| Nuclear fission has a wide range of applications in the field of nuclear science,such as nuclear reactors,spent fuel processing,isotope synthesis,astrophysics,etc.These fields have high requirements for the accuracy and quantity of neutron-induced fission cross-section data.In response to these demands,a large number of experiments measuring neutron-induced fission cross-sections have been carried out internationally.However,due to limitations in neutron source levels and measurement methods,there are still significant discrepancies between these data,particularly for highly radioactive nuclides.In China,there have been a few relevant measurement works,mainly some experimental data obtained from quasi-monoenergetic measurements in the 1960s-80s.In recent years,with the construction of the Back-streaming white neutron beam line(Back-n)at the China Spallation Neutron Source(CSNS),preliminary measurements of(n,f)cross-sections for several nuclei such as 236U,232Th based on this device have been carried out domestically,but no measurement works for(n,f)cross-sections of highly radioactive nuclei have been conducted.Accurate measurements of(n,f)cross-sections for 239Pu and241Pu,which are key nuclides in reactor nuclear fuel cycles,are necessary.In response to the significant international discrepancies in the(n,f)cross-section measurements for highly radioactive nuclei,and the insufficient research on(n,f)cross-section measurements for highly radioactive nuclei in China,this thesis mainly focuses on two works:the experimental measurement of the 239Pu(n,f)cross section at CSNS Back-n,and the experimental study of the 241Pu(n,f)cross section using the surrogate reaction method.The specific contents are as follows:1.Measurement of the 239Pu(n,f)Cross Section at CNSN Back-nCurrently,there are significant discrepancies among the evaluated 239Pu(n,f)cross-sections in different nuclear databases.Internationally,the experimental measurements of the239Pu(n,f)cross-section suffer from problems such as low measurement accuracy,large differences between measurements,and a lack of data above 20 MeV.Domestically,only limited energy regions(30 keV5.6 MeV and 14 MeV-18 MeV)of the 239Pu(n,f)cross-section have been published.Therefore,in order to clarify the discrepancies among different data and fill the gap domestically,an experimental measurement of the 239Pu(n,f)cross-section was performed using a white neutron source.The high-flux neutron beams with energy ranging from 0.4 eV to 200 eV was provided by CSNS Back-n.The Fast Ionization chamber for fission cross section Measurement(FIXM)was used to measure the fission fragment signals of 239Pu.High-purity 235U and 238U samples were used as reference samples,and the measured(n,f)cross sections of 235U and 238U from the experiment were used as relative measurements for comparison.The detector signals’ waveforms are digitized and acquired using the CSNS Back-n common electronics and data acquisition system.Offline filtering is performed on waveform data to extract information such as amplitude and time for each trigger event.The amplitude information is used to distinguish between light charged particles and fission fragments,while the time information is used to determine neutron energy through time-of-flight(TOF)methods.During experimental data processing,the detection efficiency of the detector is determined by considering the self-absorption of fission fragments and the loss of extrapolation of the detector signal.Neutron flux is corrected based on its attenuation in different samples,while isotope correction is performed based on the isotopic impurity level of the sample and the(n,f)cross-sections of these isotopes.CSNS generally operates in double-bunch mode,where two proton pulses with a time interval of about 410 ns are generated each time.This can affect the results of data corresponding to neutron energies above 10 keV.A Bayesian iterationbased spectral unfolding method is used to obtain the original fission rate spectrum.The accuracy of this experiment is determined by evaluating neutron scattering background,photo-nuclear reaction background,high-energy neutron-induced reaction background,and verifying consistency between samples of the same type.An analysis of neutron energy resolution,statistical error,and unfolding error was performed to determine the uncertainty in measuring cross-sections.The 239Pu(n,f)cross sections relative to the 235U(n,f)cross sections and the 238U(n,r)cross sections in the energy range of 0.7 eV-100 MeV and 1.8 MeV-100 MeV,respectively,are obtained.The consistency of the measurement results using two reference cross-sections between 1.8 MeV and 100 MeV was good.The measurement results are in good agreement within the error range in the energy range of 4keV-100 MeV with previous experimental data,evaluated data,and TALYS calculation results.2.Experimental Research of the 241Pu(n,f)Cross Section Using the Surrogate MethodDirect measurement of the reaction cross section of short-lived actinide isotopes suffers from difficulties in target preparation and sample handling due to their strong radioactivity.For a long time,experimental and theoretical research on neutron-induced fission cross sections of nuclei with half-lives of a few years or less has been very limited,calling for the development of new measurement methods to obtain these data.There is an urgent need to develop new measurement methods to obtain these data.The surrogate reaction method is currently one of the most promising methods and has been widely used internationally to measure the(n,f)and(n,γ)cross sections of short-lived medium and heavy nuclei.The basic principle of this method is based on the compound nucleus reaction theory,where the formation and decay of the compound nucleus are independent.This method uses a surrogate reaction to form the same compound nucleus as the desired reaction and measure the decay channel of interest.The target reaction cross section can be obtained by theoretical correction.Currently,only a small number of experiments have been conducted in China using the surrogate reaction method to measure(n,γ)cross sections,and no experiment has been carried out to measure(n,f)cross sections using the surrogate reaction method.In order to promote research on the surrogate reaction method and obtain independent experimental data on unstable nucleus(n,f)cross sections,an experimental study was conducted using the surrogate reaction method to measure the 241Pu(n,f)cross section.The experiment was performed using the HI-13 tandem accelerator at the China Institute of Atomic Energy(CIAE).The 238U(180,14 Cf)reaction was used as surrogate reaction to measure the of 241Pu(n,f)cross sections.The experiment utilized a 93.5 MeV 18O beam from the HI-13 accelerator,which bombarded a 238U thin target.The ΔE-E telescopes,composed of largearea silicon strip detectors(SSD),were used to measure and identify light charged particles,while parallel-plate avalanche counters(PPAC)were used to measure fission fragments.The detector signals were processed using NIM analog electronic modules to obtain information such as signal amplitude and timing.Additionally,the ADC and TDC based on VME bus were used for recording.The thin SSDs with a nominal thickness of 20 μm used in the experiment have significant non-uniformity.Therefore,pixel-by-pixel processing of the SSD experimental data was required.The particle’s spatial position and direction were determined based on the mapping between the data acquisition channel and detector position.All pixels of the SSD were calibrated for energy using the online beam elastic scattering peak.The obtained energy values were corrected for detector energy loss and peak position drift.Interested charged particles were selected through the ΔE-E twodimensional correlation spectrum.During the PPAC data processing,the position calibration was performed by using fission fragments produced by the online beam.Particle selection was carried out using nuclear reaction kinematics calculations.A time difference method was employed for the PPAC timing calibration,and the Fission Fragments Mass Distribution(FFMD)of the complete fusion fission fragments was obtained.The coincidence signal of the two detectors provided rich information regarding fission physics,including FFMD,Total Kinetic Energy(TKE)of the fission fragments,fission fragment angular distribution,and their correlation with excitation energy.The fission fragment detection efficiency was computed using Monte Carlo(MC)simulation based on kinematic-related physical quantities.By combining the energy spectra before and after SSD and PPAC coincide,the fission rate of compound nucleus can be obtained as well as the extracted fission barrier height.The compound-nucleus formation cross section of n+241 Pu reaction was calculated using the UNF program.Combining the measured fission rate of the compound nuclei and the Weisskopf-Ewing(W-E)approximation resulted in the 241Pu(n,f)cross sections within the range of 0.125 MeV-18.625 MeV.Overall,the experimental results were consistent with evaluation data.In this work,the neutron-induced fission cross sections of two highly radioactive nuclei,239Pu(n,f)and 241Pu(n,f),were measured using two different experimental methods.The first work established a data processing approach based on white neutron source to measure the 239Pu(n,f)cross section in the energy range from 0.7 eV to 100 MeV,which is the first measurement of fissile nuclide(n,f)cross sections in the neutron energy region from eV to hundreds of MeV in China.The second method established an experimental approach and data processing method to measure the(n,f)cross section of highly radioactive nuclei by utilizing heavy-ion transfer-induced fission reaction as the surrogate reaction.A series of rich physical information including FFMD,TKE,fission fragment angular distributions,and fission barrier heights,as well as their correlation with excitation energy were obtained.This work demonstrated the enormous potential of this method in fission physics research and obtained the 241Pu(n,f)cross section from 0.125 MeV to 18.625 MeV,which agreed well with evaluated and other experimental data,proving the feasibility of the new approach.These two works have addressed the lack of neutron-induced fission cross section data and related research in China,laying a solid foundation for subsequent research work. |
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