiolysis reactions of water depend on pH, water temperature, LET of radiation, and time. The yield of radiolysis products after 1 ps after a particle passing is called the initial yield, and after 1 ms, the output yield. With the continuous action of ionizing radiation, due to the reverse reactions of water synthesis from radiolysis products, a steady state of dynamic equilibrium is established. Therefore, even in nuclear reactors, water does not undergo complete decomposition. With continuous exposure to radiation, we can talk of stationary concentrations of radiolysis products. However, to assess the role of radiolysis of water in living cells the initial yield is important. The initial yield in neutral water for gamma radiation is presented in Table 16.1. As the LET grows from 3 keV/mm to w300 keV/mm, the yield of hydrated electrons, hydrogen atoms, and PO radicals decreases, and the yield of molecular hydrogen, hydrogen peroxide, and HO2 grows. Thus, the yield of hydrated electrons within the indicated limits of the LET change drops almost in 10 times, and the yield of HO2 increases by a factor of w100.
REFERENCES [1] I. Obodovskiy, Complex of Problems on the Experimental Methods of Nuclear Physics, Moscow Energoatomizdat, 1987, p. 79, problem 1.22 e in Russian. [2] A. Mozumder, Charge particle tracks and their structure, Adv. Rad. Chem. 1 (1969) 1e102. [3] R.J. Woods, A.K. Pikaev, Applied Radiation Chemistry: Radiation Processing, JohnWiley&Sons, 1994, 535 p. [4] E.J. Hart, M. Anbar, The Hydrated Electron, Wiley-Interscience, New York, 1970. [5] M.D. Galanin, About the reasons of the dependence of luminescence yield of organic chemicals on the ionization particle energy, OpticaiSpectroscopia 4 (6) (1958) 759e762 (in Russian). [6] A.K. Pikaev, S.F. Kabakchi, Reactive Ability of the Primary Products of Water Radiolysis, Energoizdat, Guidebook, 1982, p. 201 (in Russian). [7] S. LeCaër, Water radiolysis: influence of oxide surface on H2 production under ionizing radiation, Rev. Water 3 (2011) 235e253.
Chemical Processes Under the Influence of Radiation Chapter Outline 16.1 Radiochemistry and Radiation Chemistry 16.2 Stages of Radiation Processes 16.2.1 Physical Stage 16.2.2 Physicochemical Stage 16.2.3 Chemical Stage 16.3 Radiolysis 16.3.1 Radiolysis of Gases 16.3.2 Radiolysis of Liquids
247 247 248 248 250 251 251 252
16.4 Radiolysis of Water 16.4.1 Properties of Water and Its Role 16.4.2 Physical Stage of the Water Radiolysis 16.4.3 Reactions of the Water Radiolysis of Physicochemical Stage 16.4.4 Chemical Stage of the Water Radiolysis 16.5 Radiolysis of Solids 16.6 The Yield of the Radiolysis Products References
252 252 252 253 253 254 254 255
16.1 RADIOCHEMISTRY AND RADIATION CHEMISTRY Let us indicate the difference between the two branches of science: radiochemistry and radiation chemistry. “Radiochemistry” is the chemistry of radioactive materials; it involves study of chemical transformations of radioactive substances, dealing with actinides and transuranium elements, development of physicochemical principles of handling radioactive waste from nuclear power engineering, solving radioecology problems, developing methods for manufacturing sources of radioactive emissions, and separation of radioactive isotopes. “Radiation chemistry” deals with study of chemical transformations under the action of ionizing radiation, study of radiation-chemical processes, development of methods for predicting the radiation resistance of various materials, and development of methods for their protection against destruction. Radiation chemistry is also responsible for development of radiation-chemical methods for the synthesis of organic compounds and production of modified materials for medicine, biotechnology, and membrane technology; development of new radiation-resistant materials, radiation modification of polymers; development of radiation-induced nanostructures; production of nanoporous track filters; and many other processes occurring in matter under the action of ionizing radiation. This chapter offers the basics of radiation chemistry. Some applied problems are discussed in some chapters below, which are: Chapters 24, 27, 29 and 30.
16.2 STAGES OF RADIATION PROCESSES The sequence of the processes of ionizing radiation effect on a substance is usually divided into three stages: l l l
physical physicochemical chemical
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248 PART | I Fundamentals
16.2.1 Physical Stage The effect of ionizing radiation on a substance begins with two fundamental processesdthe ionization and excitation of molecules. These processes are described in detail in Chapter 4. The whole variety of phenomena that constitute the subject of such sciences as radiation physics, radiation chemistry, radiation biology, radiation material science, and others, starts from these two processes. The interaction of charged particles with matter, i.e., directly ionizing radiation, is described in detail in Chapter 5. Neutral radiation, i.e., gamma quanta and neutrons interacting with matter, transmits energy to charged particles, and those, in their turn, produce ionization and excitation. Therefore, they are called indirectly ionizing radiation. The features of gamma quanta interaction with matter are described in Chapter 6, and those of neutrons in Chapter 7. One more process has to be also pointed out and that is elastic collisions of particles with the atoms of matter. Usually in the balance of energy losses of charged particles, elastic collisions play just a minor role, which increases only at low particle energies and are only significant for heavy particles. The elastic interactions of incident particles in gases and liquids practically do not change anything in the properties of matter, but under certain conditions they can rather significantly affect a structure of solids. This question is considered in more detail in Chapters 15, 25 and 29. In this chapter we confine ourselves to the two indicated processes, i.e., ionization and excitation. In the process of ionization, electrons separated from atoms can receive energy, which is higher than the ionization energy. The resulting electrons with excess kinetic energy are called delta electrons. For details on delta electrons, see Section 5.1. It is essential to point out for the analysis carried out in this book, that delta electrons can carry the energy of a particle away from its path and produce ionization and excitation at some distance. The physical stage continues as long as the particle moves through the substance to a stop. The typical deceleration time of particles with energies of the order of MeV in matter with a density of water or inbiological tissue is several picoseconds (wx∙1012 s) [1]. Many processes that are traditionally attributed to the physical stage are finished much faster, in about 1015 s. This means that the wave of the physical stage processes runs through the substance with the velocity of the particle. And when at the end of the track the physical stage is still developing, whereas at the beginning of the track the reactions of the following stage already started. By the end of the physical stage, the substance contains molecular ions Mþ, free subthreshold electrons (nonexcitation electrons) e, excited molecules M*, and also so-called superexcited molecules M**, which have an energy exceeding the ionization potential of molecules. It is essential that all the excitations formed by radiation are distributed unevenly in space; they are concentrated within the particle tracks (Section 8.2). Both the statistical nature of the energy distribution in the track and the associated fluctuations influence the effect of radiation on matter.
16.2.2 Physicochemical Stage The next step in converting the energy of ionizing radiation in matter is called the physicochemical. Some physical processes continue, and some reactions begin to occur between the products of radiolysis. This stage takes about 1015 to 1012 s. Electrons formed as a result of ionization have some excess kinetic energy. Such electrons are called hot. Moving in matter, a hot electron loses energy in acts of interaction with molecules of the medium until its energy becomes of the order of thermal energy. This process is called thermalization. The thermalization process is considered in detail in Section 8.4. The average thermalization path in water is several nm [2]. In any substance, electrons polarize surrounding atoms or molecules and, therefore, appear in the field of polarization. When moving in matter (by diffusion or drift), electrons move along with this field, which significantly affects the dynamics of their movement. In radiation chemistry, the process of producing a potential well by an electron as a result of the polarization of surrounding molecules is called solvation, and the electron is called solvated. If the medium in which an electron is located is water, then the solvation of an electron is related to the orientation of the polar molecules of water and then this process is called hydration, and the electron is called hydrated. Sometimes an electron, which is off the field of polarization, is called “dry,” and a hydrated electron is called “wet.” The hydrated electron is denoted by the subscript eaq, and the hydration process is presented by the reaction [3,4] e/eaq .
(16.1) 12
s in water. Mobility m and diffusion The process of hydration of electrons formed by radiation takes about 10 coefficient D of hydrated electrons in water are much lower than in nonpolar liquids: m(eaq) ¼ 2 103 cm2/s V,
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D(eaq) ¼ 4.9 105 cm2/s; for comparison m(c-hexane) ¼ 0.24 cm2/s V, D(c-hexane) ¼ 5.5 103 cm2/s; m(liquid Ar) ¼ 480 cm2/s V, D(liquid Ar) w 70 cm2/s. The hydrated electron is characterized by a wide intense absorption band in the red region of the spectrum and by a narrow single line (singlet) in the EPR spectrum. The maximum optical absorption band of a hydrated electron in water at T ¼ 298 K corresponds to 720 nm (Emax ¼ 1.73 eV) [4]. An important role in radiation chemistry is played by so-called free radicals. Free radicals are relatively stable molecules (or atoms) containing one or two unpaired electrons on the outer electron shell. If a radical has a charge, it is called a radical ion. There are various ways to designate a radical. We mark the radical with a bold dot to the right of the chemical formula. Radicals are formed during the dissociation of excited molecules, e.g., water: H2 O /H% þ OH%;
(16.2)
in the dissociative capture of an electron, for example, by a carbon tetrachloride molecule: CCl4 þ e/CCl3 % þ Cl ;
(16.3)
in some radiation chemical reactions, for example, at the water radiolysis: H2 O/Hþ % þ OH .
(16.4)
A radical with an unpaired electron is a highly reactive particle. If radical is not charged, then its interaction with other molecules is determined by the same potential as for any neutral molecule, and the probability of encounter or, in other words, of the collision cross section for the radical and for the neutral molecule is identical. But when a radical is encountered with a molecule, it reacts immediately. The activation energies of such reactions are close to zero. In the case of the interaction of ordinary molecules, i.e., nonradicals, for a reaction to occur, thousands of collisions may be required. Actually, such a delay in the reaction can be associated with the presence of an energy barrier or in the search of the ways to give up the energy released in the reaction. The radical reacts at once. The next process that must be considered in the physicochemical stage is the capture of the electron. As a result of the capture of electrons by electrophilic molecules, negative ions appear in the substance e þ X/X .
(16.5)
In this reaction, X is an electrophilic molecule, it can be a molecule of the basic substance X ¼ M, and maybe an impurity molecule. The process of electron capture is rather complicated, in which the binding energy of an electron in a negative ion must be released. The capture of electrons is discussed in detail in Chapter 9. Electrons and positive ions, as well as encountered in matter ions of opposite signs, can recombine e þ M þ /M;
(16.6)
X þ M þ /M þ X.
(16.7)
Recombination can generate excited molecules, if the energy released during the recombination process is sufficient to excite the molecule, for example, e þ M þ /M .
(16.8)
The problems of recombination of charges in matter are discussed in detail in Chapter 11. Not only charges, but also radicals recombine CH3 % þ C2 H5 %/C3 H8 .
(16.9)
In many complex organic substances, the excitation of molecules leads to nonradiative transitions with high probability and also to their dissociation. In radiation physics this phenomenon is called fragmentation. Some of the charged particle energy turns into heat, and this leads to an instant increase in temperature in the track region. Generally, the increase in temperature can be very significant. So, the energy released on the tracks of some delta electrons is enough to form microbubbles in the liquid. This is on what a bubble chamber operation is based, i.e., the track device, which has played a very important role in the history of nuclear physics. In a superheated liquid, such seed bubbles grow to visible dimensions and allow to record the traces of charged particles. Bubble chambers operate on liquid propane, freon, hydrogen, and some other liquids.
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In the 1950s, to explain the decrease in the luminescence yield of organic substances when excited by charged particles with a large Linear energy transfer (LET), the Russian physicist M.D. Galanin suggested the idea of temperature quenching due to an increase in the excitation channel temperature [5]. Calculations carried out by M.D. Galanin showed that in a channel with a radius of 3∙103 mm, an alpha particle with an energy of 5.3 MeV in anthracene raises the temperature by not less than 100 K. According to A. Mozumder’s calculations [2], particles with small LET (fast electrons) raise the temperature on the track axis in water by about 30 K, alpha particles by about 400 K, and fission fragments by 104 K. The time during which the temperature on the track axis decreases due to the thermal conductivity by half, is equal to 6 1012 s for particles with a small LET, and 1011 s for an alpha-particle track. At high irradiation doses, an additional increase in the temperature of the substance on irradiation is also possible, if the reactions initiated by the radiation are exothermic. The final temperature depends on the absorbed dose and on the heat capacity of the substance. It is estimated that, with an absorbed dose of 10 kGy, an increase in temperature can be w5 C in water and w10 C in polyvinyl chloride. At a dose of 30 kGy, the increase is w15 and w30 S, respectively. One of the ways to remove excitation in the processes of radiation chemistry may be the emission of photons. Questions of luminescence are considered in detail in Chapter 12. Because the energy of ionizing radiation is emitted in the process of luminescence, this process competes with processes, which lead to defect formation, dissociation of molecules, and other processes of nonradiative energy transfer.
16.2.3 Chemical Stage As most of the chemical reactions discussed here occur in mixtures and solutions, the probability of interaction, i.e., the probability of reaction and, hence, the probability of recombination are determined by the mutual diffusion of the reacting partners. They must first meet to react. The rate constant of such encounters k can be calculated from the Smoluchowski equation k ¼ 4pDAB ðrA þ rB Þ;
(16.10)
where DAB ¼ DA þ DB is the coefficient of relative diffusion, and rA and rB are the radii of components A and B. If one of the interaction components is an electron, its “dimensions” are determined by the wavelength of the de Broglie wave. In the case, when the reaction components are charged, we can assume that the radii of the components are equal to the radius of the capture sphere rc ¼ e2 4pεε0 kT; (16.11) and the diffusion coefficient D is related to the charge mobility m by the Einstein relation D ¼ mkT=e;
(16.12)
then the Smoluchowski formula (16.12) transfers to the LangevineDebye formula k ¼
4peðmþ þ m Þ . 4pεε0
(16.13)
The reacting partners after encounter form a diffusion pair and enter the solvent molecules environment. In radiation chemistry, this environment is often called a “cage.” In this cage, they undergo multiple collisions before the reaction really happens. If the reaction occurs at the first meeting, as, e.g., when radicals are recombined, then this reaction is called diffusion controlled. In the chemical stage, the products of the action of radiation interact with each other and with molecules, not affected by the radiation action. One of the variants of the reaction between molecules or of the recombination process of radicals is called a reaction of disproportionation. In such a reaction, a redistribution of atoms or fragments of molecules occurs CH3 % þ C2 H5 %/C2 H4 þ CH4 .
(16.14)
Another option for a chemical reaction with products of radiation exposure is substitution reactions, when certain functional groups, that make up a chemical compound change to the other groups C2 H5 % þ RH/R% þ C2 H6 .
(16.15)
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16.3 RADIOLYSIS 16.3.1 Radiolysis of Gases In gases at pressures close to atmospheric, the structure of the track differs noticeably from tracks in condensed matter. In the process of thermalization, an electron leaves the parent ion for considerable distance. The uniform irradiation is manifested at significantly lower doses than in condensed bodies. LET significantly affects the radiation-chemical yields. Because of the low density of the medium, the probability of deactivation of intermediate products of radiolysis in collisions with surrounding molecules is small. Therefore, at low pressure the intermediate particles in gases have longer lifetimes than in liquids. As an example, we indicate the main reactions of the radiolysis of carbon dioxide CO2 and the reactions leading to the formation of ozone O3 in atmospheric air. In CO2, ionization by charged particles can lead to the appearance of four types of ions [6] CO2 þ hn/COþ 2 þ e; CO2 þ hn/COþ þ O þ e; CO2 þ hn/Oþ þ CO þ e;
(16.16)
CO2 þ hn/Cþ þ 2O þ e. Excited molecules of CO2 quickly dissociate CO2 þ hn/CO2 /CO þ O.
(16.17)
The formed products of primary interaction enter the radiation-chemical reactions among themselves and with unexcited CO2 molecules. As a result of these reactions, molecules of CO, atomic and molecular oxygen appear. The carbon formed in the reactions is rapidly oxidized, so the final radiative yield of carbon is very small. The process of radiolysis of carbon dioxide is significantly affected by water impurities. Ozone in the atmosphere is formed mainly by the hard ultraviolet radiation of the sun; this process occurs in the stratosphere. Because of thunderstorms, fires, and human industrial activity, ozone as well is formed in the troposphere. The formation of ozone follows a reaction, which, in general, can be written as 3O2 /2O3 .
(16.18)
The reaction of ozone formation is endothermic. The reaction (16.18) requires 2.96 eV (285 kJ mol)dthis energy turns to the formation of two molecules of ozone (1.48 eVd142 kJ/mol for one). Let us, at least partially, show the separate stages of the reaction (16.18): O2 þ hn/Oþ 2 þ e;
(16.19a)
O2 þ hn/O2 ;
(16.19b)
O2 þ e/O 2;
(16.19c)
O2 /O þ O;
(16.19d)
O2 þ O/O3 ;
(16.19e)
þ O 2 þ O2 /O3 þ O;
(16.19f)
þ O 2 þ O2 /O2 þ O2 ;
(16.19g)
þ O 2 þ O2 /O þ O þ O2 .
(16.19h)
and so on. The oxygen atoms released in the last reactions are connected with oxygen molecules (16.19e), resulting in the formation of ozone molecules. Let us note that the reaction (16.19c) is presented by provisional view. In the gas phase most likely occurs a three-particle capture (Chapter 9).
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The ozone molecule is relatively stable, but colliding with particles of aerosols or impurities in the liquid (in water), it decomposes easily with the release of atomic oxygen: P3 /O2 þ O.
(16.20)
Calculations show that the formation of ozone due to natural radioactivity in the near-surface layer of the Earth can reach w 1.8 105 tons/year. Substantially large amounts of ozone are generated in the atmosphere during testing of atomic and hydrogen bombs in the air, the operation of nuclear reactors and other nuclear cycle facilities, and the storage of nuclear waste. Nuclear activity of mankind supplies to the atmosphere about 109 tons of ozone per year.
16.3.2 Radiolysis of Liquids The main feature of the radiolysis of organic liquids is a large variety of reactions leading to the formation of a large number of intermediate radiolysis products. However, the results of research obtained so far show that several transformation channels are crucial, which determine the accumulation of the main products of radiolysis. The identification of these channels and the study of the regularities of the reactions proceeding make it possible to obtain the necessary information for predicting the accumulation of radiolysis products. To obtain detailed information on the processes of radiation-chemical transformations in organic compounds, we refer the reader to the special literature [3,6,7]. Here we note that, as a rule, the bonds between carbon atoms are broken with less probability than the CeH bond, and therefore one of the most important products of radiolysis is molecular hydrogen. Aromatic hydrocarbons have a higher radiation resistance in comparison with alkanes and alkenes. This is due to the fact that the excitation in some particular fragment of an aromatic molecule, due to the presence of a conjugated system p-bonds, is delocalized along the aromatic ring. As a result, rupture of the ring becomes unlikely. In these compounds the processes of excitation, transfer of charge or excitation energy, and other processes of energy dissipation, which do not lead to dissociation of molecules, predominate.
16.4 RADIOLYSIS OF WATER Let us consider the radiolysis of water as an example of radiation-chemical transformations of matter.
16.4.1 Properties of Water and Its Role Water is a special substance on the planet Earth and, probably, in the whole universe. In search of life on other planets, scientists focus on the temperature conditions of each planet. It is assumed that life is possible only in the temperature conditions, where liquid water can exist. And if there are no visible traces of water, as, e.g., on Mars, then traces of the existence of liquid water there in the past are sought. Water is of key importance in the creation and maintenance of life on Earth, for the formation of climate and weather. It is the most important substance for all living beings on the planet Earth. A significant part of a living cell is the cytoplasm, consisting mainly of water. In the intercellular space, lymphatic water also plays a significant role. Water is necessary for the life of all, without exception, unicellular and multicellular living beings on Earth. Water is the most common solvent on our planet, which largely determines the nature of chemical processes. Most of the chemistry, when it became a science, began precisely as the chemistry of aqueous solutions of substances. Water can be used as a moderator and coolant in nuclear reactors. Water is a model object of radiation physics and chemistry; just as a hydrogen atom is a model object of atomic physics, an alkaliehalide crystal is a model object in solid-state physics, etc. So, it is obvious that for mankind, it is extremely important to realize what happens to water when irradiated with ionizing radiation.
16.4.2 Physical Stage of the Water Radiolysis As a result of ionization and excitation of water molecules, molecular ions H2Oþ, free subthreshold electrons e, excited H2O*, and superexcited molecules H2O** are formed. The minimum ionization energy of water is 12.6 eV, and the minimum excitation energy is 7.4 eV. Let us present the list of radiolysis products by the end of the physical stage: H2 Oþ ; e; H2 O ; H2 O .
(16.21)
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It is evident that the system at the end of the physical stage is energetically in nonequilibrium. In the subsequent stages occur some transformations with these radiolysis products, and their number changes.
16.4.3 Reactions of the Water Radiolysis of Physicochemical Stage With the products of radiolysis formed at the end of the physical stage, the following transformations occur: 1. Dissociation of excited and superexcited molecules H2 O /H% þ OH%;
(16.22)
H2 O /H2 þ O%;
(16.23)
H2 O /H2 Oþ þ e;
(16.24)
2. Recombination of a “dry” electron with a molecular ion with the formation of an excited molecule and subsequent dissociation processes H2 Oþ þ e/H2 O /HO% þ H;
(16.25)
3. Thermalization and hydration of electrons e/eaq ;
(16.26)
4. Ionemolecule reaction with the formation of hydroxonium ion H2 Oþ þ H2 O/H3 Oþ þ OH%;
(16.27)
H3 Oþ /ðH3 Oþ Þaq.
(16.28)
5. Hydration of hydroxonium ion
Thus, at the end of the second stage, the assortment of particles in the case of radiolysis of pure water has the following form: hydrated electrons eaq, O, PO and P radicals, hydroxonium ions (H3Oþ)aq, and molecular hydrogen H2: eaq ; H%; OH%; ðH3 Oþ Þaq; H2 .
(16.29)
These particles are inhomogeneously distributed in space; they are concentrated in microregions along the particle track. In the case of gamma irradiation, these microregions are mainly the “spurs.” The distribution of particles in the “spur” is also uneven: the radicals O, H, OH, hydroxonium ions (H3Oþ)aq, and hydrogen H2 molecules are located predominantly in the center of the “spur”, and the hydrated electrons are located in the spherical layer at a distance of about 4 nm from the center [6].
16.4.4 Chemical Stage of the Water Radiolysis At the chemical stage, chemical reactions occur in the products, formed at the previous stage, with each other, which leads to the formation of atoms and molecules of hydrogen, hydrogen peroxide, and hydroxyl ions. These reactions occur, mainly, in the ionization cellsdin spurs, blobs, and short tracks. Simultaneously, diffusion of these and previously formed particles from the spurs to the solution volume occurs, which results in the blurring of the spurs and the leveling of the radiolysis products by volume, i.e., the establishment of a homogeneous distribution of the products. Radiolysis products can interact with each other and with the molecules of the solution. The time for establishing a homogeneous distribution depends on the irradiation intensity; at high intensity it can be of the order of 107 s [6]. The main reactions of the chemical stage are: eaq þ H% þ H2 O/H2 þ OH ;
(16.30)
H% þ H%/H2 ;
(16.31)
eaq þ OH%/OH ;
(16.32)
eaq þ H3 Oþ /H% þ H2 O;
(16.33)
254 PART | I Fundamentals
OH% þ OH%/H2 O2 ;
(16.34)
OH% þ H%/H2 O;
(16.35)
H3 Oþ þ OH /2H2 O.
(16.36)
New molecules, not yet produced in the previous stages, are hydroxyl OH and hydrogen peroxide H2O2. Later, under certain conditions, the OP2 radicals and O2 oxygen molecules can be formed. With a high excitation density during the chemical stage, reverse reactions play an important role when one of the radiolysis products collapses and water molecules reappear [6]. H þ OH/H2 O
(16.37)
H2 þ OH/H þ H2 O.
(16.38)
or
If a sufficient concentration of radicals is created in the volume, then one of the main reactions of radicals is their recombination. In particular, in the radiolysis of water, the ion radicals Oþ and POe are formed. As a result of the recombination of these radicals, a series of highly reactive compounds are created, such as superoxide HO2 and peroxide H2O2. In the case of the biological tissues irradiation, such compounds can interact with organic cell molecules to cause significant damage. We also note that among the products of radiolysis, some are short-lived (hydrated electrons, hydrogen atoms, hydroxyl radicals, etc.), whereas others are stable (hydrogen, oxygen, and hydrogen peroxide).
16.5 RADIOLYSIS OF SOLIDS Radiation-chemical transformations in solids have, as a rule, much smaller yields than in liquids or gases. This is due to the fact that rigid bonds between particles, characteristic of a solid, greatly impede the motion of atoms and molecules in a solid and, therefore, inhibit the chemical reactions that result from the collision of these particles. The formation of crystal defects by radiation is described in Chapters 15 and 25. Chapter 29 describes the staining of glasses and crystals by ionizing radiation, which is associated with the capture of electrons by certain color centers. In complex substances, deep chemical transformations are also possible. The changes in the structure of graphite and the related thermal phenomena are especially important because graphite acts as a moderator in uraniumegraphite nuclear reactors and is exposed to very significant fluxes of neutrons and gamma quanta. The well-known physicist E. Wigner had discovered that graphite, when bombarded by neutrons suffers dislocations in its crystalline structure, causing a buildup of potential energy. This energy, if allowed to accumulate, could escape spontaneously in a powerful rush of heat. Underestimation of changes in stored energy during irradiation of graphite resulted in several accidents of nuclear reactors. The most famous is the nuclear reactor accident for the production of weapons-grade plutonium in Windscale, England, in 1957. The accident led to the release of radioactive substances totaling 550e750 TBq (Section 42.2.3). A significant part of the researches of the effect of radiation on solids is devoted to the radiolysis of polymers and polymer composites [3].
16.6 THE YIELD OF THE RADIOLYSIS PRODUCTS To quantify the yield of radiolysis products in radiochemistry and radiobiology, the quantity G is used, the number of products per 100 eV (or per joule) of absorbed energy. Physicists, as a rule, are interested in the yield of ions and excited molecules. To estimate these values, the parameter wi is used, which is the average energy expended for the formation of a pair of ions, or the parameter wph, which is the average energy expended on the formation of a photon. There is a simple relationship between G and w G ¼ 100=w.
(16.39)
The parameter G was proposed by M. Burton in 1952 at the Leeds conference. At present, it is possible to find the yield measurement in SI unitsdmole/J [7]. A connection between different units is Gð1=100eVÞ ¼ 0; 104Gðmmol=JÞ.
(16.40)
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255
TABLE 16.1 The Initial Yield of Radiolysis Products in Neutral Water for Gamma Radiation [7] Product
eaq L
O
PO
P
O2
H2O2
H3OD
OH-
HO2
G, 100/eV
3.0
0.6
2.8e2.9
0.0067
0.45
0.75
3.3e3.4
0.5e0.6
0.002
The energy yields of reactions excited by ionizing radiation are generally lower than the energy yields of the same reactions excited by light. The point is that with a photochemical reaction, almost all of the radiation energy is used to excite a molecule to such a state that is capable to react. The action of ionizing radiation is not selective; it leads to ionization and excitation to various levels, and some of them cannot participate in the reactions. Thus, the formation of ozone in the gas phase occurs under the action of ultraviolet radiation with a wavelength of the order of 190 nm, which corresponds to a quantum energy of 6.5 eV. Under the action of such quanta, the O2 molecule is excited to the level 3Su. The quantum yield of ozone is about 2; the value of the energy yield is about 30 molecules per 100 eV. The yield of photochemical reactions of ozone formation is about 10 times higher than the radiation yield of ozone in the gas phase and two times higher than the radiation yield when liquid oxygen is irradiated. The energy yields in most radiative reactions are approximately one to five molecules per 100 eV of absorbed energy, but reactions with yields of 0.1 and 15 molecules per 100 eV also occur. In the summaries of the yields of radiationchemical reactions, reactions with yields of 104e105 molecules per 100 eV can be found. Such large yields are a sign of a chain process. For example, the reaction of chlorination of benzene under the action of gamma radiation at 25 C has a yield of 2.3 105. Yields of radiolysis reactions of water depend on pH, water temperature, LET of radiation, and time. The yield of radiolysis products after 1 ps after a particle passing is called the initial yield, and after 1 ms, the output yield. With the continuous action of ionizing radiation, due to the reverse reactions of water synthesis from radiolysis products, a steady state of dynamic equilibrium is established. Therefore, even in nuclear reactors, water does not undergo complete decomposition. With continuous exposure to radiation, we can talk of stationary concentrations of radiolysis products. However, to assess the role of radiolysis of water in living cells the initial yield is important. The initial yield in neutral water for gamma radiation is presented in Table 16.1. As the LET grows from 3 keV/mm to w300 keV/mm, the yield of hydrated electrons, hydrogen atoms, and PO radicals decreases, and the yield of molecular hydrogen, hydrogen peroxide, and HO2 grows. Thus, the yield of hydrated electrons within the indicated limits of the LET change drops almost in 10 times, and the yield of HO2 increases by a factor of w100.
REFERENCES [1] I. Obodovskiy, Complex of Problems on the Experimental Methods of Nuclear Physics, Moscow Energoatomizdat, 1987, p. 79, problem 1.22 e in Russian. [2] A. Mozumder, Charge particle tracks and their structure, Adv. Rad. Chem. 1 (1969) 1e102. [3] R.J. Woods, A.K. Pikaev, Applied Radiation Chemistry: Radiation Processing, JohnWiley&Sons, 1994, 535 p. [4] E.J. Hart, M. Anbar, The Hydrated Electron, Wiley-Interscience, New York, 1970. [5] M.D. Galanin, About the reasons of the dependence of luminescence yield of organic chemicals on the ionization particle energy, OpticaiSpectroscopia 4 (6) (1958) 759e762 (in Russian). [6] A.K. Pikaev, S.F. Kabakchi, Reactive Ability of the Primary Products of Water Radiolysis, Energoizdat, Guidebook, 1982, p. 201 (in Russian). [7] S. LeCaër, Water radiolysis: influence of oxide surface on H2 production under ionizing radiation, Rev. Water 3 (2011) 235e253.