| This paper describes a measurement method of the fast neutron spectrum by utilizing the principle of the elastic scattering of neutrons and hydrogen nuclei, and recording the results by CR-39 detector. This method has been used for studying spectrum of the leakage neutrons from 14Mev + Pb ball surface.The features of the method are: 1) Small volume of the detector, high space resolution of the measured neutron spectrum, and available in isotropic neutron field 2) based on the needs of neutron energy to be measured, constructing the combination of the detectors for the measured energy, solving the spectrum, the neutron spectrum can be obtained, comparing with other methods, such as threshold detector method, flying-time method, this method can obtain better space and energy resolution.This paper has four chapters, Chapter 1 for introduction, describing the principle, features and purposes of the ADS, and the structure and features of "Qi Ming Xing No. 1 sub-critical facility" ; Chapter 2 for introducing the history and method of neutron energy measurement, three stages: 1932~1959, 1960-1979, 1980~till now;first stage: using ionization chamber etc for measuring,second stage: using Bonner multi-ball method,third stage: computer technique introduced, neutron energy measuring method are also introduced, Chapter 3 for introducing the theoretical model and features of the combined detectors method, also the measuring experimental equipment. The theoretical model of the combined detectors method is to use polyethylene (proton producer) + Pb (or Al) (proton energy attenuation) + CR-39 detector (proton recorder). When neutrons enter into the polyethylene,elastic scattering reactions happen between neutrons and hydrogen nuclei in the polyethylene, after the recoil protons pass through Pb for attenuation, then finally enter into the CR-39 for recording. Ep=Er cos2 , En: neutron energy, Ep: recoil proton energy, : the angle of both, through measuring Epat the angle , En can be obtained. BNDF/B6 provides the elastic scattering cross sections between neutrons and hydrogen nuclei, neutron energy ranges 0~20MeV, dividing 95 energy points.Proton energy ranges 0~19. 5 MeV, divided into 60 energy points. The ranges of the protons with different energy in the polyethylene, Pb and CR-39 detector can be calculated ( see App. - ). Energy response functions and energy resolutions for different combinations have been calculated. The results show the followings:1) The energy response of the detector combination is related to the neutron energy, and the thickness of the polyethylene and Pb. For the fixed thickness of the polyethylene, the peak value of the energy response turns to smaller with the increase of Pb thickness.2) For the fixed thickness of Pb, with the increase of the thickness of the polyethylene, the peak value turns to larger, its position moves towards the high neutron energy and its shape also changes.3) The energy resolution of the detector combination is related to the thickness of Pb and the polyethylene. When the thickness of polyethylene is fixed, energy resolution turns to lower with the increase of Pb, while for the fixed thickness of Pb, energy resolution turns to smaller with the increase of the thickness of the polyethylene.2. The features of CR-39 detector have been studied; Under the certain chemical and electrochemical etching condition, the CR-39 detector can detect the proton track with the certain range of energy. The electrochemical etching can produce the tree track, and the tree track will become larger with theincrease of etching time. The detecting efficiency by electrochemical etching method is higher than that by chemical etching method.3. A set of track auto measuring device has been set up, including an optic microscope, CCD camera, picture collecting card, computer, printer, auto display platform and its controller. The error is less than 5% for the track density with the over 1000 track/cm2.4. An electric chemical etching device has been set up, including a high tension signal generator, etching...
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