Measurements Of The²³⁶U（n,f） And ²³²Th（n,f） Cross Sections At White Neutron Source

The neutron-induced fission cross section is an important data used in the development of new nuclear energy systems,as well as in fundamental and applied nuclear physics research.In response to the problem of poor quality of fission cross section data for key nuclides in the new thorium-uranium fuel cycle and the lack of corresponding experimental data in our country,this paper carried out several fission cross section measurements based on the China Spallation Neutron Source(CSNS)Back-n white neutron source(Back-n).The measurement setups and data analysis techniques were continuously improved.The neutron-induced fission cross section measurement methods for 236U and 212Th were established,and the high-precision and wide-energy-range neutron-induced fission cross section measurement techniques were mastered.By measuring the standard fission cross section ratio of 238U relative to 235V,Uhe reliability of the measurement system and data analysis method was verified.Based on this,measurement experiments and data analysis were carried out using the time-of-flight method and multi-layer fast fission chamber,and the highprecision and wide-energy-range neutron-induced fission cross sections of 236U and 212Th relative to 235U were obtained.High-performance detector is the important foundation of high precision fission section measurement.Based on the multi-cell fission chamber principle prototype,the multi-cell fast fission chamber spectrometer has been completely optimized and upgraded.The upgraded multi-cell fast fission chamber has the advantages of strong electromagnetic shielding,fast signal rising time,stable performance and light weight.The performance test on Back-n white neutron source shows that the upgraded multicell fast fission chamber has good electromagnetic shielding and can effectively distinguish the spontaneous decay alpha signal from electronic noise.The signal rising time of the improved multi-cell fast fission chamber is faster,and the rising time of the fission signal is less than 30 ns,which reaches the international first-class level.Based on a small solid angle counting device,high precision physical quantitative experiments were carried out by measuring the counting rate of alpha particles emitted by the high-purity,large-area fission nuclide targets in a solid angle.The quantitative uncertainties were 0.9%-1.3%.Firstly,the measurement and data analysis of the standard fission cross section ratio of 238U relative to 235U were carried out.Fission cross sections of 238U relative to 235U in the energy range of 0.5 MeV-200 MeV were obtained in single-bunch and double-bunch accelerator operation modes.The experimental data agrees well with the evaluated neutron standard cross section,with a difference of less than 0.5%,which confirms the reliability of the experimental setup and data analysis methods in this paper.Based on the Back-n white neutron source,the measurement of the neutroninduced fission cross section of 236U relative to 235U were carried out using the TOF method and the prototype of the multi-layer fast fission chamber.In the data analysis,flight distance and flight time calibration,detailed analysis of detection efficiency and background,double-bunch spectra unfolding,and multiple corrections were carried out,and the neutron-induced fission cross sections of 236U relative to 235U in the fast neutron energy range of 0.4 MeV-200 MeV and the 5.34 eV-5.56 eV resonance peak region were obtained.The uncertainties of the ratio in single bunch mode were 2.1%3.5%in 2 MeV-100 MeV region and 3.5%-6.4%in 100 MeV-200 MeV region.The measured fission cross sections agree with the theoretical calculation and international mainstream evaluation libraries at 1ost energy points,but the 236LU fission cross section was generally overestimated in the second chance fission energy range.In addition,at the 5.34 eV-5.56 eV resonance peak region,the measured data are in agreement with the existing experimental results,theoretical calculations and data of JENDL-5 and BROND-3.1 evaluations.In single bunch mode of the Back-n white neutron source,the fission cross section measurement of 232Th were carried out using the upgraded multi-cell fast fission chamber.In the data analysis,the flight distance was calibrated using the 8.77 eV resonance peak of the 235U(n.f)reaction,and the neutron flight time was calculated using the constant ratio timing method based on the fission signal and gamma flash signal.The fission cross sections of 232Th relative to 235U in the energy range of 1 MeV-200 MeV,with experimental uncertainties of 2.9%-4.0%in the 2 MeV-20 MeV range,4.0%-5.2%in the 20 MeV-100 MeV range.and 5.2%-7.7%in the 100 MeV200 MeV range were obtained.The 232Th fission resonance peaks observed in the 1 MeV-3 MeV energy range are consistent with previous experimental results and evaluation databases within the uncertainties.The measured 232Th fission cross section is better in agreement with the ENDF/B-Ⅷ.0 evaluation database than other mainstream evaluation databases and compared with the previous measurments and theoretical calculation.Within the experimental uncertainty range,the measured results of this experiment are consistent with the measurement data of Shcherbakov et al.in the wide energy range of 1 MeV-200 MeV.Comparative analysis was carried out between the experimental data and theoretical calculation results,international previous experimental results,and mainstream evaluation databases.The experimental results of neutron-induced 236U and 238U fission cross sections from the fission threshold to 200 MeV energy region have been recorded in the IAEA EXFOR database,which have verified the fission cross section data of the mainstream evaluation libraries at most energy points and provided important experimental data support for clarifying existing data discrepancies,especially in the high energy range above 20 MeV,where experimental data is scarce.The experimental results can provide key nuclear data for the research and development of new nuclear energy systems based on the thorium-uranium fuel cycle.