In nuclear reactors, radioactive tracers can be produced by nuclear reactions with neutrons or by the reprocessing of spent fuel elements (fission products and trans­uranium elements). Fission products can also be obtained by the radiation of ura­nium as the target. In accelerators (cyclotrons or linear accelerators), the targets are irradiated with positively charged particles. The general characteristics of nuclear
reactions are discussed in Chapter 6. The most important radioactive tracers are shown separately in Section 8.6.

The preparation of radioactive tracers consists of two steps: the preparation of the isotope by nuclear reaction and the preparation of the desired compound by chemical processes. In most irradiation processes, the chemical species of the target and the product are different, that is, the chemical species needed for the applica­tion cannot always be produced directly. Two main processes are responsible for this. First, the radiolysis of the target can take place during irradiation, resulting in the change of the chemical species. Second, the other constituents of the target can also be transformed in nuclear reactions, and the product can contain other radioac­tive isotopes too. In addition, further reactions that may lead to the formation of undesired side products are the nuclear reactions of the other isotopes and chemical impurities of the target, and secondary nuclear reactions with the already produced radioactive isotopes.

The target has to be selected so that the quantity of the polluting product is kept at a minimum. For this reason, the target has to be a very pure substance and, if possi­ble, in elementary form. Oxides and carbonates are also suitable because the nuclear reactions of oxygen and carbon can be ignored, and the products are stable isotopes.

When the other nuclides of the irradiated element enter in nuclear reactions producing undesirable radioactive isotopes, the target has to be enriched after irradi­ation, that is, the concentration of the isotope has to be increased. For example, nat­ural silver consists of two isotopes: 107Ag and 109Ag. By irradiation of silver with neutrons, 108Ag and 110Ag isotopes are produced in a (n, Y) reaction. When only one of these isotopes is needed, silver isotopes can be separated by mass spectrometry.

Enriched targets are used when the concentration of the target nuclide is very low in the substance with a natural isotopic ratio. For example, 18F isotope is pro­duced from 18O by proton irradiation. In the case of enriched targets, the specific activity of the product nuclide also increases.

In some nuclear reactions, secondary nuclear reactions can also take place. For example, in the production of 125I, the subsequent nuclear reaction of 125I occurs: 125I(n, Y)126I. The effect of the secondary nuclear reactions can be limited by con­trolling the irradiation time or cooling the undesirable isotope if its half-life is shorter than that of the main product.

The desired radioactive isotopes can be separated by radiochemical methods (such as chromatography, ion exchange, distillation, sublimation, precipitation, and thermochromatography). The simpler the method is, the better.

As mentioned previously, the radioactive isotopes have to be manipulated fur­ther to obtain the chemical compounds needed for the specific application, which includes the production conditions (pH, redox potential, etc.), chemical reactions, and purification procedures.

It is important to remember during the production of the radioactive isotope that a carrier-free or just minimally carrier-containing isotope has a high level of spe­cific activity. Carrier-free isotopes can be produced in nuclear reactions in which the atomic number changes or the daughter nuclide of the product is also radioac­tive, and they can be separated from the parent nuclide produced by the nuclear

reaction. For example, the Szilard—Chalmers reaction can be used to produce cer­tain carrier-free radioactive isotopes. This method is based on the recoiling of the produced radioactive isotope, which leads to breaking its chemical bond. In this way, a new chemical compound is formed, and the target and the product, contain­ing different isotopes of the same elements, can be separated by chemical proce­dures because the radioactive and the inactive isotopes are in different chemical compounds. For example, in I(n, Y) I nuclear reactions, the iodine in the target

can be an organic compound or iodate, and the radioactive iodine is present as iodide ion. Bromine and chlorine isotopes have similar nuclear and Szilard—Chalmers reactions. In addition, the same reactions can be used for the inactive chromium, manganese, phosphorus, and arsenic isotopes in chromate, manganate, phosphate, and arsenate ions. No-carrier-added radioactive isotopes with high specific activity can be produced by nuclear reactions with high cross sections, especially when the half-life of the product is too short to allow irradia­tion for a suitably long time, that is, the maximum activity of the radioactive prod­uct can be approached (see Section 6.1 and Eqs. (6.9) and (6.11)).

As discussed in Section 6.2.1, the nuclear reactions with neutrons can easily be created in nuclear reactors. Radioactive isotopes can be produced by the irradiation of a target substance located on the irradiation channels of the nuclear reactors. The other possibility for the production of radionuclides in nuclear reactors is the reprocessing of spent fuel elements. Fission products and isotopes transuranium elements can be obtained in this way. The two methods can be combined: a target containing 235U isotope can be irradiated in the irradiation channels of the reactor, and then the radioactive isotopes can be separated from the target. This procedure is significant in the production of fission products with short half-lives.

As discussed in Section 7.3.2, the first step of reprocessing of spent fuel ele­ments (or the irradiated 235U) is the separation of transuranium elements, in most cases by extraction with tributyl phosphate, followed by subsequent chemical pro­cedures. The number of fission products is about 300, including the isotopes with longer half-lives. These fission products are the isotopes of many chemical ele­ments; therefore, the chemical procedure is usually complicated. At first, the chem­ically similar fission products are separated by such methods as extraction, ion exchange, and precipitation, and then the individual isotopes are separated from the groups of the chemically similar elements.

As an example of the separation of fission products, the separation of 140Ba is shown here. Lead nitrate solution is added to the solution of fission products, and then lead sulfate containing 140Ba(II) ions is precipitated with sulfuric acid (coprecipitation):

140Ba21 1 Pb(NO3)2 + H2SO4 ! (140BaPb)(SO4) 1 H2O (8.17)

The precipitate, polluted with 90Sr, is digested with KNaCO3 and dissolved in nitric acid. Then barium— lead carbonate is precipitated with ammonium carbonate and dissolved in nitric acid again. The precipitation with carbonate and dissolution with nitric acid is repeated until the radioactive purity of the precipitate becomes

high. When the desired purity is attained, concentrated hydrochloric acid is added to the solution at 0°C. Lead ions are precipitated as lead chloride, and barium ions remain in the solution. The residual lead ions are eliminated by electrolysis. By this method, carrier-free 140Ba isotopes are obtained.

Radioactive isotopes can be produced by irradiation with charged particles (as discussed in Section 6.2.3) in cyclotron (see Figure 8.7), or in linear accelerators (see Figure 8.8). This method is older than the nuclear reaction with neutrons in nuclear reactors. As discussed in Section 6.2.6, the heavier transuranium elements have been produced by irradiation with charged particles. During the isotope pro­duction in accelerators, the target becomes very hot; therefore, the cooling is very important, and even cryogens are applied, if required (see Figure 8.9). The require­ments to the target are the same as in the nuclear reactors.

Some radioactive isotopes are produced by spallation reactions too (see Section 7.3.2).

In Figure 6.7, the different possibilities that lead to the production of a nuclide with a Z atomic number and an A mass number are summarized, including the for­mation of the nuclide by radioactive decays. When selecting a method for isotope production, the general nuclear reactions, the requirements of the isotope in terms of purity and use, and the available techniques are to be taken into account.