The most frequent sources of nuclear reactions generating artificial radioisotopes are neutrons produced through uranium fission. Based on their kinetic energy, these neutrons can be classified into three groups (as discussed in Section 6.2.1): thermal, epithermal, and fast neutrons, and each of these generate different nuclear reac­tions. The ratio of the neutrons belonging to the three groups is different at various points of the reactor core, so it is possible to select the neutron energy necessary for the required nuclear reaction either by selecting the appropriate irradiation channel or by shielding the target with cadmium foil.

Neutron flux in research reactors (i. e., in reactors not used for energy produc­tion) is typically in the range of 1012—1015 n cm-2 s-1. While in reactors at the lower end of this range, low-activity radioisotopes can be generated, and reactors with higher neutron flux result in products with high specific activity and provide a means of cost-effective radioisotope production. For example, a research reactor with 20 MW power has neutron flux in the range of 1—3 X 1015 n cm-2 s-1. Other, low-power reactors (e. g., training reactors) are not suitable for regular, technology — based isotope production, but only for physics measurements and neutron-activa­tion analysis.

Considerations for target selection were discussed in Section 8.5.2, while time — dependence of the neutron activation is found in Section 6.1. Besides, the irradia­tion time belonging to a given flux has to be calculated based on the activity to be achieved and on the target mass.