Summary Report on "An Analysis of Incident Neutron Energy Dependence of beta delayed Neutron"

Full text: We discussed incident neutron energy (En) dependence of beta-delayed neutron yields (DNYs) produced by neutron-induced fission. According to nuclear data evaluations on the open market and several theoretical calculations, the energy dependence can be separated into three parts. The first part is moderate change from thermal and resonance regions to En ~ 4 MeV, and the second is steep decrease from En ~ 4 to 7 MeV. The third is slow decrease at high En region. We first discussed the moderate change from thermal to MeV region. In this energy region, Yi at resonances were totally smaller than that at the thermal neutron energy. As a result of this resonance dependence, the DNY gave slight increase toward En ~ MeV region. Next, we discussed the steep descent from En~4 to 7 MeV. We gave several examples which were able to explain the energy dependence. First one was the effect of second- & third-chance fissions, but it did not give any conclusive influences on the energy dependence. "Would-be Precursor" and Odd-Even effect provided a reasonable result for the energy dependence. In particular, excitation energy and charge distributions to fission fragments were shown to be important to obtain accurate energy dependence. We also pointed out the effect of decrease of fission fragment being precursor. Then, we discussed the slow decrease at high En region. In this energy region, there are only a few experimental values. Therefore, simulation calculation may be a powerful tool. Considering the energy dependence of Yi, namely, increasing fraction of symmetric fission, the energy dependence of DNY should give a slight decrease. We plan to examine this energy dependence by the next CRP meeting. We also introduced some preliminary results of our microscopic calculations. Our formalism is based on SHF+BCS & proton-neutron QRPA. T1/2 of Z=50, 82 isotopes and N=126 isotones were calculated. In addition, Pn of Z=50 isotopes and N=82 isotones were shown. T1/2were reasonably reproduced, while the results of Pn were not good accuracy comparing with experimental data. We also showed problems in the decay-scheme around heavy nuclei where first-forbidden transition becomes important. The microscopic calculation may help providing Pn data, however several improvements were still required. We plan to calculate all the even-even nuclei systematically and extend our formalism to odd-nuclei, looking for appropriate improvements. (author)