The Szilard-Chalmers Process

Pipette a volume V, cm of the active solution (with a safety pipette) into a 25 cm beaker. Heat nearly to boiling on a water bath and add V2 cm of 0.4 M iron complex solution dropwise and with stirring. Allow the suspension to settle and filter under suction through GF/F paper in a filter assembly, Fig. 18.2, collecting the filtrate in a clean dry receiver. Pipette 9 cm of the filtrate (safety pipette) into a liquid G-M counter. Transfer it to the lead castle and count for an adequate time ( 60 min) setting the H.V.of the counter at 420 V. 

Calculate the count rate in each case after subtracting the background. To obtain comparable results, multiply the count rate by (V,+V2)/9. Plot the count rate against the volume of titrant V2xlOA ]. Taking die concentration of iron(III) complex as 0.40 mol dm, calculate the concentration of La(N03)3 in mol dm. Hence work out the number of molecules of water of crystallisation in the solid. 

9 THE SZILARD-CHALMERS PROCESS IN SOLID SODIUM lODATE 

This involves breaking of a covalent chemical bond when a radioactive atom, formed by neutron activation, recoils as a y ray is emitted. When the active hot atom formed is separated from the large excess of inactive target nuclei, its formation is demonstrated by its radioactivity. Separation is possible since the hot and die inactive nuclei are in different oxidation states. 

Special precautions when using open radioactive sources 

---

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. 

This process is known as the Szilard-Chalmers reaction and was discovered when, following the irradiation of ethyl iodide with thermal neutrons, it was found that radioactive iodide could be extracted from the ethyl iodide with water. Moreover, when iodide carrier and silver ions were added to this aqueous phase, the radioactive iodide precipitated as silver iodide. The obvious interpretation of these results is that the neutron irradiation of the ethyl iodide, which caused the formation of ruptured the bonding of this atom to the ediyl group. The bond energy of iodine to carbon in C2H5I is about 2 eV. Since this exceeds the recoil energies of neutron capture, the bond breakage must have resulted from the 7-emission which followed neutron capture and not the capture process itself. The reaction can be written ... 

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. 

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]. 

---