Szilard-Chalmers reactions

The study of Szilard-Chalmers reactions places considerable demands on the separation methods used. These molecular compounds are only weakly polar, if at all, and thus are not easily separable. Moreover, many of the compounds and especially the radicals and other intermediates formed are not very stable, often requiring the absence of air, moisture, and even light. Combining this with the fact that many of the product species are present in extremely small amounts, we have a nearly impossible situation. Nonetheless a good deal of reliable work has been done. 

Harbottle, G., Sutin, N. The Szilard-Chalmers Reaction in Solids. Advan. Inorg. Chem. Radiochem. /, 267—314 (1959). 

A sample of iodine-128 was produced in a Szilard-Chalmers reaction by irradiating a sample of ethyl iodide in a neutron source. The radio-iodine was extracted with sodium thiosulfate solution and then counted in a Geiger counter at various time Intervals. Use the tabulated data of t in minutes against C counts/min to find the rate equation and the half time. 

Williams, R. R. The Szilard-Chalmers Reaction in the Chain Reacting Pile. 

The Szilard-Chalmers Reaction in Solids Oarman Harbottle and Norman Sutin... 

Ela,stic-coIlision model, Szilard-Chalmers reaction and, 1 268-269... 

Elastic-inelastic collision model, Szilard-Chalmers reaction and, 1 269 Electrical conduction, in organic superconductors, 29 278-286 Electrical conductivity of chalcogenide halide compounds, 23 331 of Group IB, 23 337-339, 342, 346-349 photoelectric effects, 23 368, 410 semiconductors, 23 368, 390, 395-396, 400-402, 410-412 superconductors, 23 375-377 of graphite intercalation compounds, 23 290, 294, 309-310, 312, 317-318 Electric discharges arc type, 6 146-147 chemical reactions in, 6 189-191 chemical reactions in, 6 143-206... 

Szilard-Chalmers reaction, recrystallization and, 1 290-292 HoxN protein, 47 287 HoxW protein, 47 288 —HoxX protein, 47 290 hoxY gene product, 38 409-410 HjPOj-, 33 106... 

Ramsey theory, 22 201-204 Random-fragmentation model, Szilard-Chalmers reaction and, 1 270 Random-walk process, correlated pair recombination, post-recoil annealing effects and, 1 288-290 Rare-earth carbides, neutron diffraction studies on, 8 234-236 Rare-earth ions energy transfer, 35 383 hydration shell, 34 212-213 Rare gases... 

For several decades there has been research directed toward the attainment of enrichment, or separation of the product isotope of a nuclear reaction, as a direct result of the nuclear reaction itself. In principle this problem was solved in 1934 by the Szilard-Chalmers reaction. In this instance advantage is taken of the reaction product s recoil energy, which is sufficient to break a chemical bond. Thus, the product atom may be converted to a chemical state unlike that of the original, unreacted target atoms, and hence be chemically separated. 

In practice, the Szilard-Chalmers reaction has not been very successful for production of substantial quantities of any isotope although it often works well for tracer or low irradiation levels. When greater levels of neutron irradiation became feasible with development of nuclear reactors, it was found that both enrichment factors and product yields decreased as neutron exposures increased to the values required for significant isotope production. 

In the work reported here, which was directed toward the attainment of an isotopic enrichment of the trivalent actinide and lanthanide elements, the problem was compounded by the fact that these elements do not readily form appropriate compounds, like iodine in ethyl iodide. They do form some stable organic chelates, and, indeed, it is possible to obtain a Szilard-Chalmers reaction with such compounds. However, their radiation damage resistance does not appear adequate to permit useful production of an isotope like 247Cm, which requires a thermal neutron exposure ap-... 

Inorganic compounds may be more stable to radiation damage than organic compounds, and their decomposition products are less deleterious. Accordingly, a search was made for inorganic compounds which would contain these trivalent ions in a nonexchangeable state and, at the same time, allow recoil atoms ejected from their position in the structure to be separated. Szilard-Chalmers reactions have been reported for certain inorganic systems, for example clays ( ), but yields and enrichment factors were both low. 

In the classical Szilard-Chalmers reaction a specific bond or group of bonds is broken by the recoil energy. This reaction is analogous, but it is not clear whether such specific bonds are broken, or if physical factors related to pore trapping are involved. The mechanism of the interaction between the trivalent metals and zeolites is not completely understood. 

Szilard-Chalmers reactions are applicable to elements existing in different stable oxidation states or forming substitution-inert complexes. Exchange reactions between the oxidation states or with the complexes should not take place during irradiation and chemical separation, because they would cause a decrease of the specific activity. Therefore, substitution-labile complexes are not suitable. 

Szilard-Chalmers reactions are characterized by the enrichment factor (i.e. the ratio of the specific activity of the radionuclide considered after separation to the average specific activity before separation), and by the yield (i.e. the ratio of the activity of the radionuclide obtained after separation to its total activity). Enrichment factors of up to about 1000 or more may be obtained, and yields of about 50 to 100% are of practical interest. 

Examples of Szilard-Chalmers reactions are given in Table 9.4. Radionuclides of the halides may be obtained in high specific activities by neutron irradiation of alkyl or aryl halides or of the salts of the oxoacids. Radionuclides of other elements may also be produced in high specific activities by neutron irradiation of covalent compounds. 

Szilard-Chalmers reactions are of special interest for the investigation of nuclear isomers, because they offer the possibility of separating isomeric nuclides. 

Table 9.4. Yield and retention for some Szilard Chalmers reactions.

Szilard-Chalmers reactions may also be applied to obtain radionuclides of high specific activity after (n, y) reactions (section 9.6.). 

The recoil phenomenon is often seen with (n,y) reactions which are known in terms of the Szilard-Chalmers reaction (1934). Although the capture of a thermal neutron... 

For instance 1 produced by a (n,y) reaction on ethyl iodide is recoiled and hence, when shaken with water, it is transferred into the water phase in carrier free state (ideally speaking). This is the first example of Szilard-Chalmers reactions which are now extensively studied and used for the production of some isotopes in high specific activity. 

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