RADIATION CHEMISTRY

GENERAL INTRODUCTION

BY F. S. DAINTON Chemistry Department, The University, Leeds 2

The absorption by matter of electro-magnetic radiation in the wavelength range 2000-7OOOA is generally a simple process. The destruction of the energy quantum occurs in a single act involving the quantum and the absorbing molecule only, and is governed by well-recognized laws. The absorption is selective, the primary products of the absorption process can often be identified unambiguously, and are found to be of similar reactivity, and their rate of formation and spatial distribution can usually be specified with some exactness and certainty.

In these respects the primary photochemical act differs completely from the primary act in chemical reactions which are induced by the absorption of high energy quanta (say radiation A < 50 A) or by the slowing down of rapidly moving charged and uncharged particles of atomic and subatomic nature. The mechanism of energy transfer from the radiation or the particles is complex, selective and im- perfectly understood; it is not possible to make anything more than very approxi- mate and qualitative predictions as to the number, nature and initial and final distribution of the entities formed in the primary process. Moreover, in the most important reaction medium, namely water, reactions initiated by one of the par- ticles of the primary act may be reversed by one of the others. Despite the fact that at the turn of the century the development of radiation chemistry was com- parable with that of photochemistry, the present status of the former subject is similar to that of photochemistry 30 years ago. At the present time the most useful conclusions as to the primary act are still obtained by inference from the nature of the ultimate products. The last few years have seen a considerable strengthening of this nexus due primarily to a greater understanding of the chem- istry of free radicals and unstable ions, and it now seems that species of this kind must be intermediary between reactants and products.

The main purpose of this Discussion, which is the first to be held on this subject by the Faraday Society, is therefore to appraise the present position, to attempt what synthesis is possible of the views of the physicists, chemists and biologists who, for varying reasons, have contributed ideas and methods to the subject, and to suggest future lines of development. In the first three papers, the authors summarize some of the current ideas concerning the physical processes involved in the formation of the primary products. The lacunae in our knowledge of the mechanism of energy loss by fast charged particles are emphasized by Professor Spiers. We are still ignorant of the Wvalues (i.e. energy required for creation of one ion pair) for liquids, and of the relation of the ionization potentials of the isolated molecules, which gain energy by inelastic collisions with the impingen t particle, to this quantity W and the empirical quantity, the mean excitation poten- tial l? which is employed in the Bethe theory in its origrnal and modified forms. Nevertheless we do know in qualitative terms how the mean ion density of com- monly-used radiations varies with the energy, mass and atomic number of the fast particle. The conclusions reached here are still the foundations on which are erected all hypotheses concerning the dependence of the reaction product in both

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INDEX

TO

PH

OT

OG

RA

PH

TA

KE

N A

T G

EN

ER

AL

DIS

CU

SSIO

N

ON

A

T L

EE

DS

UN

IVER

SITY

. 8T

H-f

OT

H

APR

IL.

1952

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AD

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ION

C

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n. P

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sT .

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mph

lett.

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C . B

. .

Axf

ord.

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Bac

q. P

rof . 2

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. B

arb.

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B

arna

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Dr . D

. B

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dale

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Bas

kett.

Dr . A

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. B

ates

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. B

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10 G E N E R A L I N T R O D U C T I O N

quality and quantity on the particle mass and energy. Thereare plenty of examples of this providedin the papers of Allen, Hardwick, Bonet-Maury, Haissinsky and others.

We still derive most of our knowledge concerning the properties of positive and negative ions (i) from experiments in gaseous systems (discharges, swarm experiments and mass spectrometry), and (ii) for the simplest ions from wave mechanics. Although several authors utter warnings concerning the " carry- over " of conclusions derived from such attcnuated systems into the liquid phase, there seem to me to be many such justifiable extrapolations. As two examples, I will cite the variation of the cross-section for the formation of a particular positive ion with the energy of the incident electron and the fact that dissociative electron capture processes such as AB + e -+A $- B- are resonance phenomena. This latter conclusion is of value because by photochemical means we can produce electrons of low energy in liquid systems adjacent to the molecules by which they can be captured, and thereby gain useful experimental information concerning the ultimate fate of the negative ion. By representing the process in terms of potential energy curves, we can predict relations in conformity with this notion which are susceptible to experimental verification, and also lead to interesting conclusions concerning certain thermal reactions which can take place. An example in point is the reduction of polyvalent cations by the hydroxyl radical in aqueous solution.

Most radiation chemical experiments are carried out in polar media, and it is here that the cautious approach is most required, because the solvation energies and entropies are of such considerable magnitude and vary so greatly from ion to ion that processes considered to be of low probability in the gas phase may be of very high probability in polar media, and vice versa. Another warning is necessary here. The magnitudes of changes in thermodynamic functions accompanying solvation are derived from eqiiilibriurn measurements, whereas there is reason to believe that many charge transfer processes are so rapid that the reorientation of solvent dipoles which occurs is a slower process. This may make prediction of reaction probabilities, based on the consideration of the energetics of initial and final states, of little value. It is probably preferable at the present time to proceed empirically, studying various series of reactions so chosen as to reveal relationships between rates and some parameter such as a varying ionization potential or dis- sociation energy.

Whilst it is not for me to predetermine the course of the Discussion, I would like to draw attention to some questions which seem to me to transcend others in importance.

1 . DOSIMETRY AND ACTINOMETRY .-Before effective comparison between the results of workers in different laboratories can be made, we must have an agree- ment about methods of assessing energy absorption. If this is not easily possible, the case for a substandard by radiation actinometry is very strong. 1 feel sure I am right in expressing the gratitude of the meeting to Dr. Miller and Mr. Wilkinson for their efforts in this direction, and I hope it will be agreed that with X- and y-rays and using the dose rate range 1-103 r/min we will agree to use the aerated FeSO4 actinometer. Associated with this problem, but of less importance, is the desirability of expressing the total amount of chemical change per unit of energy absorbed, e.g. as the so-called G-value or radiation yield per 100 eV. Whilst a case can be argued for the retention of the use of ionic yield for gaseous reactions, there seems to be little advantage in this for liquid phase reactions as long as the chosen value of W for the liquid concerned has little more warrant than that it is the author's persona1 preference.

2. THE PRIMARY RADIATION YIELD.-Views have been advanced that a propor- tion of the energy absorbed is used in forming ultimate molecules and another fraction in forming radicals. No one doubts the latter but many will have some reservations about the former and about the validity of assigning immutable values G, and GR respectively to the yields. It has been said that the only authentic value of G, will be that corresponding to " precise agreement between independent

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F. S . D A I N T O N 11

reactions ”. But if solutes are chosen which are modified by one radical in such a way that this modification can be reversed by another, then G R obs will necessarily be a function of the solute chosen. And even if solutes are employed for which one radical does not reverse the effect of the other, the proportion of the total energy used to create radicals as judged by the chemical reactions they initiate may still depend on competition for the radicals of the solute with other reactions leading to radical destruction. The true value of G R may therefore be much larger than found by such methods, and, if this argument is valid, maximum values of G R obs are the more significant. We may then find a high efficiency of energy utilization.

3. TRACK DENSITY EFFECTS.-Earlier in this introduction 1 mentioned that track density considerations underlie most of our thinking in this field. In single-hit processes, differences between x-rays and X-rays have long been ascribed to the gross differences of mean track density. There are now indications that for a given type of radiation, differences in track density might arise from quite small changes of energy with consequential variations of the ionic yield. For example, with X-rays the mean track density appears to be substantially constant in range 30-200 kVp and evidence will be presented that chemical changes in this energy range may differ significantly from those induced by say, 1 MeV electrons where the mean track density is considerably less. If this should be established it is a point of considerable importance to the radiobiologist.

initial distribution of the primary products, the majority of authors still deduce overall rate expressions based on the assumption of a uniform concentration of initial radicals. In some cases this treatment fails to account for the observed kinetic behaviour, but in others it is surprisingly successful. This fact poses some interesting problems. At very high dose rates we may expect complete track overlap and the initial distribution may in fact be almost uniform, but at low dose rates this may rarely be so. Chain reactions which are initiated by the radicals offer a promising field of investigation, the study of which may be expected to provide interesting information concerning track distribution. A simple example may make this point clear. If the mean distance from a radical in one track to a radical in an adjacent track is x, the probability that a chain of life r sec carried by reaction centres possessing a mean diRusion coefficient in the medium of I) will be terminated by mutual interaction with a reaction chain initiated in an adjacent track, will be high if 2Dr > x2. This is the reason, stressed elsewhere, why the kinetics of the radiolysis and photolysis of H202 are identical, and it is justifiable to apply “ homogeneous kinetics ” to the radiolysis, and why chain reactions in gases, e.g. the spin isomerization of hydrogen, can be treated in this way.

Other systems are known which, when initiated photochemically (and therefore uniformly) may cause reaction to proceed at the same overall rate as when initiated radiochemically, and yet show entirely different reaction kinetics in the two sets of circumstances.

and more will be said to-day about this subject. Views of great diversity have been expressed, but in all the theories which have been proposed, it is always conceded that the action of radiation on water is to produce inter alia some species capable of oxidation and other(s) of reduction of certain solutes, and that most of these species would have a transient existence in water even if the solute were not present. The basis for effective theorising on this subject would seem to be know- ledge of the following for each reaction of this kind : (i) a complete material balance under all conditions under which the reaction is carried out ; (ii) a thorough study of yields and equilibria as a function of dose rate and pH; (iii) measurement of the actual redox potential of the system under ail these conditions; (iv) the effect of additives which complex with the oxidized or reduced form of the solute.

6. NON-AQUEOUS SYSTEMS.-such a large proportion of current published work in radiation chemistry is concerned with water or aqueous solutions that it is often

4. UNIFORM OR NON-UNIFORM KINETICS.-DeSpite the known nOn-UnifOrm

5. OXIDATION AND REDUCTION REACTIONS IN WATER.-MUCh has been Written

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12 G E N E R A L I N T R O D U C T I O N

overlooked that in some respects the investigation of non-polar liquids and vapours offers a better opportunity for elucidating the general principles of radiation- chemical changes. This difference was better realized 25 years ago by such pioneers as S. C . Lind and W. Mund. The outstanding advantage appears to be the virtual absence of secondary processes involving electron transfer ; and the papers by Burton and Gordon, and by Prevost-Bernas, Chapiro, Cousin, Landler and Magat are modern examples of the exploitation of this advantage, which one would like to see emulated. In each paper, very interesting general points are raised, such as the astonishing range of variation of radical yields (table 2 of Magat’s paper), and the energy transfer mechanism found in aromatic systems, which require discussion.

7. BIOLOGICAL SYSTEMS.-These are of particular interest in radiation chemistry, because arising from their study have come several new ideas, e.g. protection and sensitization, which may find application in simpler systems. Furthermore, many of us must always have at the back of our minds the very great importance of the effects of radiations on living cells. We cannot help but feel that had Dr. Douglas Lea still lived, Itis contributions to this discussion on all topics-but this one in particular-would have been both helpful and stimulating.

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