Carrier-free production

The basis of co-precipitation and adsorption methods for the purification of carrier-free radioisotopes is the use of a non-isotopic carrier for the required product. The carrier must behave chemically similar to the product in enough reactions to enable purification to be effective, its bulk being necessary for manipulation in precipitations. Its chemistry must be sufficiently different, however, to enable a simple separation of the carrier and carrier-free product to be obtained when a satisfactory purity of the radio-active material has been reached. A good example... 

Figure 12. Variation of heating value of dry, carrier-free product gas with oxygen-coal ratio...

The term carrier-free stands for isotopically pure. However, the carrier-free product, obtained from either fission or tellurium target, generally contained small amounts of inactive and long lived The contamination of and however, can hardly act as a carrier for the as long as no carrier is intentionally added. 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

Failure to measure chemical yields and to identify products are sometimes difficult to rectify, especially in the case of unexpected carrier-free compounds. Unfortunately, the amounts involved are so small that even the... 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

Scott, K. G., Overstreet, R., Jacobson, L., Hamilton, J. G., Fisher, H., Crowley, J., Chaikoff, I. L., Entenman, C., Fishler, M., Barger, A. J. AND Loomis, F. (1947). The Metabolism of Carrier-Free Fission Products in the Rat, Report No. MDDC-1275 (National Technical Information Service, Springfield, Virginia). 

The same problems of separating radioactive materials occur of course with the fission products of uranium where the task is often to separate a much larger number of different carrier-free radio-elements than occurs in normal targets. The mixture is complex and consists of elements from zinc to terbium and several hundred radioactive isotopes of varying half-life. 

The problem is usually the one of preparing so-called carrier-free material, which means material where all the atoms are radioactive (apart from the solvent, of course, if in solution). This, however, is entirely theoretical and in practice there are always a number of inactive atoms present from impurities, even if they have not been deliberately added in the form of carrier. In some cases, as in the fission products,... 

It is common practice for analytical purposes as in the analysis of carrier-free mixtures, such as the fission-products of uranium, to add the isotopic carriers for the main constituents (37), but the exercise is then one of normal analytical practice. This aspect will not be discussed here nor will the classical investigations of radioisotope behaviour with carriers such as is discussed in several books (30), (90), (130). 

A less specific type of adsorption can sometimes be used if the required product forms insoluble hydroxides but the target element does not. In this case, the solution is made alkaline, and the carrier-free radio-colloidal product is readily absorbed on to filter paper in good yield, when, after washing, it can subsequently be dissolved in acid. This has been used for the separation of magnesium from aluminium, scandium from calcium and for several other elements (17), (26), (42), (44), (66), (103), (104), (105), (106). 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

That diolefins play a role in benzene formation has also been shown over over a nickel-on-alumina catalyst. Product composition from 1-heptene as a function of the catalyst amount is shown in Fig. 3. This points also to diene intermediates 50). The same was found with carrier-free nickel and platinum 51). 

For the 0(p,n) F nuclear reaction, the oxygen-18 target material normally consists of highly enriched (>95%) liquid [ 0]water, but [ 0]dioxygen gas has been used as well [19,37]. Appropriate cyclotron targetry allows a batch production of several Curies of [ F]fluorine in a single irradiation of a few hours. While the theoretical specific radioactivity of carrier-free fluorine-18 is 1.7 x 10 ... 

C. Crouzel, D. Comar, Production of carrier-free F-hydrofluoric acid, Int. J. Appl. Radiat. Isot. 29 (1978) 407-408. 

Hg-195m/Au-195m Th Daughter High photon yield Acceptable production rate Carrier free parent isotope Ik Parent somewhat short... 

Leach rates for elements other than those listed in Table II can also be determined by this method. In fact, any element in the periodic table that is solid at room temperature and has an activation product with a half-life sufficiently long to allow leach testing can be studied with this technique. This method can also be applied to the study of the leach rates of alpha emitting actinides present in waste. In this case, standard carrier-free radiochemical procedures, coupled with low background alpha counting, would be invoked. 

C. J. Orth and W. R. Daniels, A Rapid Procedure for the Separation of Carrier-Free Thorium from Uranium and Fission Products pp 1-154-155, in Collected Radiochemical and Geochemical Procedures, 5th Edition, compiled and edited by Jacob Kleinberg, DOE Report LA-1721 (May, 1990) for Section C. 

Pillai and colleagues63 improved the method described above and were able to obtain astatinated vinylsteroid hormones 17-a-astatovinylestradiol and its 11-methoxy derivative (25) under carrier-free conditions with yields of ca 80%. In the presence of iodine carrier the yield could be increased up to about 90%. The products were separated from the reaction mixture by extraction, purified and identified by HPLC. 

In this mechanism, pH is the organic compound which decomposes thermally, YH another compound with a labile H atom, m and 3H are the main reaction products (if chains are long), p- is a chain carrier free radical which can decompose by an unimolecular fission, whereas chain carriers of the / type cannot decompose and can only react in bimol-ecular processes. It is assumed, at least as a first approximation, that the radicals Y are thermally stable, i.e. their decomposition by unimolecular fission can be neglected. Notice that transfer processes, intermediary between initiation and propagation processes are not written indeed, these processes can be neglected if chains are long, i.e. at low temperature. 

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