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Neutron-induced uranium fission reactions play an essential role in advanced nuclear energy research, basic and applied nuclear science. Based on the China Spallation Neutron Source (CSNS) Back-streaming white neutron source (Back-n), the measurement of the 236,238U fission cross sections relative to 235U from the threshold to 200 MeV was carried out by using time-of-flight (TOF) method with a multi-cell fast fission ionization chamber in double-bunch mode. The TOF spectra in double-bunch mode were unfolded by using Bayesian method. The fission cross section ratios of 238U/235U and 236U/235U were obtained. The experimental data of the 236,238U/235U fission cross section ratios measured in single-bunch mode were re-analyzed in high energy region and the upper limit was extended to 200 MeV for comparison. The experimental results in double-bunch mode are in good agreement with those in single-bunch mode and the main evaluation data within experimental uncertainties. The 236,238U(n,f) cross sections were obtained from the measured ratios and the standard 235U(n,f) cross sections and they agree well with the theoretical calculation, previous experiments, main evaluations and IAEA NDS. The experimental results can provide technical support for further experimental researches in double-bunch mode and relevant nuclear data evaluations. [ABSTRACT FROM AUTHOR]

Aluminum and its alloys are widely used in the nuclear industry. Therefore, it is essential to precisely measure and accurately know the neutron total cross section of aluminum in the wider energy region. The measurement is performed by using the transmission method at the Back-n White Neutron Source of CSNS. Two aluminum samples 70 mm in diameter and thicknesses of 40 and 60 mm, respectively, were positioned at 55 m from the neutron source. The transmission detector consisted of a multi-layer fast fission chamber loaded with 235 U and 238 U, and it was located at the 76-m measurement station. By applying the time-of-flight technique, it was possible to extract the n+ 27 Al total cross section in a wide energy region, from 1 eV to 20 MeV, after the correction for the double-bunch mode of the CSNS accelerator. The total cross sections obtained with the two Al samples are consistent and the results obtained with the 235 U fission cells are in good agreement with that with 238 U in the energy range of 1–20 MeV. The uncertainty of neutron total cross section measured with 235 U for 40 mm and 60 mm thick aluminum is 0.7–22.3% and 0.6–12.4% in the energy range of 10 keV–20 MeV. Results are in fair agreement with respect to previous data and evaluations. [ABSTRACT FROM AUTHOR]

China Spallation Neutron Source (CSNS) has been built and commissioned successfully in 2018 which is implemented by impinging 1.6 GeV protons onto a tungsten target with 25 Hz repetition frequency. An associated white neutron beam line exploiting the back-streaming neutrons, named Back-n, with a flight path of 55 m in endstation 1 (#ES1) and 76 m in endstation 2 (#ES2) was built mainly for nuclear data measurements. The Back-n beam line started running since the beginning of 2018 and the neutron energy spectrum was measured at the very beginning since it is an essential information for feasibility study and analysis of any measurement. In this paper, the measurement of the neutron energy spectrum of Back-n #ES2 is reported. The first result of the neutron flux from 1 eV to 100 MeV measured by a fission chamber based on the 235U(n, f) reaction is presented. The total neutron flux normalized to the proton beam power of 100 kW is 7. 03 × 10 6 neutrons/cm2/s. The results show a good agreement with that of the Monte Carlo simulation as well. [ABSTRACT FROM AUTHOR]

• Measured the 238U/235U fission cross section ratio in 1–20 MeV. • The nonuniformity of the neutron beam profile was considered. • Experimental uncertainties of 2.3%–3.6% in 1.4–20 MeV while 4.1%–11% in 1–1.4 MeV. • Provide information and data support in relevant nuclear data evaluation. Based on the China Spallation Neutron Source (CSNS) – Back-streaming white neutron source (Back-n WNS), the measurement of the U-238/U-235 fission cross section ratio in 1–20 MeV neutron energy region has been carried out by a multi-cell fast fission ionization chamber using the time-of-flight method. The measured resonance peaks of the 235U(n, f) reaction are in good agreement with those of the ENDF/B-8.0 database. The measured six groups of the U-238/U-235 fission cross section ratios in 1–20 MeV neutron energy region agree with those of the ENDF/B-8.0 database in trend, and the average discrepancies between experiment and database are from 1.9% to 2.6%. The relative experimental uncertainties are 2.3%–3.6% (1.4–20 MeV), and the measured data agree with those of database within experimental uncertainties at most energy points. The measured U-238/U-235 fission cross section ratio could provide information and data support in relevant nuclear data evaluation. [ABSTRACT FROM AUTHOR]