The Szilard-Chalmers effect

For this method to be useful, there must be no rapid exchange reaction between target and product (equation 2.25) and hence an alkyl halide rather than an alkali metal halide is chosen for the irradiation. 

In forming artificial radioactive isotopes, problems of isolation are often encountered. For example, a product may decay quickly with the result that the initial product is contaminated with the daughter nuclide. 

The methods used to separate a desired isotope depend on whether or not the starting material and the product are isotopes of the same element (e.g. equation 2.14). If they are not, the problem is essentially one of chemical separation of a small amount of one element from large amounts of one or more others. Methods of separation include volatilization, electrodeposition, solvent extraction, ion-exchange or precipitation on a carrier . For example, in the process f Zn(n,p) Cu, the target (after bombardment with fast neutrons) is dissolved in dilute HNO3 and the Cu is deposited electrol5dicaUy. This method is successful because of the significant difference between the reduction potentials °(Cu /Cu) = +0.34 V and °(Zn +/Zn) = -0.76 V (see Chapter 7). 

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The earliest studies in this field were conducted largely to benefit from the Szilard-Chalmers effect—namely, the separation of radioactive atoms from the bulk material—in order either to make nuclear chemical study of radioactive nuclides or to effect an enrichment of radioisotopes. In Table II are listed some selected works of this type. 

Early Studies of Isotope Enrichment by the Szilard-Chalmers Effect... 

When a radionuclide is produced by fission or activation processes or in radioactive decay of a radionuclide to its progeny, the product has a different form than its antecedent and moves from its original site. That is, the nuclear reaction may rupture the chemical bond between the radioactive atom and the molecule of which the atom was a part, and the newly created radioactive atom may have several new electron configurations. This result is described as the Szilard-Chalmers effect. 

The Szilard-Chalmers effect permits separation of radionuclides at high specific activity and purity from the matrix in which they are produced. Preparation of the pure product is an empirical process because of the complex interaction of the three sequential steps producing the free radioactive ion or atom, maintaining the product in its new form in the sample matrix, and separating the product from the matrix. Success of the process is evaluated empirically in terms of the specific activity of the product relative to the matrix or, alternately, the fractional yield of the product. 

Practices on how to prevent contamination of a sample material before bombardment have been described by Smith (874) and by Thiers (934). More recent reports by Broadhead and Heady (128), Brune (131,132), Moore and Leddicotte (627), and Poey and Leddicotte (722) have discussed the possible interchange of trace substances in the sample and the constituents of irradiation and storage containers prior to bombardment, during bombardment and after the activated materials are stored. The discussions by Brune (131,132), in particular, emphasize that these crosscontamination effects could occur as a result of recoil reactions, or the Szilard-Chalmers effect. 

Szilard-Chalmers effect The rupture of the chemical bond between an atom and the molecule of which the atom was a component, as a result of a nuclear transformation of that atom. 

The chemical effects observed after neutron irradiation of ethyl iodide have found great practical interest, because they allow general application to various compounds and chemical separation of isotopic products of nuclear reactions. Above all, isotopic nuclides of high specific activity can be obtained by Szilard-Chalmers... 

The elementary cooperative e-,ot-,P-,y-nuclear processes in atoms and molecules were considered in the pioneering papers by Migdal (1941), Levinger (1953), Schwartz (1953), Carlson et al. (1968), Kaplan et al. (1973-1975), Goldanskii-Letokhov-Ivanov (1973-1981), Freedman (1974), Law-Campbell (1975), Martin-Cohen (1975), Isozumi et al. (1977), Mukouama et al. (1978), Batkin-Smirnov (1980), Law-Suzuki (1982), Intemann (1983), and Wauters-Vaeck et al. (1997) [5-17]. Naturally, in this context, the known Mossbauer, Szilard-Chalmers, and other cooperative effects should be mentioned [7]. 

This recoil energy is so large compared to chemical energies that there seems to be no question about the atom s breaking loose from its bonds and travelling a considerable distance before coming to rest. The effect observed by Szilard and Chalmers is thus readily explained. 

Chemical effects of nuclear reactions were first observed by Szilard and Chalmers in 1934 when irradiating ethyl iodide with neutrons. They found several chemical species containing 1 that are produced by the chemical effects of the nuclear reaction l(n, y) 2 1. In the following years, chemical effects of radioactive decay were observed in gaseous compounds, liquids and solids. 

The chemical effects of nuclear reactions in liquids have been investigated in great detail with alkyl halides. The first example was studied by Szilard and Chalmers in 1934. They irradiated ethyl iodide with neutrons and were able to extract about half of the 1 produced by the nuclear reaction I(n, into an aqueous phase. Similar results are obtained in the case of (d, p), (n, 2n) and (y, n) reactions and of other alkyl or aryl halides appreciable amounts of the radioisotopes of iodine or other halogens obtained by these reaction can be extracted into aqueous solutions. 

The chemical effect of a nuclear transformation was observed by Szilard and Chalmers (78) in 1934. They irradiated liquid ethyl iodide with neutrons and found that radioactive iodine could be extracted into water. The effect was attributed to the rupture of the carbon-iodine bond by the mechanical recoil imparted to the iodine nucleus by the incident neutron. Subsequently Fermi et al. (/) showed that the recoil energy given to the nucleus by the emission of gamma rays following thermal neutron capture was sufficient to break the bonds holding the capturing atom to the remainder of the molecule. 

Also in 1934, Leo Szilard and T. A. Chalmers observed that neutron capture is accompanied by chemical effects (Szilard and Chalmers 1934) when ethyl iodide was irradiated with neutrons, about half of the produced 1 was not organically bound, but could be extracted with water in the form of iodide. The recoil energy transferred to the iodine atom by the emitted y ray exceeds by more than one order of magnitude the energy of the carbon-iodine... 

Szilard L, Chalmers TA (1934) Chemical separation of the radioactive element from its bombarded isotope in the Fermi effect. Nature 134 462 Tanihata I, Hamagaki H, Hashimoto O et al (1985) Measurements of interaction cross sections and nuclear radii in the light p-shell region. Phys Rev Lett 55 2676... 

Following the above mechanical use of nuclear recoil, purely chemical effects of nuclear recoil were observed by Szilard and Chalmers (1934a, b) in 1934. They used (n,y) reaction of iodine in ethyl iodide. The product of neutron capture, I, could be chemically extracted into an aqueous phase after mixing ethyl iodide with water. 

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