应用于非弹性中子散射谱仪的重复频率倍增方法研究

Inelastic neutron scattering is the process of the neutron energy change before and after the scattering, which is widely used in the condensed matter physics research. For traditional inelastic neutron scattering instruments, there is only one incident neutron beam pulse in each time-of-flight cycle, as the incident neutron energy broadening is generally small after the sample’s inelastic scattering, the neutron detector will not have counts in most time of one measuring cycle, and the data collection efficiency is very low. Repetition rate multiplication method can manifold increase the count rate without the decrease of energy resolution, which is very important to the performance of the inelastic scattering instruments. The key lies in the use of pulse shaping neutron chopper, it can produce multiple pulsed neutron beam in each time-of-flight cycle, which will produce multiple inelastic scattering time-of-flight spectrum in the detector without time-of-flight frame overlap between each other, and the actual measurement rate increases N times. This project will carry out the repetition rate multiplication method’s design and experimental work in the China spallation neutron source (CSNS), and will get the results based on the beam characteristics of CSNS, which can be used to guide the design and optimize the physical performance of the future inelastic scattering instruments of CSNS.

非弹性中子散射是指散射前后中子能量有变化的散射过程，广泛应用于凝聚态物理研究。传统的非弹性中子散射谱仪上每个飞行时间测量周期内只有一个入射脉冲中子束流，由于样品上非弹性散射对入射中子的能量展宽通常比较小，在一个测量周期的绝大部分时间内，探测器上都没有中子计数，数据采集效率很低。重复频率倍增方法可以在不降低能量分辨率的前提下成倍提高计数率，对提升非弹性散射谱仪的性能有重大的作用，其关键在于采用脉冲成形中子斩波器在每个飞行时间测量周期内产生多个入射脉冲中子束流，从而在样品上产生多段非弹性散射谱，彼此之间没有飞行时间重叠，相当于实际的飞行时间测量频率增加了数倍，本项目将在中国散裂中子源上开展重复频率倍增方法的设计和实验工作，得到基于中国散裂中子源束流特性条件下的重复频率倍增方法的研究结果，可用于指导下一步规划的非弹性散射谱仪的设计，对优化谱仪的实际物理性能具有重要作用。

非弹性中子散射是指散射前后中子能量有变化的散射过程，广泛应用于凝聚态物理研究。重复频率倍增方法利用脉冲成形中子斩波器，可以在不降低能量分辨率的前提下成倍提高计数率，有效地解决传统非弹性中子散射谱仪数据采集效率很低这一难题。本项目开展了重复频率倍增方法的设计和实验工作，设计研制了一台脉冲斩波器原型机，在中国散裂中子源20号束线上进行束流测试验证了重复频率倍增方法的可行性。实验结果显示，脉冲斩波器原型机在最高转速300Hz下成功截取了多个脉冲中子束流，脉冲形状近似梯形，时间宽度与预期相符，误差小于8μs@300Hz，通过率大于93%。基于中国散裂中子源束流特性条件下得到的重复频率倍增方法研究结果，可以用于指导中国散裂中子源下一步规划的多台非弹性散射谱仪的设计和建设，优化谱仪的实际物理性能。