Measurement Of ²³²th Neutron Capture Cross Section Based On CSNS Back-n Facility

Accurate knowledge of neutron-nucleus reaction cross sections is of primary importance in studies of fundamental nuclear physics and in applications of advanced nuclear energy and nuclear technology.Important information of the nuclear structure and reaction models can be obtained directly from high-resolution neutron resonance cross sections.However,the studies on basic nuclear physics and applications such as thorium-based nuclear fuel cycle have pointed out that the measurements of neutron nuclear reaction cross section are of insufficient accuracy and sometimes even lacking for many nuclides.Therefore,to provide nuclear data for basic and applied research,it is very important to carry out the measurements of the neutron-induced reaction cross section at advanced neutron sources.Compared to the traditional uranium-plutonium fuel cycle,the importance of thorium-based nuclear fuel has become increasingly prominent because of its excellent breeding capability in both thermal and fast reactors,lesser long-lived minor actinides resulting from fission,and abundant reserves of thorium.At present,Th-U fuel cycle based on advanced critical reactor nuclear energy system（TMSR）and on subcritical accelerator-driven subcritical systems（ADS）attracts growing attention.The design and development of these innovative nuclear energy systems require precise neutron nuclear data of the key isotopes,as the simulation reliability of the performance for these systems strongly depends on the cross sections of neutron-induced reaction.The Th-U fuel cycle is based on the fissile ²³³U and fertile ²³²Th,formed by neutron-induced capture on ²³²Th.²³²Th is the‘beginning’nuclide of the thorium-uranium cycle chain,which makes the neutron capture data of ²³²Th an important input for the design and safe operation of the reactor core.In addition,a sensitivity analysis of nuclear data for the Th-U fuel cycle in molten salt reactor shows that a 1%disturbance of the ²³²Th neutron capture cross section would result in a 0.35%change of the_efffor the core system,which makes the accurate measurement of²³²Th（n,γ）cross section important for serving the neutron nuclear data requirements of the TMSR system.The available data of ²³²Th（n,γ）cross section are mainly from evaluated nuclear data libraries or existing experimental data included in EXFOR.Although most of these measurements were measured with high precision,the discrepancies between different data still exist and cannot be ignored.The white neutron source based on the time-of-flight method is an important platform for neutron capture cross section measurement.However,the measurement of ²³²Th（n,γ）reaction cross section in the resonance region is empty due to the lack of suitable white neutron sources in China before the Back-n facility（Back-n）was bulit.Back-n is a newly built neutron time-of-flight spectrometer at the China Spallation Neutron Source（CSNS）,which has been constructed with the aim of studying the neutron-induced reaction cross section for supporting the eminent demands for an enhanced nuclear database in China.With a wide neutron energy spectrum,high neutron flux,and high energy resolution of the Back-n facility,it provides an opportunity for the measurement of²³²Th（n,γ）reaction cross section in the resonance region.The measurement of the ²³²Th（n,γ）reaction cross section is carried out at CSNS Back-n facility in this work,and the（n,γ）reaction cross section in the resonance region of ²³²Th is studied for the first time.Back-n provides an important platform for the measurement of ²³²Th（n,γ）reaction cross section,however,this work also faces a series of challenges in the experimental measurement of cross section,the determination of energy resolution function and the evaluation of resonance parameter.The main contents of the paper are organized as follows:（1）The ²³²Th（n,γ）reaction cross section was measured at the Back-n facility,and the measurement scheme and data analysis method of the（n,γ）reaction cross section based on the time-of-flight method and the C₆D₆liquid detector were studied and summarized in detail.The detected particles of the（n,γ）reaction cross section areγphotons.Considering the multiplicity of theγ-ray cascade,a capture event may generate multipleγrays,so the C₆D₆detector efficiency needs to be corrected.To reach the correction of the detection efficiency,the pulse height weighting technique is used,and a weight function is weighted to the pulse height spectrum of the C₆D₆detector based on simulation,which makes the detection efficiency related to the kinetic energy of the neutron.Then,the neutron capture yield and capture cross section are obtained combining with the background analyses.（2）The energy resolution function of the Back-n facility is studied and calculated based on the Geant4 Monte-Carlo simulation,which provides data reference for the resonance parameter evaluation of the capture cross section data.The energy resolution function of the time-of-flight spectrometer describes the relationship between the energy resolution and the neutron energy measured at the facility.The energy resolution function represents the intrinsic broadening effect of the resonance peaks,which is crucial to the determination of resonance parameters.For the constructing of resolution function,the most crucial component need to be investigated is the neutron production and transport in the spallation target,which could only be inferred from detailed Monte-Carlo simulations.In present work,the production and transport of neutrons in the target–moderator–reflector assembly of the CSNS are simulated by Geant4 tookit,and the simulation data are analyzed to deduce the distributions of the time spent of neutrons’transport in the assembly.The RPI energy resolution function was used to fit and analyze the time distribution of the moderated neutrons of the Back-n,and a set of parameters were obtained for determining the energy resolution function of the facility,which offers data reference for the correction of resolution broadening effect for the data measured at Back-n.（3）The preliminary evaluation of the experimental data of the ²³²Th（n,γ）reaction cross section was completed using the multi-level and multi-channel R matrix code SAMMY based on Bayes’equations.SAMMY is a widely used tool for theoretical analysis of neutron resonance cross-sections.However,this code is rarely used in China due to various constraints.Therefore,the code SAMMY is described in detail in this paper,and the nuclear data evaluation process and important inputs based on the white neutron source are summarized.The theoretical capture cross section was calculated by the SAMMY code and fitted with the experimental data.The resonance parameters in the resolved resonance region and the unresolved resonance region are preliminarily analyzed,which provides a reference for the further study of resonance parameter in the future.