Production of radionuclides

Three primary problem areas exist in dating groundwater. These are (1) Formulation of realistic geochemical-hydrodynamic models needed to interpret data which are generated by field and laboratory measurements, (2) development of sensitive and accurate analytical methods needed to measure trace amounts of various stable and unstable nuclides, and (3) theoretical and field oriented studies to determine with greater accuracy the extent and distribution of the subsurface production of radionuclides which are commonly assumed to originate only in the atmosphere. 

Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to 14C dating of water which is more than 40,000 years old. 

Most importantly, the radionuclide and the stable nuclide must undergo isotopic exchange. In practice, this means that the tracer and the stable atom must be in the same redox state. By heating or using redox cycles, the experimenter must assure this to be true. Anomalous experimental results have frequently been traceable to the chemical form of the administered radiotracer. Since reactor production of radionuclides often results in side reactions (see Chapter 10), various oxidation states may be present when the sample is produced. In one case involving phosphate-32P uptake in plants, the unexpected experimental results were explained by the fact that a large percentage of the tracer dose was actually in the form of phosphite-32P. 

Figure 1. Chart of nuclides from carbon to sodium, illustrating vai ious processes for production of radionuclides (10)...

For production of radionuclides during the case of single-stage, continuous exposure without erosion, burial or uplift during nuclide production, Lai (1991) has shown that concentration of the radionuclide scales as ... 

Spallation the production of radionuclides such as 14C, produced by reaction of cosmic rays with atmospheric atoms such as N2, O2, and others to produce broken pieces of nuclei. 

In modem machines, protons, deuterons and a particles with energies of several 100 MeV up to about 1 GeV are available. Proton linacs serve frequently as injectors of 50 to 200 MeV protons into proton synchrotrons. For the production of radionuclides, relatively small cyclotrons are used by which particle energies of the order of 10 to 30 MeV and ion currents of the order of 100 pA are available. Radionuclides obtained by reactions with protons exhibit decay or electron capture (s). 

The influence of irradiation time on activation is illustrated in Fig. 8.8. For production of radionuclides, irradiation times of up to several half-lives are sufficient. About 99% of the saturation activity is obtained after irradiating for seven half-lives. 

For the production of radionuclides, (n, y) reactions are of greatest practical importance, because in many cases the cross sections are relatively high and high fluxes of neutrons are available in nuclear reactors. 

A survey of neutron-induced reactions used for the production of radionuclides is given in Table 12.1. (n, 7) reactions are preferably triggered by thermal neutrons giving isotopic products of limited specific activity. 

Reactions with fast neutrons, such as (n, 2n), (n, p) and (n, a) reactions, are only of minor importance for production of radionuclides in nuclear reactors. However, special measures may be taken for irradiation of samples with high-energy neutrons. For instance, the samples may be irradiated in special fuel elements of ring-like cross section as shown in Fig. 12.1, or they may be irradiated in a receptacle made of enriched uranium. In both cases, the fast neutrons originating from the fission of enter the samples directly and their flux density is higher by about one order of magnitude than that at other places in the reactor. 

Table 12.5. Reactions with deuterons applicable to production of radionuclides...

Activation analysis is based on the production of radionuclides by nuclear reactions. The specific activity is given by the equation of activation,... 

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