Experimental Study Of Neutron Capture Cross Section For ¹⁹⁷Au And ^natSe

In nuclear astrophysics,the origin of the cosmic elements has been a hot research topic.About 99%of the elements heavier than iron in stars are formed by neutron capture processes,and this fraction is produced by the slow neutron capture process(s-process)and the rapid neutron capture process(r-process);the remaining 1%of the elements heavier than iron are produced by the p-process.Precisely measured neutron capture cross section is an important parameter to determine the abundance of the stellar elements;and it is also valuable in nuclear power,accelerator driven sub-critical system(ADS),nuclear waste transmutation,and nuclear data.Back-n of China Spallation Neutron Source(with wide neutron energy region,high flux and good energy resolution)provided an advanced platform for high-precision neutron capture cross section measurements.The C6D6 detector spectrometer of Back-n has a low sensitivity to neutrons and a fast response time,and is widely used as a total energy detection system for neutron capture cross section measurements.The capture cross sections of 35 p-nuclei,such as 74Se,78Kr,and other key elements in the p-process,and the capture cross section of key heavy elements in the s-process,such as 79Se,85Kr,89Y,159Tb,169Tm and other decay branching nuclei,are of great importance.They are the key nuclides in the nuclear reaction path,and their capture cross sections and beta decay rates will determine the reaction path and become the key branching points;the study of the capture cross sections of these nuclides provides the key nuclear physics parameters(neutron energy 0.3-300 keV,corresponding to the cosmic temperature kT=8-90 keV)required for nuclear astrophysics.It is important for understanding the origin of heavy element abundances in the solar system,the evolution of the universe,and the nucleosynthesis process.Neutron capture cross section measurements of 197Au and natSe were performed using the deuterated benzene(C6D6)detector spectrometer on the B ack-n beamline in China Spallation Neutron Source.The thesis is mainly based on the pulse height weighting technique(PHWT)and Monte Carlo simulation method for experimental data analysis;and finally the experimental data in the resonance energy region are fitted by R-matrix code SAMMY,then the resonances parameters such as neutron width and Gamma width are extracted.The main conclusions are as follows:1.The neutron capture cross section of 197Au in the energy region from 1 eV to 100 keV was analyzed by PHWT method;and compared with the ENDF evaluation database and experimental data from from n_TOF and GELINA.Our results are in good agreement with evaluated data and the experimental data of n_TOF and GELINA;thus demonstrating the rationality of the experimental and data analysis methods and providing methodologies for subsequent experiments.Then we fit the experimental data in the 1 eV to 1 keV resonance energy region by SAMMY.2.The neutron capture cross section of natSe in the energy from 1 eV to 100 keV was measured and analyzed for the first time,and the resonance parameters from 1 eV to 1050 eV were fitted by SAMMY to confirm the contributions of each isotope.Three resonance peaks at 27.1 eV(1/2+),270.7 eV(1/2+),and 1018 eV(1/2+)are confirmed for the p-nuclei 74Se contribution,and their fractional decay width parameters are crucial for calculation of nuclear astrophysics production rates.While most of the resonance peaks are contributed by 77Se,some resonance peaks of 77Se have low cross sections due to the thickness of natural Se target(2 mm).It is recommended to measure the capture cross section of the isotope 77Se by using different thickness of samples to further examine and verify the resonance parameters.The resonance cross section at 377.2 eV(76Se,1/2+)was found to be much lower than that of the ENDF/B-Ⅷ.O evaluation database.In addition,the difference between the third resonance peak of 74Se(1018 eV)and that of ENDF/B-Ⅷ.O is about 11 eV,while the difference between the other resonance peaks is 1-2 eV,for which further experiments are need.Back-n is one of beamline on the first spallation neutron source(China Spallation Neutron Source),and the fight length of more than 70 meters is advantageous for the capture cross section measurements in the resonance energy region.However,due to the neutron influence of the resonance peak of the spallation target,and the in-beam Gamma background,there is a certain background deduction error in the accurate measurement of the resonance peak of the low cross section.The neutron capture cross section measurements of natural samples have provided some key neutron nucleus data,which provide some basic data for the enrichment of neutron nucleus database and theoretical calculations of related nuclear physics and nuclear astrophysics.In the future,further precise measurements of isotope cross sections,such as 74Se,76Se,77Se,etc.,will provide more accurate and detailed information on neutron resonance cross sections and resonance parameters.