The Study Of The Measurement Technology Of Neutron Total Cross Section At The Back-n White Neutron Beam Of CSNS

Neutron total cross section is the basic quantity of nuclear data.It is the sum of the probability of interaction between neutron and nucleus.Neutron total cross section can provide basic information about the internal structure of nucleus and its components,and play an important role in the development and application of nuclear science and technology.Based on the white neutron source(Back-n)of China Spallation Neutron Source(CSNS),an initial neutron total cross section spectrometer(NTOX)is established.NTOX uses Back-n as the neutron source,the neutron energy is in the energy range of 1 eV?200 MeV,and the neutron flight path is about 76 m.This work is performed in the double-mode with an interval between the two bunches of approximately 410 ns,whereby the full width at half maximum(FWHM)of a bunch is 41 ns.NTOX uses a multi-cell fast ionization chamber as a neutron detector.Three235U and three 238U cells are located in the detector.By counting and analyzing the fission events of 235U(n,f)and 238U(n,f)reactions,the neutron counts can be determined.NTOX also includes an electronics system,data acquisition system,and sample changer.In this thesis,NTOX is used to carry out the measurement technology research of neutron total cross section of carbon and aluminum at Back-n.The current situations of neutron total cross section evaluated data of commonly used nuclides in nuclear reactor design are grasped,the reliability of NTOX is verified,and the corresponding fission count-neutron energy spectrum,transmission,and total cross section are obtained.In this paper,the data processing program ROOT developed by CERN is used for data processing and analysis.The flight path is calibrated by fitting the resonance time of flight of 235U(n,f)as a function of its corresponding neutron energy.And the fission count-neutron energy spectra for open beam and sample-in are obtained after energy calibration.The fission count-neutron energy spectrum is compared with the fission-energy of 235U(n,f).And there is a good agreement for all the fission resonances in the energy range of 5?20 eV.The statistical uncertainty of the neutron spectrum for open beam in the measurement of the total cross section of carbon is mostly less than 3%at 1?20 MeV.In addition,the background of the neutron energy spectra is also analyzed.In order to make the neutron transmission in the more interesting energy range between 0.5?0.7,the thickness of graphite used for measurement is 4 cm,and the thickness of aluminum is 4 cm and 6 cm.Based on the neutron energy spectrum,the neutron transmission distributions of the graphite and aluminum in the energy range of 1 eV?20 MeV are obtained.The neutron total cross sections of carbon and aluminum in the energy range of 1 eV?20 MeV can be obtained based on the neutron transmission and the atomic density of the samples.The measured total cross sections of aluminum samples with different thicknesses are in good agreement,which verifies the reliability of the measurement system and method.In order to obtain accurate experimental results,deeper data processing and analysis are made in this work.A database broadening program based on Gaussian is developed to make the single/double broadening of neutron total cross section evaluated data.And an unfolding procedure that can unfold the double-mode neutron spectra is developed by a member of the cooperation.The neutron total cross sections before/after unfolding of carbon and aluminum have a good agreement with the double/single broadening of ENDF/B-?.0 library.The measured total cross sections and other existing experiments are also in good agreement.In addition,the uncertainty analysis of the measured total cross section of 4 cm and 6 cm thickness aluminum can be seen in this thesis.The uncertainty of neutron total cross section measured by 235U is 0.70%?22.26%and 0.63%?12.38%in the 10 keV?20 MeV energy range,respectively.There are a total of 330 energy points at 10 keV?20 MeV,there are 77 energy points in the range of 0.15 MeV?1.5 MeV with an uncertainty of less than 1%for 4 cm aluminum,and there are 115 energy points in the range of 0.16 MeV?3 MeV with an uncertainty of less than 1%for 6 cm aluminum,which reaches the initial measurement goal.The uncertainty of neutron total cross section measured by 238U for 4cm and 6cm aluminum is 1.58%?6.66%and 1.33%?4.89%in the energy range of 1MeV?20MeV,respectively.The neutron total cross section measurement technology in this paper provides a reference for the total cross section measurement at Back-n.The measured results are important for the evaluation and verification of nuclear data and the development of nuclear energy and nuclear technology.