Szilard—Chalmers process

H. A. C. McKay, The Szilard-Chalmers Process, in Progress in Nuclear Physics (Ed. O. Frisch), Vol. I. Pergamon, Oxford, 1950... 

The loses its kinetic energy and is stabilized as an iodine atom or iodide ion it can also be recaptured by the C2H5 radical (retention of activity in C2H5I). In addition to the necessity that the recoiling species have sufficient energy to rupture the bond, it is also necessary for a successful enrichment of specific activity by the Szilard-Chalmers process that there is no rapid exchange at thermal energies between the active and inactive iodine atoms in ethyl iodide ... 

Before nuclear reactors became available for radioisotope production, the Szilard-Chalmers process mentioned in Sect. 24.1 was very important for its availability to prepare radioisotopes with high specific activity. This unique technique survived for many years after the first nuclear reactor started to operate in 1942. The activity A of a radionuclide produced by activation can be expressed as... 

In the dawn of the history of nuclear science, the neutron flux density (/) of Ra-Be source was only 10 -10 n cm s The total activity produced by such neutron sources via (n,y) reaction was very low and so was the specific activity of the radionuclides. The Szilard-Chalmers process, however, could dramatically increase the specific activity the improvement could reach orders of magnitude. In the measurement of P radioactivity, which was a frequent task in early days of nuclear science, samples with low specific activity brought sometimes troublesome problems of self-absorption corrections. By the introduction of the Szilard-Chalmers process, however, this difficulty could be avoided, because the measurement could be performed within small statistical errors using a sample with high specific activity. Therefore, the Szilard-Chalmers process became one of the useful means of preparation of radioisotopes for measurement, as Szilard and Chalmers (1934b) recognized the importance of this technique in their early work. 

It is true, however, that the advent of modem nuclear reactors changed this situation. The neutron flux attained to 10 —10 n cm s . So, the total activity and the specific activity increased strikingly. Without the Szilard-Chalmers process, the specific activity of the aimed radioisotope was high enough for many purposes. 

A French radioisotope production group provided radionuclides such as Cr, Fe, Cu, Zn, and As by the Szilard-Chalmers processes (Henry 1957). At the Japan Atomic Energy Research Institute, this process was applied to obtain pure from neutron-irradiated potassium phosphate. Ordinary products using (n,p) reaction in a nuclear reactor contain an impurity isotope P in P, but P produced by (n,y) reaction in neutron-irradiated phosphate does not contain P. Hot atom chemically obtained P by (n,y) reaction was therefore appropriate for some special experiments in which contamination of P with different half-life and P-particle energy had to be excluded (Shibata et al. 1963). 

Although the Szilard-Chalmers process using nuclear reactors can normally ensure high concentration of the desired radioisotope, it suffers from radiation decomposition problems of the target compound. At higher neutron flux values, radiation decomposition also increases which sets a limit to the enrichment factor (concentration factor). When the enrichment factor (E) is defined by the ratio of the specific activity (Si) of the separated part of the Szilard-Chalmers process to that (S2) before separation. 

Note that in Table24.3 phtalocyanines are useful target materials. Fe, Co, Cu, Zn, Ga, Mo, Pd, In, Dy, Os, and Pt radioisotopes can be separated from neutron-irradiated metal phthalo-cyanines. Metal chelate compounds such as metal oxinates are other examples. Ca, V, Mn, Ni, Cu, In, and W are separated by the Szilard-Chalmers process from their oxinates. Salts of oxyacids... 

Enrichment of radioisotopes by the Szilard-Chalmers process, (ppt precipitate)... 

The other radioisotope Cu has been produced by the Szilard-Chalmers process at the Japan Atomic Energy Research Institute in the 4 x 10 Bq/mg level. This radioisotope is usefiil for medical purposes, but the total amount of copper should be limited because of its toxicity to biological systems. Therefore, enrichment procedure by the Szilard-Chalmers process using copper phthalocyanine is recommended. 

In selected cases, there is a technique that can be utilized to improve the specific activity of (n,y)-produced radionuclides. This method is known as the Szilard-Chalmers process (Szilard and Chalmers 1934). The Szilard-Chalmers process depends upon the fact that, following neutron absorption, prompt y-rays are emitted, which may cause nuclear recoil and subsequent molecular bond disruption. This excitation sometimes leaves the resulting hot atom in a chemical state different from that of unreacted atoms, which makes it chemically separable. This separated fraction is relatively enriched in radioactive atoms and has a specific activity higher than that of the rest of the target. 

THE SZILARD-CHALMERS PROCESS IN SOLID SODIUM lODATE... 

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