| Neutron spectrum is an important characteristic parameter of radiation field of a critical assembly.The measurement of neutron spectrum is an important content of the neutron integral experiment.The main significances of neutron spectrum measurementsare as follows:1)In the neutron physics experiment,the measured neutron spectrum is compared with the theoretical calculation result,so as to test or improve the theoretical calculation methods,procedures and parameters.2)In ICSBEP,the measured neutron spectrum can be used to test the theoretical calculation results.3)The reasonable division of the group constants depends heavily on the measurement accuracy of the neutron spectrum.4)The difference of displacement ratio of radiation damage to Kerma factor between several keV and several MeV neutrons with different energy is two orders of magnitude,and accurate measurement of energy spectrum is the basis of the research of anti-nuclear strengthening of electronic.5)The neutron dosimetry research carried out on critical assembly and the calibration of neutron detector need accurate energy spectrum parameter.A critical assembly is a nuclear reactor device forcritical experiments.Its core is mainly composed of 235U or 239Pu material with high concentration,and there are no thermalized and cooling systems.Therefore,the slow-thermal neutron component is relatively small.More than 80%of the main neutron energy is in the range of 0.1 keV?4 MeV,and the average energy is about 1MeV.It is also called fast-neutron critical assembly.At present,in order to verify the theoretical calculation results,the neutron physics experiments put forward higher requirements for the neutron spectrum:the measurement range is 10 keV?10 MeV,and the uncertainty in measurement in the range of 0.1?4 MeV is less than 10%,It is difficult to obtain wide neutron spectrum range and high measurement accuracy,especially the spectrum below 0.3 MeV is basically blank.The commonly used methods of neutron spectrum measurement are neutron activation technique and semiconductor sandwich neutron spectrometer technique.Among them,the activation foils with a threshold below 1.2 MeV are very few,and the neutron measurement in the range of 10 keV?1.2 MeV cannot be realized,and the poor accuracy of activation reaction cross section and resolution spectrum results in the poor accuracy of the activation method.The traditional 6Li sandwich semiconductor spectrometer based on Au-Si surface barrier detectors and NIM electronics is a classical instrument used for the measurement of neutron spectrum in critical assemblies.However,the main problems existing in this technology are as follows:1)the poor consistency between two Au-Si surface barrier detectors and NIM electronics results in poor resolution of thermal neutron "sum" peaks,which affects the lower limit range of spectrum measurement.2)The problem of energy spectrum drift caused by fast neutron radiation damage of the Au-Si detectors is severe,which makes the results from the traditional 6Li sandwich semiconductor spectrometer questioned for a long time.3)Because of the poor consistency of the traditional Au-Si detectors,the background response is inconsistent,and this affects the background subtraction accuracy.4)The traditional NIM electronics system is complex,and the settings and measurement are inconvenience,and the accompanied gamma rays seriously disturb neutron signals.Therefore,the resolution of the thermal neutron "sum"peak of the traditional 6Li sandwich spectrometer was generally more than 300 keV(6LiF thickness?500 nm),the range of neutron spectrum measurement was 0.3 MeV-7.5 MeV,and the uncertainty was 13%in the range of 0.5 MeV?4 MeV.In this paper,we propose a new type of 6Li sandwich digital spectrometer for neutron spectrum measurement,which is based on PIPS(Passivated Implanted Planar Silicon)detectors.The PIPS detector has the advantages of strong radiation resistance,low leakage current,stable PN junction and good consistency.It can improve the thermal neutron response resolution and background subtraction accuracy.Digital spectrometer can accurately acquire the energy deposited from the PIPS detectors.The digital analysis and processing of the alpha and triton particles produced by 6Li(n,?)T reaction can simultaneously realize the measurements of tritium spectrum and "sum" spectrum.Therefore,the 6Li sandwich digital spectrometer based on PIPS can improve the measurement range,accuracy and reliability of neutron spectrum.The 6Li sandwich probe and background probe based on PIPS detector were successfully fabricated.The leakage current of PIPS detectors was less than 10nA with full depletion at 30V.The energy resolution of 226Ra-a source was less than 0.96%.For the 239Pu-a source with energies of 5.15 MeV and 5.499 MeV,the relative standard deviations of the alpha peaks of the 18 PIPS detectors were 0.06%and 0.08%,respectively.This proved the excellent alpha response consistency of PIPS detectors.The technology of 6LiF film deposition on the surface of the PIPS detectors by electron beam evaporation had been achieved.And 6LiF film with thickness of 500 nm had been successfully fabricated on the PIPS surface.A 6Li sandwich digital spectrometer based on PIPS detector was successfully developed.Six pulse amplitude spectra were obtained by once measurement.The settings of gain,threshold and coincidence resolution time of the digital spectrometer were more accurate and convenient.The parameters of the 6Li sandwich digital spectrometer were as follows:the resolutions of triton peak and "sum" peak with the thermal neutron response were 45.9 keV and 221.8 keV,respectively.And the detection efficiency of thermal neutron was 0.4%.Two methods of 226Ra-a source and thermal neutron response were used to improve the accuracy of energy calibration of the digital spectrometer system.The channel width of the spectrometer system was(11.±0.1)keV.The Linearity was observed up to 0.9999,indicating excellent energy linearity of PIPS semiconductor detectors for charged particles.The energy deposition of alpha and triton charged particles and accompanying gamma rays were obtained from the neutron response produced by the accelerator.The optimum operation conditions were determined as follows:biasvoltage at 6V,discrimination threshold at 1.5MeV,and 1mm Cd sleeve on the probe surface.The six spectra of 6Li(n,?)T reaction with six single energy neutron points in the range of 144keV?5MeV were obtained.The 6Li sandwich digital spectrometer was used to measure the neutron spectrum in a critical assembly.Simultaneous measurement of Trion-spectrum and "sum"-spectrum had been realized.The energy deposition process of neutron and gamma response in PIPS detector has been clearly obtained.A method for gamma rays discrimination had been established.The resonance peak near 250keV had been measured for the first time by a higher resolution T-analysis technique.The effect spectrum and background spectrum at power of 0.53W and 0.92W were obtained.The neutron counting rates were 10.00 cps and 16.93 cps,respectively.The background spectrum ratio was 7.7%for each power.The influence of irradiation damage on the measurement of the effect spectrum was studied.The relative standard deviation of the peak of the "sum" peak was 1.4%after five runs of irradiation,and the change of the probe performance was?1%after fast neutron irradiation with fluence of 3.4×1011/cm2.it was proved that the radiation resistance of PIPS detector was better than that of Au-Si surface barrier detector,and the spectrum drifting existed in the traditional 6Li sandwich spectrometer was solved.Reducing the coincidence resolution time could greatly improve the anti-gamma accumulation ability of the probe.Measurements showed that the gamma discrimination ability wih coincidence time of 100 ns was 44 times that of 600 ns.The neutron spectrum in the range of 5 keV-10 MeV was obtained by using ROOUNFOLD program unfolding.The ratio of 0.1 MeV to 4 MeV was 87%(relative uncertainty:?9.1%)and the average neutron energy was 1.09 MeV(? 0.1 MeV neutron).The measured results were in good agreement with the theoretical results.Through the above research,a 6Li sandwich digital spectrometer based on PIPS detectors was established.The problems of energy accurate calibration,background precise subtraction,gamma rays accumulation removal and the influence of radiation damage on neutron spectrum measurement were effectively solved,and the measurement range and accuracy of neutron spectrum of critical assembly were improved.The necessary conditions of neutron spectrum measurement for the research of neutron physics experiments have been established.Through the research of this paper,the main conclusions are as follows:(1)The PIPS sandwich probe with strong radiation resistance,low leakage current,high resolution and good consistency was successfully developed.The resolution of triton peak and"sum" peak of thermal neutron response were 45.9 keV and 221.8 keV,respectively.(2)The simultaneous measurement techniques of triton and "sum" spectrum were established,and a 6Li sandwich digital spectrometer was fabricated,and the energy deposition process of neutron and gamma response was obtained.(3)Two energy calibration methods,226Ra-? source and thermal neutron response,were established.The channel width was(11.3+0.1)keV,and the fitting linearity was 0.9999.(4)An accurate measurement method of background response deduction was established.The effect spectrum and background spectrumat at power of 0.53W and 0.92W were obtained,and the background response share was 7.7%.(5)The anti-irradiation ability of PIPS sandwich probe was improved.After fast neutron irradiation with fluence of 3.4×1011/cm2,the change of probe performance was?1%.The problem of neutron spectrum drift caused by radiation damage of Au-Si surface barrier detector did not appear.(6)Using the 6Li sandwich digital spectrometer,the neutron spectrum measurement method of the critical assembly was established,which improved the measurement range and accuracy of the neutron spectrum.The neutron spectrum distribution of the critical assembly in the range of 5keV?lOMeV was obtained.The proportion of the neutron spectrum in the range of 0.1MeV?4MeV was 87%,and the relative uncertainty of the measurement was less than 9.1%. |
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