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Radiochemistry Volume 2
A Review of the Literature Published Between August 1971 and December 1973
By G. W. A. Newton
The Royal Society of ChemistryCopyright © 1975 The Chemical Society
All rights reserved.
Nuclear Recoil Chemistry in Gases and Liquids
BY D. S. URCH
Nuclear reactions produce ionized atoms with very high kinetic energies. Szilard and Chalmers in 1934 were the first to discover that such atoms have a unique and distinctive chemistry. The purpose of this Report is to consider the progress that has been made in the study of this specific branch of chemistry, especially in recent years. The chemistry of atoms and ions produced by nuclear reactions is important in at least three very different branches of science. The first is in the design of and choice of materials for nuclear reactors, or indeed any other apparatus in which nuclear reactions may take place; the second is in the direct production of molecules labelled with a radioactive isotope for tracer studies in medicine and chemistry; and the third is in the study of reaction kinetics, where the high energy which the recoil atom brings to the collision enables new reactions to take place and allows the role of excess energy in a chemical reaction to be investigated. The first of these topics is concerned with reactions in solids and so lies outside the scope of this Report, but the second and third items will be discussed in detail. In order that the reactions of interest and importance in the production of labelled molecules or in kinetic studies can be isolated it will of course be necessary to study the whole field of nuclear recoil chemistry. First it is necessary to consider in a very general way the factors which influence the chemistry of a particle produced in a nuclear reaction. Nuclear reactions are of two types, spontaneous and induced, and for the purpose of this review the only difference is that in the latter type of reaction the sample being studied is usually bathed in a flux of the particles or photons which are to induce the reaction. A nuclear reaction results in the liberation of energy which is shared between the resultant particles, or particle and photon, with conservation of energy and momentum. The kinetic energy of the particle is its recoil energy. Light particles will have large recoil energies and heavy particles small recoil energies, and in particular, in a reaction which results in a particle and a photon, the photon will carry away the bulk of the energy as a γ-ray (or rays) whilst the particle will have only a very small recoil energy of, say, hundreds of electron volts. Since nuclear reactions involve energies of a million or more electron volts, the recoil energies are usually of the order of many thousands of electron volts. Such energies are clearly in vast excess of the energies normally encountered in chemical reactions. Also the products of nuclear reactions will be heavily ionized. The commonest reaction of a nuclear recoil particle is therefore to cause the ionization and/or disruption of any molecule it encounters. In this way the recoil species loses energy and will, in the fullness of time, acquire enough electrons to be recognized as an atom or a positive or negative ion. The exact charge state as a function of energy will of course vary from atom to atom and also with different chemical environments, and also with the number of collisions it has made. Furthermore, it is probable that at any given energy there will be considerable variation in the charge state and degree of electronic excitation of the recoil atoms. There will inevitably come a time in the brief life of each recoil atom when its recoil energy will be so degraded that only a few, maybe ten or twenty, electron volts remain. It is at these energies that chemical reactions now become possible by virtue of kinetic energy alone. Such reactions are 'hot' reactions and form a special and most interesting branch of the chemistry of species produced in nuclear reactions. Other types of reaction might be initiated by the residual charge on the particle, and/or its degree of electron excitation. These reactions could involve recoil species which had lost all their extra translational energy, or some reactions might depend upon some combination of translational, electronic, and ionic excitations. The field of possibilities is unfortunately wide and, as will be seen in the subsequent sections, many different experimental techniques have been employed in order to try to eliminate particular types of reaction. There are other complicating factors which must be taken into account. The substance in which the recoil atom finally comes to rest will be undergoing a variety of chemical reactions which are broadly lumped together as 'radiation damage'. The primary source of this damage is the passage of the recoil particle itself but in the case of artificially induced reaction the radiation itself may well induce damage in the sample being studied. The radiation damage produced by the recoil particles themselves puts an upper limit to the number of particles which should be produced in an experiment if it is to be considered as a study of recoil particles with a particular pure substance. These limitations are considered in more detail in the experimental section.
From these considerations it can be seen that a general understanding of nuclear recoil chemistry is not going to be easy. It will be necessary to draw upon results from many other branches of chemistry, especially ion–molecule studies, radiation chemistry, kinetics, photochemistry, molecular beam studies of excited atoms, etc. The limitation as to the number of recoil events that can be reasonably allowed in any experiment had led to few of the many possible recoil atoms being studied in any detail. It is fortunate that most atoms produced in nuclear reactions are themselves radioactive, so that their chemical reactions give rise to labelled compounds. The recoil atoms can therefore be detected by the most sensitive method, that of their own radioactive decay, i.e. an autotracer technique. This is true for solids, liquids, and gases, but the physical constraints which the phase places upon the way in which the recoil energy is dissipated or can be used to initiate reactions are such that solids are best considered separately. This was done in the first volume of these Specialist Reports devoted to Radiochemistry. This Report will therefore deal with reactions in liquids and in gases. Gas-phase reactions are of particular interest since the collision between a substrate molecule and a recoil species can be presumed to take place in isolation, whereas in the liquid phase, radicals and other potentially reactive species will be held near the site of the collision by surrounding molecules. It is often of great interest to compare and contrast reactions of a recoil atom in the gaseous and liquid states, and so these two phases will be considered together in the subsequent sections of this Report. An important distinction, from the chemical point of view, is between univalent and multivalent atoms; the former can react in a single reaction step to produce a stable molecule whereas multivalent atoms will usually produce radicals as a result of their first reactive encounter with a substrate molecule. It therefore seems reasonable to divide the Report in the following way: a general experimental section, univalent atoms (hydrogen, halogens), and then multivalent atoms in the order of their increasing valency. The final section will consider what general conclusions can be drawn and discuss practical applications of hot-atom chemistry.
Throughout this Report 'hot' will be used to denote translational excitation exclusively and 'atom' will refer to a neutral atom in its ground electronic state. 'Recoil particle' or 'recoil species' means any atom or ion electronically excited or one that has resulted from a nuclear reaction. The scope of this Report is not as wide as that encompassed by the phase 'chemical effects of nuclear transformations' since radiation chemistry will not be explicitly dealt with here. Many other reviews on hot atom chemistry, recoil chemistry, etc. have appeared in the past as well as conference reports and at least one book.
The emphasis in this Report will be on recent results but reference will be made to older work in order to try to obtain the right perspective.
2 Experimental Techniques
Whether experiments are carried out in the liquid or the gaseous phase, a consideration of the half-life of the recoil species to be detected is of paramount importance. If it is too short (say < 0.5 min) any experimental separation of labelled products will be almost impossible, whilst if the half-life is too long (say > 104 yr) it would be necessary to produce a very large number of recoil atoms in order that their radioactivity might be detected. This is undesirable because of the large amount of recoil energy that would be deposited in the substrate, to its detriment. Table 1 shows some typical recoil species whose chemistry has been investigated, together with relevant nuclear data. The general experimental methods that are used in recoil chemistry are remarkably similar, the main distinction being those imposed by phase. Many of the reactions are initiated by neutron irradiation and the general remarks that follow are for this type of irradiation.
Liquid Phase. — Liquids are usually irradiated in enclosed capsules of quartz or glass. After irradiation these capsules are broken under a suitable solvent or mixture of solvents. The simplest type of mixture is of organic and inorganic materials. The distribution of radioactivity between the two solvents is then determined and the result expressed as the percentage 'organic yield' (Y),
Y% = activity in organic solution × 100/ activity in organic and inorganic solutions
The amount of radioactivity present in each solution is usually measured with a scintillation counter. A more sophisticated type of analysis involves the use of gas–liquid chromatography to separate and to identify the various labelled compounds. This type of analysis can usually be most easily applied to the components of the organic yield. Inorganic products are more often determined by the use of particular additives which will react with one but not another of the anticipated compounds to remove it from the inorganic solution. The change in inorganic activity can then be ascribed to a particular compound.
Gas Phase. — For the study of most recoil reactions in the gas phase, quartz has proved the most useful material for the manufacture of sample ampoules. A typical volume would be about 20 cm3, with a diameter of l0 or 20 mm. Such ampoules are made with a capillary tip, which can be easily broken after the irradiation, and a constriction, where the ampoule can be easily sealed off when it has been filled, in the tube leading to the vacuum line. Quartz is virtually transparent to thermal neutrons and many other radiations which can be used to initiate nuclear reactions. Quartz does, however, have one disadvantage which is of especial importance in recoil tritium chemistry. It has an open lattice and a considerable number of tightly adsorbed hydrogen atoms (presumably as — OH groups). The recoil atoms (X) can react with t his hydrogen to form XH, some of which then enters the ampoule volume. This difficulty can be overcome by using perdeuterio-compounds, either directly, or to enable the amount of XH that arises from reaction with adsorbed hydrogen to be determined. Another solution is the use of 1720 glass which, at least in the case of tritium, docs not permit the release of H to the gas phase. However, this glass contains boron, which attenuates the thermal neutron flux that can reach the sample. A quantitative determination of neutron dose is therefore difficult.
The ampoules arc filled from a calibrated vacuum line. Usually the quartz bulb is immersed, almost to t he constriction, in liquid nitrogen. Gases are then transferred to the ampoule in measured quantities, where they are allowed to condense. Materials volatile at 77 K are added last; if more than one such substance is to be added two alternative procedures can be used. Either the component with the highest vapour pressure is added last and allowed to sweep all the other vapours into the ampoule, or some time can be allowed before the stopcock is closed, so that equilibration can take place. If mixtures of gases are to be added it is particularly important to allow this equilibration to occur; effusion rates will vary through the constriction, giving a very different gas composition in the ampoule from that intended. The actual composition of the sample can always be checked after irradiation by gas chromatography. When all the gases required have been added to the ampoule it is sealed off at the constriction and can be dispatched for irradiation. Thermal neutrons are widely used for initiating nuclear reactions and in many cases it is very convenient and useful to be able to irradiate many samples simultaneously. This in turn requires rather a large volume, a volume within which the thermal neutron flux will vary quite considerably. To overcome this problem the samples can be rotated in a carousel in the presence of two or three randomly placed monitors or in the presence of a standard sample ampoule which can be used as a reference. Another technique is to use a larger number of accurately positioned monitors, which can then be used to map out the neutron flux within the facility where the irradiation took place. It is also an advantage if the temperature of the samples can be controlled during the irradiation, especially if liquids are being used.
Depending upon the half-life of the atom being investigated, the post-irradiation sample handling may or may not be leisurely. It is usual to attempt a thorough analysis of all labelled products, using gas–solid and /or gas–liquid chromatography. The ampoule is placed in a suitable vessel which can be evacuated and in which the ampoule can then be broken. Aliquots of gas from the ampoule are then analysed in sequence using a variety of gas-chromatographic columns. Special taps have been described for ensuring a clean sample injection. Provided consistent behaviour for a particular compound is observed on two or three different columns then this is usually taken as sufficient evidence for its chemical identification. If, as in some experiments, a group of very small ampoules has been irradiated, each one can be used for a single chromatographic analysis by crushing the sample in the gas flow. Completely automatic sample analysis equipment is used in some laboratories. The choice of chromatographic columns is dictated by the anticipated products; in the case of olefinic compounds, and also isotopically labelled materials, very long columns (or repeated automatic switching between two short columns) have enabled labelled isomers to be differentiated, (e.g. TCH2CH=CH2 and CH3CH=CHT).
The gas flow from the chromatographic column is led directly either to a scintillation or to a proportional counter in order that the amount of labelling in each molecule may be determined. In the case of tritium the β-radiation is so weak (max. 18 keV, average ~6 keV) that a windowless counter must be used, i.e. the sample must flow in the counter gas itself. The carrier gas for chromatographic analysis is not usually directly suitable as a counter gas; in the case of helium, methane is, for example, added so that a helium-methane mixture then flows through the proportional counter. The presence of macroscopic amounts of sample passing through the counter can sometimes cause considerable problems in counter stability. Such problems are: variation in flow rate due to the additional material ; alteration of the counter's detecting characteristics; and the possibility of quenching of the counter (oxygen, alkyl halides, and to a lesser extent the noble gases and hydrocarbons can all cause quenching). These problems can be overcome by continuously monitoring the flow rate and by determining the effect on counting rates caused by various amounts of added gas. This can most easily be done using a standard counter which has tritium-polymer deposited on its walls. Such a counter is also particularly useful when setting up the counting apparatus before an analysis is started. The whole problem of the use of internal proportional counters for the detection of tritium has been reviewed.
Tritium is, fortunately, a rather special case, and most other radioactive species can be detected much more easily since the radiation or the particles which they emit are so much more powerful. This allows the gas flow containing the labelled compounds and the counter itself to be physically separated, either by a thin Mylar window or, in the case of γ-rays, by glass. A variety of different proportional counters have been described in which the gas to be counted flows through a central tube or section which is surrounded by two or more proportional counters. It is also possible to pass the gas flow between two scintillators and thus detect γ-emission. The problems associated with stability of an internal counter are clearly removed in all these cases; however, the problem associated with variations in flow rate due to the passage of macroscopic amounts of sample remains, and the flow rate should be continuously monitored.
Excerpted from Radiochemistry Volume 2 by G. W. A. Newton. Copyright © 1975 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
ContentsChapter 1 Industrial Applications of Radioisotopes By J. A. Heslop,
Chapter 2 Activation Analysis in Archaeology By G. Harbottle,
Chapter 3 Preparation of Radiopharmaceuticals and Labelled Compounds using Short-lived Radionuclides By D. J. Silvester,
Chapter 4 Sample Preparation Procedures for Liquid Scintillation Counting By B. W. Fox,
Author Index, 133,