| Neutron imaging,as a non-destructive testing technology complementary to X-ray imaging technology,has important applications in the fields of basic scientific research,medical diagnosis,industrial radiography,nuclear industry,antique identification and so on.Neutron resonance imaging technology can achieve position-sensitive nuclide identification.With the development of the new generation of pulsed spallation neutron sources at home and abroad,neutron resonance imaging technology is considered as a potential development direction of high-efficiency non-destructive testing technology.Currently,the exploration of white neutron resonance transmission imaging experiment with all nuclide identification ability has been performed at China Spallation Neutron Source(CSNS)Back-n White Neutron Source with its high flux and wide spectrum white neutron beam.Among a variety of neutron imaging techniques,the detector based on the neutron sensitive microchannel plate(MCP)has the characteristics of high counting rate,high time resolution and spatial resolution,which can more efficiently perform neutron resonance imaging covering the neutron energy range of 0.5 eV-10 MeV.The research on the multi-channel readout electronics for MCP detecter has been carried out.In the white neutron resonance imaging experiment based on MCP,the counting rate of neutrons in the energy region above keV is more than 10 kHz,especially the counting rate of neutrons above 0.1 MeV is above MHz.The challenge of this work is the signal pile-up and multi-channel data transmission with high counting rate.According to the intensity distribution of white neutron beam,this paper introduced a dual-mode trigger scheme to improve the neutron detection efficiency and reduce the pressure of data transmission.For dense neutron events in high energy region,T0 trigger provided by the particle accelerator is used for completing waveform readout.And for neutron events in low-energy region,trigger generated by neutron hit signal is used for waveform parameter extraction.Without increasing the data buffer and transmission bandwidth,the complete waveform of neutron events in the high-energy region is preserved,while the stable operation of the sy stem at the average count rate of 100 kHz in the low energy region is guaranteed.In order to verify the feasibility of the neutron case reading scheme based on dualmode trigger,the lead-out design of the anode signal is first carried out.Taking the balance of the counting rate and the number of channels,the signal lead-out board based on cross strip is adopted,and the anode design of the verification version is preliminarily completed.The cross strip anode can greatly reduce the number of channels without affecting the imaging efficiency and spatial resolution of the MCP detector.For multi-channel charge measurement and time measurement of the MCP deteeter,a modular readout electronics has been put forward:low power consumption,high integration,multi-channel ASICs(Application Specific Integrated Circuits)are used for integration amplification,filtering and shaping to improve the signal-to-noise ratio;Multi-channel ADCs(Analog to Digital Converters)are used to digitize the waveforms of all channels simultaneously;FPGAs(Field Programmable Gate Arrays)are used for real-time data filtering and compression.Based on the scheme above of the readout electronics,a 256-channel waveform digitization readout system has been designed and implemented.The detailed performance tests have been carried out.The integrated nonlinearity of the readout electronics system in the dynamic range of 180 fC is less than 1%,and the baseline noise level(RMS)is less than 0.3 fC,which meets the demand of charge measurement of the white neutron resonance imaging system;The time measurement accuracy is better than 3 ns,better than the accuracy required for element identification;The counting rate of high-energy neutron above 20 keV can reach above MHz,and the peak counting rate in low-energy region can reach 500 kHz,which is suitable for high counting rate neutron detection of white neutron resonance imaging system.After the electronic performance tests,a joint test of readout electronics and MCP detector in the neutron beam was carried out in Back-n.The time-of-flight spectrum and neutron hit image were successfully reconstructed.By analyzing and processing the resonance absorption peaks of neutron spectrum,different heavy metal elements of the target samples in the hit image were successfully distinguished,which verified the feasibility of the white light neutron resonance imaging system. |
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