Study Of The Neutron Chopper Performance At Spallation Neutron Source

Neutron chopper is one of the most important neutron optical devices installed on the time-of-flight neutron scattering spectrometers.It is used more widely in spallation neutron source than reactor neutron source.Three neutron spectrometers have been built and put into use in the first-phase project of China Spallation Neutron Source(CSNS),each with its own set of neutron chopper system.Through reasonable physical design and parameter configuration,the function of the neutron chopper system can fully meet the experimental requirements of the spectrometer samples test.The study of the performances of the neutron chopper can verify its function and uncover potential problems during its usage,which contribute to the installation and improvements of the neutron choppers on neutron spectrometers.Given that the spectrometers to be built in numbers in CSNS would be widely equipped with T0 choppers and bandwidth choppers,it is of great significance,scientifically and engineeringly,to study the performance of the neutron choppers.This paper starts with an introduction of the application of neutron chopper in spallation neutron sources worldwide,then describes the physical principles of T0 chopper and bandwidth chopper,which are currently used on the neutron scattering spectrometers at CSNS.Based on these physical principles,numerical calculations,Monte Carlo simulations with McStas,neutron beam experiments are used to study the performances of the neutron choppers on the General Purpose Powder Diffractometer(GPPD).In this study,we mainly carried out: 1)Assessment of T0 chopper's blocking ability to high energy gamma rays and fast neutrons produced at the zero time of the pulse,as well as its phase optimization;2)Study on the performance of the chopper,including its neutron transmittance,selection of wavelength range and the filtration of long-wave overlapping background;3)A new model based on GPPD was built to study the influence of different spacing between the two disks of the double-disk chopper on the neutron intensity loss,beam divergence distribution and neutron intensity distribution at 33 m of the model.The main research results are as follows:1: When operating at 25 Hz,T0 chopper can reduce the high energy gamma rays and fast neutrons generated at the zero time of the pulse by approximately 537 times.T0 chopper can well block the background noise within the phase time range of 0-50?s,with 50?s being the optimal value.When operating at 50 Hz,the blade of T0 chopper can predictably block theleaked fast neutrons and gamma rays for an additional 2mm on each side in width.2: The neutron absorption coating on the bandwidth chopper disk can absorb the unwanted neutrons well.The relative error between the actual neutron bandwidth and the target bandwidth 4.8? is within 0.5%,which indicates that the bandwidth chopper can define the range of wavelength accurately.Bandwidth chopper system can significantly reduce the frame overlap and improve the signal-to-noise ratio.3: When the distance between the two disks falls within 4m,the neutron flux loss at 33 m away from the surface of the moderator is below 0.4%,with no influence on the divergence distribution and intensity distribution of the beam.