The Research Of The Neutron Dose Equivalent Measurement In Pulsed Neutron Field

With the accelerator of different energy widely used in many diverse fields, the research of the detection of the pulsed neutron field around the accelerators is more and more important. At present, no neutron detector or measuring method of neutron dose-equivalent is both real-time and accurate, therefore in this thesis a real-time measuring method that can reduce the counting loss of neutron influence and dose-equivalent in the pulsed neutron field is investigated, and an improved neutron dose-equivalent meter based on this method is built.The pulsed field has the feature of low duty ratio and concentrated beam. When the A-B neutron dose equivalent meters, which usually measure the uniform neutron field, are used in pulsed neutron field, several neutrons may enter the detector simultaneously, resulting in the increment of the amplitude of the output pulse of BF3 proportional counter tube. However, only one pulse is recorded by the counter. Therefore, the counting loss exists. The activation detector has no counting loss, but is not real-time measurement. The measurements in the pulsed neutron field by a digital oscilloscope and a multi-channel analyzer (MCA) verify the counting loss, and the pulse amplitude spectrum obtained by the MCA shows that the peaks in the spectrum correspond to the energy of one neutron reaction, and two or three neutrons reacting at the same time. The method proposed in the thesis to reduce the counting loss is to analyze the spectrum obtained by MCA. By setting several Regions of Interest (ROI) in the pulse amplitude spectrum measured with MCA for one-neutron pulse, two-neutron pulse and three-neutron pulse; and then individually integrating the counts in these ROIs and multiplying with proper coefficients, the actual amount of neutrons can be obtained and converted to Sv or Sv/h.In this thesis the influence of the pulsed neutron field around the accelerator and the moderating process are investigated by the Monte Carlo simulation, and the overloading correction coefficient is gotten. Based on the results of the calculation of the overlapping pulse amplitude spectrum, the correction formula of counting loss is obtained. The verification experiment of the correction formula is then carried out by: 1) Measuring the neutron influenceФ0 and the counting rate of pulse amplitude spectrum q0 respectively by GM counter tube covered by silver foil and the BF3 counter tube in the same uniform neutron field, and then calculating relative detection efficiency k 0 =Φ₀/q₀ in uniform neutron field. 2) Setting up these two detectors at the same point in the pulsed neutron field for measuring the neutron influenceΦ' and the pulsed amplitude spectrum respectively. The total counting rate of the spectrum was q' . Applying the overlapping correction and the overloading correction to calculate the corrected counting rate q af, the relative detection efficiency in pulsed neutron field was obtained by k_af =Φ'/q_af. 3) The result k_af≈k₀ indicating that the counting loss of A-B neutron dose equivalent meter in pulsed neutron field can be corrected by the correction formula.The method can accurately measure the real-time neutron influence and dose-equivalent in pulsed neutron field.