热中子诱发^(239)Pu裂变的^(85)Kr m、^(87)Kr和^(88)Kr产额测量研究

Determination of^(85)Kr^(m),^(87)Kr and^(88)Kr Yields of^(239)Pu Fission Induced by Thermal Neutrons

中子诱发^(239)Pu裂变的^(85)Kr^(m)、^(87)Kr和^(88)Kr的产额是重要的核参数,目前国外实验数据较少而国内尚未见实验报道。基于西安脉冲堆跑兔系统辐照Pu靶开展了热中子诱发^(239)Pu裂变的^(85)Kr^(m)、^(87)Kr和^(88)Kr的产额测量研究。纯化后的钚溶液通过滴定后阴干的方式制靶,靶辐照后结合γ无损分析和气-固分离制源测量等方式测量裂变产物。采用有机玻璃扁平面源等效石英管源、不锈钢大面源等效气体源,并结合蒙特卡罗模拟实现了3类实验样品的γ能峰探测效率曲线的等效法刻度。以相同方式制备的^(235)U靶开展气-固分离制源实验验证了钚靶中^(85)Kr^(m)、^(87)Kr和^(88)Kr气体释放率的一致性。根据实测目标产物与^(99)Mo的相对产额,以ENDF/BⅧ.0评价数据库中^(99)Mo产额为基准,经非主中子场和非主同位素影响等校正后,得到了热中子诱发^(239)Pu裂变的^(85)Kr^(m)、^(87)Kr和^(88)Kr的产额,分别为(0.559±0.020)%、(0.992±0.034)%和(1.264±0.037)%,测量结果与数据库评价值在不确定度范围内一致,与文献测量值相比,不确定度明显降低。所建立的实验方法及干扰扣除方法可为后续更多中子能点的气体裂变产物产额的测量研究提供参考。

The yields of^(85)Kr^(m),^(87)Kr and^(88)Kr of neutron-induced fission of^(239)Pu are important nuclear parameters.They have a moderate half-life,areβ-decay and emissionγ-rays with a certain probability,so it is appropriate to useγ-ray spectrum method to determine their yield.At present,there are few experimental data with large deviation abroad and no experimental results have been reported in China.The yield measurements of^(85)Kr^(m),^(87)Kr and^(88)Kr of thermal-neutron-induced fission of^(239)Pu were carried out based on the irradiation of plutonium target by Xi’an Pulsed Reactor rabbit system.The purified plutonium solution was then titrated and dried to prepare the silica-encapsulated plutonium target.Three kinds of measurement experiments were designed:γ-ray nondestructive method for the relative yield of^(88)Kr and^(99)Mo,gas-solid separation method for^(85)Kr^(m),^(87)Kr and^(88)Kr andγ-ray nondestructive method for the relative yield of^(85)Kr^(m),^(87)Kr and^(88)Kr.After the target irradiation and cooling,the fission products were measured by the above-mentioned methods.The efficiency of the three kinds of experimental samples was calibrated by the equivalent efficiency curve method.The plan source of organic glass package was used to equate the efficiency calibration of quartz tube source,and the plan source of stainless steel package with the same thickness and material as the measuring source was used to equate the efficiency calibration of 50 mL gas source.By Monte Carlo simulation,the suitable equivalent position was selected,and the efficiency value was fine corrected(less than 0.8%).The uranium target was prepared in the same way of plutonium target.The relative yields of^(85)Kr^(m),^(87)Kr and^(88)Kr of^(235)U fission induced by thermal neutron measured by gas-solid separation after irradiation are consistent with the recommended values of the evaluation database,which indicates that the release rates of^(85)Kr^(m),^(87)Kr and^(88)Kr in the plutonium target are the same.After modified by the influence of non-primary neutron field and non-primary isotope,the fission yields of^(85)Kr^(m),^(87)Kr and^(88)Kr were obtained based on the measured relative yield with reference product^(99)Mo and the yield of^(99)Mo given by ENDF/BⅧ.0 evaluation database.The measured yields are(0.559±0.020)%,(0.992±0.034)%and(1.264±0.037)%,respectively,which are consistent with the evaluation value of the database and the uncertainty is significantly reduced compared with other literature value.In this paper,the retrieval and deduction methods ofγ-ray energy peak interference ratio in measurement are universal,but the uncertainty of theoretical yield may be added.The experimental method in this paper can also provide reference for the measurement of fission product yield at different neutron energy points.