High Precision Measurement Of 181Ta Neutron Capture Cross Sections Based On China Spallation Neutron Source

The neutron radiative capture cross section is an important part of nuclear data.Tantalum and its alloys as possible structural materials for nuclear facilities,and the important position of Tantalum-181 in the main s-processes of stellar nucleosynthesis,make the neutron radiative capture cross section of Tantalum-181 valuable for applications in the field of nuclear facility design and nuclear astrophysics research.However,few researchers have ever measured the neutron radiative capture cross section of Tantalum in the resonance energy region.This brings difficulties and unreliabilities to the research on Tantalum-related nuclear facility design,nuclear astrophysics research,and computer simulation,which require high precision.China’s first spallation neutron source,China Spallation Neutron Source（CSNS）,was completed and put into operation in 2018.A back-streaming white neutron source（Back-n）for the measurement of nuclear data such as neutron radiation capture cross section,fission cross section,and neutron total cross section has been established at CSNS.Back-n has been in operation for nearly four years,during which time many nuclear data measurement experiments have been carried out,including the measurement of neutron radiation capture cross sections,which provides a feasible experimental method and data analysis method for this experiment.In this work,based on the Back-n of CSNS,the neutron radiation capture cross section measurement of Tantalum in the resonance energy region and unresolved resonance region is carried out.This experiment uses four C₆D₆scintillator detectors to directly measure theγ-rays emitted by the sample after capturing neutrons,and the neutron radiative capture cross section of the sample is calculated based on theγ-ray count rate,detection efficiency,sample parameters,and neutron energy spectrum.Among them,the sample parameters are directly obtained by measurement,and the neutron energy spectrum is given by the CSNS energy spectrum measurement group.Theγ-ray count rate is also measured by the C₆D₆detector,and the counts of empty targets,carbon targets and lead targets are used to simulate the environmental background,scattered neutron background and in-beam gamma background in the experiment in different ways,and the count of the Tantalum sample is deducted from these backgrounds,and thus the true count of Tantalum samples was obtained.The detection efficiency is then determined by the Pulse Height Weighting Technique（PHWT）.The weighting function of the C₆D₆detector is simulated using the Geant4 toolkit and applied to the pulse height spectrum of the detector so that the detection efficiency is proportional to the total energy of the cascadedγ-rays（i.e.,the sum of neutron binding energy of the sample and the incident neutron energy）,resulting in an accurate detection efficiency.The double-bunch unfolding method was also used in the data analysis to attenuate the effect of the double-bunch mode.In addition,corrections were made for multiple scattering of neutrons within the target.This work presents the neutron capture yield and cross section of Tantalum-181 from 1 to500 e V.The yield uncertainty is below 8.8%and the cross section uncertainty is below 10.2%,mostly due to the uncertainty contributed by the neutron spectrum.The experimental results are consistent with the evaluated databases such as ENDF/B-VIII.0 and JENDL-5 in most areas.For the differences between the five resonance peaks at 34e V,56e V,157e V,287e V and328e V between different evaluated databases,corresponding judgments are also given according to the experimental data.The experimental data and the evaluated databseare quite different at the resonance peaks of 90e V,348e V,473e V,etc.We discuss the specific reasons and give possible explanations.In addition,we also give the experimental results in the continuous energy region（2.4ke V-1Me V）,and make a detailed comparison with the evaluated databases such as ENDF/B-VIII.0,JENDL-5 and the experimental data on the EXFOR database.At the same time,the maxwellian averaged cross section at k T=5-100ke V is also given,including the value of k T=30ke V that is concerned by nuclear astrophysics,and compared with several evaluated databases and the existing experimental data,possible explanations are given for some of the differences.Later,we will perform R-matrix fitting to give the resonance parameters and cross section values of the all-energy region recommended by our experiments.