Experimental Research On The Dependence Of ～(235)U Fission Yields On Incident Neutron Energies

Fission yields are required data for measuring burn-up of reactor's fuel rods, determing power of nuclear weapons and nuclear inspect. Since in the most of devices the neutrons are in white spectrium, it is important to study the dependence of ~235U fission yields on incident neutron energies. The fission yields study group in China Institute of Atomic Energy has been measuring fission yields for many decades. In recent years we are mainly measuring yields of fission products like ~95Zr, ~140Ba, ~147Nd, produced in ~235U's fission induced by neutrons.Fission was found and affirmed by Harn and Strassman in 1938 and 1939. The measurement of fission yields are never ceased. Since the limitation of detect means, radiochemical methods had been main ways to separate products and get pure ones to be detected, so the measured yields were limited in long life products. Electromagnetic devices can separate fission products on line, it can separate product nuclides with lives as short as less than one second. Because fission rate cannot be determined directly, fission yields were get by relative method for every isotopes with same proton numbers. After Ge(Li) detectors with high resolution were made and used for acquiring energy spectra of γ rays, many fission yields data were produced.The experimental study of fission yields at different incident neutron energies has been performed for many years. However strong enough flux of monoenergetic neutrons in region between 3 and 14 MeV are hard to be obtained , the precisions of fission yields in this region are not good enough to get a nice curve of yields versus neutron energies. The data from different labs often differs much more than error bars. So it is necessary to measure fission yields accurately, to clarify the data discrepancies and get credible fission yield- neutron energy curves.We measured fission yields of ~235U by monoenergetic neutrons like thermal neutron,3MeV, 5MeV, 8MeV and 14.8MeV with the direct Y ray spectrometric method. Sample foils with diameter of 16mm, abundance of 235U is 90.2%, mass of 0.7 to 1.6 gram, covered by pure aluminium foil with mass thickness of 54mg/cm2. The experment at thermal neutron was carried out at the research reactor, For other neutron sources, accelerator neutron sources were used. The 235U targets were placed at 5cm distance from neutron sources. Using double fission foils ionizing chamber to count the fission rate while irradiation. Using extrapolation of discriminate threshhold to get fissions below threshold (about 3%). After irradiations every sample was measured by HPGe spectrometry for about 2 months. Efficiency of spectrometry was determined with distance transition by sources with same diameter as samples. Self absorption of Y ray in samples was corrected by measurement of transmission rate. The corrections were also done for influence of low energy neutrons, isotopes of uranium and independent yields. At last we get credible results of fission yields.Plotting the results on semilogarithmic chart, it could be seen that data of 95Zr and 140Ba basically change with neutron energy linearly. But for 147Nd, the linear relationship was not suitable, data points are higher up to 10% from the line at region of 3 to 5 MeV. Further analysis and explanation for these has been done After quantitative calculation with model analysis, almost the same results as experimental data were reproduced.