| Neutron measurements are very useful in many fields,in particular,in the advanced nuclear energy system.Because neutrons are not charged particles and neutrons are unable to electromagnetically interact with any material,neutrons can only be detected with the methods of the secondary charged particles produced by some reactions.These neutron-induced reactions include nuclear reactions,nuclear fissions,nuclear recoils,nuclear activations,and so on.Since fission can produce two heavy charged fragments,very strong signals are output from the neutron detectors based on the method of nuclear fission.These fission detectors have been used widely in very harsh environments,such as in the nuclear reactors.In this thesis,two types of neutron detectors are developed based on the method of nuclear fission,where one is a parallel plate avalanche counter（the so-called fission PPAC）and the other one is an ionization chamber（the so-called fission ionization chamber）.Parallel plate avalanche counters（PPACs）have characterized themselves with large sensitive area,good resolution in time and position,and negligible parasitic reactions in the detector.The fission PPAC detector developed in this thesis consists of 7 layers.The left（right）three layers are used as three electrodes of the first（second）PPAC.Thus,each PPAC has a central anode and flanked by two cathodes.The fourth layer is used to separate two sets of PPACs.All the electrode planes are kept in parallel to ensure a uniform electric field.The gaps between the electrodes are 3 mm each.In order to measure the fast neutrons,the 238U（U3O8）coating has been attached at the central anode of the second set of PPAC.Gamma compensation is accomplished by the electronic arrangement of two sets of PPAC,because Gammas can be detected by two sets of PPAC,while neutrons can only be detected by the second set of PPAC.Monte Carlo software Geant4 has been used to find the best thickness of the 238U（U3O8）coating and calculate the detection efficiency of the fission PPAC detector.In addition,the software Garfield++for gas ionization analysis has been used to simulate the electron drift velocity and average gain of several working gases.The working gas C4H100 which is most suitable for fast time and low pressure detectors is selected.The influence of magnetic field on the electron drift velocity and fission PPAC output signal is also simulated by Garfield++.In order to evaluate the performance of the fission PPAC detector,the experimental measurements have been performed with a 241Am source and a 252Cf source.During the measurement the filling gas of the detector is Isobutane(C4H10)with a pressure of 6mbar or 10mbar in a flow-gas mode.First,the counting rates of the PPAC detector at different voltages have been measured with 241Am neutron source,in order to find the best working voltage.Then,the time resolution of the PPAC detector and the energy deposition of the fission products from 252Cf source in the PPAC detector have been measured with the 252Cf source.The 252Cf source emits a spontaneous fission fragment and a neutron,where the spontaneous fission fragment may be detected by PPAC 1 and the neutron may be detected by PPAC 2.By using the signal from spontaneous fission fragment as the stop time and the signal from the neutron as the start time,the time resolution of 164ps has been measured for the fission PPAC detector.Due to the high requirement of fission PPAC for working gas and working pressure,in order to reduce the working cost,a fission chamber which can work in different gases is developed for detecting incident neutron fluence.The electrodes of fission chamber are composed of two concentric cylinders,the inner cylinder is an anode,the outer cylinder is a cathode,and the distance between the electrodes is 2mm.By electroplating 545μg/cm2 238U（UO 2（OH）2）on the outer layer（inner layer）of the inner cylinder（outer cylinder）as fission targets,the stable detection efficiency of fission chamber(average 8×10-7)is measured in Air atmosphere and Ar in different atmospheric pressures over 3000 hours.In order to study the energy spectrum measured by fission chamber more carefully,the energy spectrum of fission products（Alpha particles and fission fragments）from inner and outer targets in working gas at different pressure of Ar gas is simulated by Geant4 software.Comparing the simulated and measured values of detection efficiency,the results show that the difference between them is less than±5%.The fission PPAC developed in this work has the advantages of good time resolution,large signal amplitude and stable long-time operation.It can measure neutron energy outside the reactor by Time-Of-Flight method.The fission chamber has the advantages of low requirement for working gas and stable long-time operation.It is suitable for neutron flux monitoring outside the reactor. |
