Neutron capture cross sections were determined on the basis of Westcott’s conven­tion [1] by an activation method. The results for LLFPs [2—23] are listed in Table 5.1, and for MAs [24—31] in Table 5.2, together with previously reported data. Here, the symbols oeff, o0, and /0 denote the effective cross section, the thermal neutron capture cross section, and the resonance integral, respectively; o0 is the cross section at the neutron energy of 25.3 meV.

Nuclear waste sometimes contains a large amount of stable nuclei having the same atomic number as that of long-lived fission products. These stable nuclei absorb thermal neutrons during the neutron irradiation of the nuclear waste and affect the neutron economics; the reaction rate of the target nuclei is reduced. Moreover, some of these stable nuclei breed more radioactive nuclei by the neutron capture process. It is also necessary for transmutation study to accurately estimate these influences caused by stable nuclei involved in the FP targets. The cross sections of the stable nuclei, such as I [14] and Cs [20], were also measured; the results are shown in Table 5.1.

As seen in Table 5.1, the thermal cross section for 137Cs is about twice as large as the previous data reported by Stupegia [2]. As for 90Sr, its thermal cross section is

found to be much smaller than the data reported by Zeisel [6]. As seen in Table 5.2, the cross section of 238Np is obtained for the first time. Thus, the joint research activities of the Japan Atomic Energy Agency (JAEA) and universities have measured the cross sections for important LLFPs and MAs by the activation method.