| The D-T fusion reaction is the primary fusion reaction in the research of thermonuclear weapons and fusion energy. Therefore, the researches focused on the neutron produced by D-T fusion reaction and the secondary neutron from the D-T neutron source become important research subjects automatically. Angular neutron flux spectra, namely, the energy distribution and angular distribution of neutrons, is one of the most basic physical quantities to understand the neutron behaviors in the bulk material system corresponding to the nuclear engineering. In the design phase of the Fusion reactor or the blanket neutronics of the Fusion-Fission hybrid reactor, the angular distribution of the secondary neuron plays an important role in the neutron transport calculation. Compared with the neutron behaviors in the fission reactor, because of the anisotropic scattering of the D-T fusion neutron after interacted with the bulk light atomic nucleus material is so sharp that the angular distribution of the secondary neutron must be considered in the neutron transport calculation. The angular distribution of the secondary neutron emitted from the macroscopic objects is different with the microscopic objects due to the high energy neutron would be multiple scatted in the macroscopic objects and it is much more sensitive to the evaluated cross section data. Furthermore, the angular distribution is not only concerned with the scattering nuclide and neutron energy, but also relative with the macro size and geometry of the objects. Therefore, no matter the designment of nuclear engineering, the verification of the nuclear database and the calculation method of the neutronics design or the integral verification of the physical modeling, the measurements of the angular neutron flux spectra corresponding to the relative materials are extremely important. It is urgent to set up a suitable neutron energy spectra measurement system and data processing and analysis method to content the requirement of the macroscopic neutron integral experiments of the relative materials.Polythene is an important neutron shielding and neutron moderation material. It is composed of C and H elements, whose nuclear reaction cross section data are quite perfect. The neutron integral experiments using a bulk polythene sample can not only verify the neutronics transport theory, calculation method, nuclear database and MC program, but also carry out the experimental test of the established neutronics benchmark experimental measurement system. In this paper, we studied the experimental measurement using proton recoiled method for the angular flux spectra of the secondary neutron emitted from the polythene slab bombarded by the D-T neutron in detail. The detailed study on the experimental technology and the data processing and analysis are summarized as below.In the experimental technology aspect: (1) By using aγ?γcoincident technology, the accurate position of the Compton edge on the Compton recoiled electron spectrum measured by aΦ50.8×50.8mm BC501A scintillation detector was identified to be at 0.90±0.05 and the energy resolution function was also obtained. At the same time, the energy linear results by using different position of the Compton recoiled electron spectrum to process the energy calibration were analyzed and compared. As a result, the best energy linear by using the accurate position of the Compton edge is affirmed which reduces the uncertainty of the effective measurement bias setting and the energy calibration in the measurement process of energy spectra and increases the experimental precision. (2) By using a 252Cf fast fission chamber, a time of flight technology and offline data analysis method, the detailed study about the relative detection efficiency and response functions of theΦ50.8×50.8mm BC501A scintillation detector were carried out. The detector response function that transforming the pulse height spectrum with equivalent electron energy to recoiled-proton spectrum in the energy range of 0.5MeV to 5MeV was experimentally confirmed. (3) The n-γdiscrimination performance of two n-γpulse shape discrimination electronic modules based on the rise-time method and zero-crossing method separately were compared carefully. The former is based on the commercial pulse shape discriminator ORTEC-552 while the latter is based on a set of electronic modules from ORTEC. After comparison, the zero-crossing method turns out to be a good n-γdiscrimination performance, which offers a good alternative n-γdiscrimination method for the setup of the neutron energy spectra measurement system. Under a time statistical model of the photoelectron emission process in the scintillation counters, the intrinsic capability of the n-γdiscrimination performance under optimal condition was evaluated. It provides a theoretical guidance for the further study of the n-γdiscrimination. (4) For the source neutron flux monitor, in addition to the absolute measurement achieved by the associated alpha particle method, a 3He long counter was used to supply the relative measurement, and at the same time, based on the LabVIEW virtual instrument development platform and GPIB instrument control and transmission bus protocol, an auto-timing counts virtual instrument system with ORTEC-974 was developed to monitor the changes of the neutron flux and the running status of the accelerator during the experimental measurement.In the data processing and analysis aspect: (1) Based on the ROOT– a data analysis framework developed by CERN (European Organization for Nuclear Research), a set of offline data analysis programs especially for the SPARROW multi-parameter data acquisition system were written. The deduction ofγray events in the n-γpulse shape discrimination spectrum and pulse height spectrum was completed by offline, which avoided the loss of neutrons or the mixing ofγrays due to the inapposite of the bias setting in the traditional online method. After normalized to the source neutron, the high and low segment recoiled-proton spectra were joined together. (2) For the transformation of proton recoiled energy spectrum to neutron energy spectrum, a simple linear neural network was designed by using the MatLab neural network toolbox for the first time. A set of monoenergetic detector response functions in the energy range from 0.25MeV to 16MeV with an energy interval of 0.25MeV are generated by the Monte Carlo code O5S in the training phase. The MC data and the experimental data using Am-Be neutron source and D-T neutron source were used to achieve the performance test of the trained neural network. It is shown that the trained linear neural network can accomplish the unfolding of unknown spectra from monoenergetic or continuous, simulated or measured neutron source efficiently. Consequently the solving problem of the incident neutron energy spectrum from the proton recoiled neutron spectrum which was measured directly by the scintillation detector was carried out.On the basis of these detailed studies, we set up a neutron energy spectrum measurement system by using a BC501A scintillation detector, a SPARROW multi-parameter data acquisition system and a set of ORTEC electronics instruments. The secondary neutron spectra at 0 degree, 20 degree and 40 degree for the 9cm thick polythene slab and at 0 degree, 20 degree, 40 degree and 60 degree for the 18cm polythene slab were measured. The energy of the D-T neutron is about 15MeV, from T(d, n)4He reaction to be generated by the PD-300 Cockcroft-Walton accelerator neutron generator in the Institute of Nuclear Physics and Chemistry in the Chinese Academy of Engineering Physics. In order to ensure the energy linear of the detector, the measurement was divided two segments in which high segment covers the 2MeV~15MeV and low segment covers 0.5MeV~4MeV, the two segment spectra was joined at the closed point in the range of 2MeV to 4MeV. Compared with previous analogous experimental results, in all cases, the weight of the background neutron and the uncertainty were reduced evidently, especially in the low energy below 3MeV. Meanwhile, the measured effective neutron low threshold was expanded from 1MeV to 0.5MeV.The MCNP5 Monte Carlo calculation program was used to accomplish the MC simulation for the whole experimental measurement system by adopting a total model simulation and a simple model simulation respectively. By C/E ratio between the experimental results and the MC simulated results was compared and analyzed. Through the detailed study in this thesis, the experimental technique of the secondary neutron angular spectra was improved and developed, the accuracy of the experimental measurement system and the data processing and analysis method was verified, which offers a good experimental approach for the development of the neutronics integral benchmark for the physics design of the fusion-fission hybrid reactor and the relative neutron physics items. |
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