| The thermal neutron cross section of a fission product (FP) is a critical nuclear parameter. The further experimental determination of the thermal neutron cross sections is not only of theoretical significance in the fields of nuclear configuration and nuclear fission mechanism, but also of practical value in the areas of reactor design, isotope production, neutron activation analysis etc.The measurement of thermal neutron cross sections of long-lived fission products (LLFPs) also has practical application value in nuclear waste disposal. Presently, the Partitioning-transmutation of nuclear waste is a worldwide "rotspots", which is of vital importance to the improvement of the ecological environment and thus the sustainable development of nuclear energy. Before being transmuted of a nuclide in reactors, accelerators or other facilities, the first important thing to know is the accurate reaction cross section of transmutation of this nuclide. Therefore, work is being carried out actively in recent years in the studies and measurement of nuclear data (including the thermal neutron cross section) of the LLPs in many labs in the world, including many American and Japanese labs. But the measurement of nuclear data of the long-lived nuclides has not yet started in our country. So it is very important to start the study in the field of measurement of nuclear data for the long-lived nuclides.For the measurement of the thermal neutron cross section of the FP nuclide, the thermal neutron across sections of some long-lived nuclides, whose half-lives are more than 20a and fission yields are little more than 0.01%, are not fully measured hitherto, especially for 126Sn, 107Pd, 93Zr, 79Se, 93Nbm and 113Cdm. So far these nuclides' thermal neutron cross sections are still to be measured. There exist three major problems: The first is that their content is comparatively very low in the high level liquid waste, causing many difficult technical problems during separation and purification; the second is the difficulties in the measurement of their atom numbers. Some isobars or matrix may interfere in mass spectrometry analysis, and the low background technology is needed for radiochemical method .The third is that their products are either short-lived or stable nuclides, resulting in difficulties of measuring their radioactivities or mass. Based on our existing condition, we think that it is possible to measure thermal neutron cross sections of some LLFPs. Thethermal neutron cross section of 126Sn is firstly selected to research and measure by experiment, and to fill the gap of nuclear data.In the measurement of the thermal neutron cross section of the reaction of 126Sn(n,y) 127Sn8 and 127Snm, the main work involved is as follows:(1) Examination of impurities in reagents and the purification of reagents. The content of some impurities (especially sodium) in some reagents used are measured, such as in deionized water, high purity HCl and HN03, etc. The result shows that Na content from higher to lower is in the sequence of HN03> HC1> deionized water, and the sodium content in these reagents is so high that can interfere the measurement of 126Sn and it's activation products after irradiation. So these reagents must be purified. The deionized water is redistillated in a quartz vessel; HCl and HNO3 are purified by evaporation on natural or heating condition. The concentration of purified HCl and HNO3 is lower than 6mol/L and 3mol/L, respectively. The sodium content of purified deionized water, HCl and HNO3 is then lower than 10"9g/g, and other metals' content is also decreased greatly. So the disturbance to the measurement of 126Sn and its activation product becomes very low.(2) Processing of polyethylene cases. These cases are used for irradiation and for direct measurement after irradiation to avoid the geometric effect on radiation measurement. Meanwhile, the use of polyethylene as case material does not produce any radio nuclides in irradiation.The target material used is PbS, because PbS does not produce -emitting nucl...
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