Radiochemistry Volume 1

A Review of the Literature Published Between July 1969 and August 1971

By G. W. A. Newton

The Royal Society of Chemistry

Copyright © 1972 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-254-5

Contents

Chapter 1 Chemical Effects of Nuclear Transformations By G. W. A. Newton,
Chapter 2 Superheavy Elements By J. D. Hemingway,
Chapter 3 Radiochemical Methods of Analysis By G. R. Gilmore,

CHAPTER 1

Chemical Effects of Nuclear Transformations

BY G. W. A. NEWTON

1 Introduction

This review is concerned mainly with solids, because this reflects the Reporter’s interests. A small section is devoted to solutions. It is hoped that future volumes will contain a more comprehensive coverage of the subject and include the gas- and liquid-phase studies.

Material has been arranged within the sections of this chapter such that the mention of each recoil atom follows the order of elements in the long form of the Periodic Table. It is clear that much of the work has been concentrated on a few elements, with cobalt complexes being the clear leader in terms of the extent of work published. With the exception of 32P and 35S, there is a paucity of information on p-block element systems.

Sources of information on earlier work include the I.A.E.A. publications of symposia, the review by Harbottle, and the review by Maddock and Wolfgang. The proceedings of the Chemical Society Symposium at Cambridge in July 1969 were not published. More recently, a book has been published on ‘Hot-atom Chemistry’, and the subject has been reviewed by Matsuura. Several articles have been written giving a general introduction to recoil chemistry, and outlining significant factors of such in chemistry and in the solid state.

Much interesting work has been carried out with recoil particles separated from, or interacting with, materials. The construction of a low-energy (5 — 10 keV) ion accelerator for hot-atom chemical research has been described.

The importance of the analytical procedure used must still be emphasized, because within the period of this Report there have been reports of difficulties with several of the systems which had been in use. One must remain suspicious of intercomparison of different systems, for example cation and hydrate effects, because often different sources of the same compound give different results. Much information can be obtained from studying variables on the same compound, particularly if this is a pure and well-defined system. The use of ion implantation has far-reaching implications in industry as well as in recoil chemistry research. Volume 311 (1969) of the Proceedings of the Royal Society contains several articles on ion implantation; in particular, a general theory of the slowing down of ions is given as well as empirical measurements of the range and energy loss of implanted ions.

Calculations have been made of the range of light ions in solids and of the spectra of energy deposited by ‘heavy particles’ (protons through to oxygen) in tissue-equivalent material. In beta decay there is evidence of multiparticle interaction during the deceleration of slow atoms in solids. Empirical methods have been given for measuring energy loss and path lengths of recoils in solids. In the determination of recoil ranges in gases, diffusion effects should be considered. The measurements of recoil distances have been used to obtain lifetimes of excited states in nuclear reactions.

The recoil effect is useful for separating radionuclides. A fully mechanized apparatus has been described for the continuous separation of short-lived radionuclides. The use of chemical reactions of recoil atoms has been utilized for on-line separations of the required isotope in studies of nuclear reactions, using electromagnetic isotope separators. The systems studied were:

[FORMULA OMITTED]

With the exception of antimony, reasonable yields were obtained using this technique, and these could be improved by passing small amounts of carrier or reaction gases over the irradiated target during product evaporation.

The identification of element 102 by a double-recoil technique following alpha decay is well known. Much of the American evidence for the existence and isolation of element 104 hinges on the identification of the element 102 daughter particle which could be separated from the element 104 parent by a recoil method (see for example ref. 26). The recoil technique has been further exploited in the separation of fission fragments. The measurement of short half-lives (ca. 300 μs) in a radioactive series is facilitated by recoil separation techniques. Recoils have been used to enrich 47Ca and 132Cs as well as to produce Xe compounds.

Another important property of a recoil, and related to its range, is its charge. The Bohr-Lindhard theory used for the calculation of electron capture and loss, applicable to heavy ions passing through solids, has been modified. The conclusion is that the variation in charge of heavy ions as they traverse solids and dilute gases is mainly due to the Auger process, which occurs after ions leave the solid. The importance of the Auger process in determining the charge of the stable product in a recoil process will be discussed more fully in the discussion of Mössbauer spectroscopy in Section 2. The significance of when the Auger process occurs in recoil chemistry has been considered. Charged and neutralized species of 56Mn interacting with mylar, KCl, and KC104 always gave more than 98% Mn0; or Mn2+; this gives some indication of the possible importance of radiolysis in recoil phenomena. It has also been shown that charge plays an important role in the capture of recoil atoms by surfaces in (n, γ) reactions in solids. de Wieclawik has measured the charge of recoil atoms following alpha decay, using a time-of-flight method for the systems 212Bi, 212Po, 227Th, and 223Ra. The charge distributions were centred on 13, 20, and 25 for 1, 2, or 3 vacancies in internal electron shells, respectively. It is well known that the dislocations occurring in single crystals are charged. The charge per unit length on edge dislocations in NaCl containing 116 p.p.m. Mn2+ has been measured. It would seem that the charge does not exceed 2+ per lattice unit for cleaved crystals, but could be as high as 5 + per lattice unit for rapidly quenched crystals.

2 Physical Methods Used to Study Recoil Chemistry

This section is only concerned with the aspects of these topics relevant to recoil chemistry.

The photons emitted by the de-excitation of nuclear levels that are populated in the course of radioactive decays can be resonantly scattered. Nuclear resonance fluorescence experiments can give information on the velocity distribution of recoil atoms and the chemical modifications following transmutations and on the slowing-down process of hot atoms. This technique can be applied in gaseous, liquid, and solid systems, giving an advantage over Mössbauer spectroscopy. Nuclear resonance fluorescence has been reviewed, with particular reference to the following systems:

[FORMULA OMITTED]

Other physical techniques that have been used for the study of recoil particles include mass spectrometry, perturbed angular correlations, and Mössbauer spectroscopy. This section is mainly concerned with those aspects of the latter which are relevant to recoil phenomena.

The difficulties of determining the chemical nature and environment of a recoil atom arise because of the very low concentrations of such atoms. In situ measurements can only be achieved by nuclear methods, which are perturbed by the chemical environment. Perturbed γ–γ angular correlations give information on the symmetry and nature of the environment of the recoiling atom. By using Mössbauer spectroscopy, the oxidation state and the nature of the environment of the newly formed species can be deduced. For example, 57Co feeds the Mössbauer state of iron, 57mFe, by electron capture and γ decay. If a 57Co-doped iron compound is used as a source, and a standard iron compound (e.g. stainless steel) as an absorber, then the Mössbauer spectrum reflects the oxidation state and environment of the 57mFe at the time of photon emission; this is ca. 10-7 s after its formation.

In general, the charge on the 57mFe is often the same as that on the iron atom source compound. This means that any high charge states produced by Auger charging mechanisms must be neutralized in the time-scale involved (l0-7 s in the [FORMULA OMITTED] system). The importance of time-scale on the chemical form of the recoil in (n, γ) reactions has been reviewed. However, different or ‘anomalous’ charge states have been observed, and their origin has caused considerable speculation. Wertheim and Buchanan ‘prove’, by the use of external gamma radiation, that the stabilization of high charge states [e.g. Fe3 + in FeII(NH4)2(SO4)2,6H2O] is due to radiolysis of water by Auger electrons. Although it is easy to see how high charge states can be produced, directly or indirectly, by Auger processes, it is more difficult to understand how ‘anomalous’ low charge states are produced. In the case of FeII in CoIII complexes, it is suggested that this is a pressure effect, arising because the cobalt complex is more dense than the corresponding iron complex. The evidence is that the spectra of the FeIII compound under pressure and of the ‘anomalous’ FeII compound are very similar. The pressure causes a redistribution of charge among the available orbitals, and from the electroneutrality principle the energy required is not large. Doubt has been thrown on this explanation because the emission spectra of potassium trisoxalatocobalt(III) and potassium trisoxalatoiron(III) are very similar; there is a pressure effect in the first compound only. The alternative explanation invokes redox reactions of radicals produced by autoradiolysis with Auger electrons. This effect was simulated by using an electron beam from an external radiation source. Fe2+ appeared when the sample was irradiated with external radiation, ‘confirming’ the autoradiolysis mechanism.

Autoradiolysis has also been considered to be an important mechanism in the stabilization of high charge states. Much work has been concerned with obtaining an understanding of the presence of high charge states following nuclear transformations.

A series of papers on the charge state of iron in 57Co-doped oxides has appeared. A detailed molecular orbital theory has been worked out, involving Co vacancies, to account for the concentration of Fe3+ in 57Co-enriched CoO. The presence of high charge states, e.g. Fe3+ and Fe4+ in 57Co(Cr2)O4 and Fe3+ in 57Co(Cr)2S4, has been attributed to the presence of cation vacancies in the source. It would seem that the particle size of the source does not affect the shape of the spectrum. Evidence has been presented, using delayed coincidence techniques, for the presence of Fe+, Fe2+, Fe3+, and Fe4+ in 57Co-doped Cu2O, Fe2+ and Fe3+ in MgO, and Fe2+, Fe3+, and Fe4+ in Al2O3.

High charge states have also been observed to exist within fluorides; e.g. Fe3+ and Fe4+ in K3CoIIIF6 and Fe2+ and Fe3+ in KCOIIIF3. The authors claim that thermodynamic lattice parameters play a significant role in the stabilization of higher charge states, and that these states are not produced by partial neutralization of even higher charge states formed after the Auger cascade. They quote the example of Cs2CoF6, which has a structure derived from K3CoF6, as a stable complex of Co4+.

The species Fe2+ and Fe3+ were observed in 57Co-doped CoF2 and CoF2,4H2O. The proportion of Fe3+ that was found to exist was higher in the hydrate, and was also a function of temperature. This was interpreted as being due to the annealing of defects or to radical reactions. There is a close parallel between these experiments, and chemical measurements of recoil species in anhydrous and hydrated compounds. Further support to the suggested importance of crystal defects and impurities in the stabilization of high charge states was given by the work of Mathur et al. They observed Fe2+ and Fe3+ in K 57CoF3, the high charge state being stabilized by a volume effect, with charge compensation.

Several authors have investigated the Mössbauer effect for 57Co-doped cobalt halides and sulphides. Charge states are usually attributed to Auger after-effects, which may or may not be extinguished in a time shorter than the nuclear lifetime.

It has been shown that many cobalt complexes [e.g. edta, (acac)3, bissalicylaldehyde, triethylenetetramine, and indenylchelates] fragment in a large majority of the events following electron capture in 57Co. This results in the formation of degraded ionic species such as Fe2+ and Fe3+. On the other hand, highly conjugated complexes such as cobalt phthalocyanine and Vitamin B12 escape fragmentation in all of the Auger events. CoIII(bipy)3(ClO4)3, 3H2O also escapes fragmentation in a large number of such events. The large amount of excitation energy is presumably dissipated in less than 10-13 s through neighbouring molecules. Alternatively, a fraction of the coordinated iron in the bipyridyl complex could come from the re-entry of a multiply charged degraded iron ion into the lattice. In any event, it would seem that these experiments provide strong evidence for the suggestion that an electronic mechanism determines the state of an atom following nuclear transformations in solids. Evidence for exchange has been presented in a Mössbauer study of CoIII(bipyh)3(ClO4)3,3H20. In Figure 1 are shown spectra of the complex, doped with 57CoCl2, before and after storage at 25 °C for 4 days. Chemical analysis showed that 1% of the 57Co was initially present in the complex, and that 99 % of the radioactive cobalt isotope was complexed after the complex had been stored. The spectra change from one which is essentially typical of FeII to one essentially typical of FeIII.

The authors claim that this exchange process is a redox phenomenon:

[FORMULA OMITTED]

If this is true, then this process could be induced by an external electron beam, using 60Co2+ as tracer. It is possible that Fe3+, rather than Co+, is the exchanging species. Radiochemical methods (discussed later) suggest that an exciton-induced exchange of Co2+, rather than Co+, is possible. A study of a covalent matrix 57CoIII(phen)3(ClO4)3 2H2O (phen = 1,10-phenanthroline) gave a spectrum in which the best fit was with low-spin FeIII and high-spin Fe2+. As expected, there was no evidence of non-equilibrium high charge states; the Fe2+ probably arose as a result of fragmentation caused by the Auger cascade followed by electron capture.

The chemical consequences of the nuclear reactions 58Fe(n, γ) 59Fe and 57Co(E.C.) 57Fe have been investigated, using Prussian Blue as the iron-containing compound. KFe[Fe(CN)6,]H2O (1) was prepared with 58Fe as the cation or incorporated in the complex anion; this was then irradiated with neutrons and analysed for free and complexed 59Fe. Parallel experiments were carried out with K4[Fe(CN)6],3H2O (2). In (1), the retention of 59Fe within the complex anion was ca. 5%, and this increased only slightly on annealing; in (2) the retention was ca. 20%, and this increased to 30% on annealing. It was thought that the low retention in (1) was the result of competition between the 59Fe recoil and the inactive Fe3+ for re-formation of the complex. When (1) was doped with 57Co in the cation positions the resulting 57Fe did not enter the complex anion. When (1) was doped with 57Co in the anion, the 57Fe appeared in different complex anionic species; this was possibly due to ligand loss, e.g. to produce [Fe(CN)5]n-.

It is interesting to note that neutron irradiation of 56Fe in iron metal and in Fe–Al alloys gave a recoil isomer shift for the 57Fe product in the latter case only.

Experiments with potassium trisoxalatoferrate(III) have shown that the compound thermally decomposes above 380°C via Fe3O4 or Fe2O3 in air or Fe in vacuo. Radiolytic decomposition gives a dinuclear anion having a quadridentate bridging ligand, [(ox)2FeO2C2O2Fe(ox)2]6-, which can also be produced by thermal decomposition. The very high G value of 7.1 is attributed to very efficient trapping of electrons by the iron atoms. The formation of 57Fe2+ in Mössbauer sources of potassium trisoxalatoferrate-(III) is attributed to an autoradiolysis mechanism, the first step being:

[FORMULA OMITTED]

The trihydrate and anhydrous salt give the same result; therefore water of hydration does not seem to play an important role in the autoradiolysis mechanism, which appears to be different from the mechanism that exists when decomposition occurs as a result of the application of external radiation. In the case of ferric oxalate, lactate, citrate, and malate, external gamma irradiation seems to give rise to a simple reduction mechanism, giving FeIII species similar to those which are formed by the autoradiolysis of K3FeIII(C2O4)3.

Studies of the Mössbauer effect in tin oxides, particularly following neutron irradiation, indicate that there is retention of the recoil particles at normal lattice sites. This has been explained by a disruption of the correlation between recoil momentum and energy, any ionizing effects during reactor irradiation being small. In neutron-irradiated SnO, retention of 119mSn increased on annealing in He at 300°C; at higher temperatures the SnO disproportionated to Sn and SnO2. The isomer shift was attributed to SnII with an admixture of 5p-electrons; this annealed irreversibly above 600°C, with an activation energy of 3 — 4 eV. The annealing was thought to involve a defect complex of SnIII and an associated charge-compensating oxygen vacancy, e.g. Sn2+:O2-. A series of controlled mirror experiments at 80 K show that spectra using a neutron-irradiated SnO2 source show no evidence of ‘anomalous’ charge states, i.e. Sn0 or SnII. There is evidence that SnO2 is formed in neutron-irradiated SnO, probably as a result of irradiation effects rather than the isomeric transition of the tin atom. The observation of the presence of SnII in the isomeric transition of 119mSn-labelled K6Sn2-(C2O4)7,4H2O is of interest because it further refutes the idea that the observation of ‘anomalous’ low charge states is the result of pressure effects.

(Continues…)Excerpted from Radiochemistry Volume 1 by G. W. A. Newton. Copyright © 1972 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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