样品转动方式对中子全息成像结果的影响

在中子全息成像实验中, 为避免透射中子干扰, 入射中子束方向与样品-探测器方向不能在一条直线上, 同时采用探测器移动结合样品转动或仅样品转动的方式, 避免探测系统的大范围移动. 因此在样品的转动过程中, 入射中子束和探测器相对于样品的位置同时发生改变, 内源全息项和内探测器全息项作为变量被记录在同一幅全息图中, 并在重建过程中相互干扰. 本文对基于中子三轴谱仪和四圆谱仪的三种不同转动方式进行了中子全息记录和重建模拟研究, 并讨论其修正方法. 结果表明, 各转动方式都可以通过适当的方法消除或减轻相关影响, 而其中基于三轴谱仪的纯样品转动方式可以使用两个探测器记录的方式, 避开入射中子束方向与样品-探测器方向不能在一条直线上的限制, 得到完整的全息图, 同时通过数据处理能基本消除相对转动造成的干扰, 达到理想的重建结果, 在条件允许的情况下应予优先采用.

Neutron holography simulation based on different sample rotations

Neutron holography is a new imaging technique based on the recording of the interference pattern of two coherent waves emitted by the same source, which allows observing the spatial order of microscopic objects like molecules or atoms in crystal sample. Two approaches can be used in neutron holography measurements. One is called inside-source holography, in which both the reference wave and object wave come from embedded atoms in the sample and propagate toward the detector outside the sample. The second approach called inside-detector holography is the inverse method of inside-source holography, in this case the reference wave is the initial neutron beam coming from a distant source outside the sample, while the atoms embedded in the sample act as detectors. In an ideal inside-source holography experiment, the sample should be fixed and the detector moves on a sphere, which is not practical because the detector system is usually heavy and far from the sample. In order to minimize the operation space, the detector always moves on a circle around sample or is located at a fixed position, while the sample rotates in an appropriate way to imitate the motion of the detector in a sphere. However, the orientation of the sample relative to the incident neutron beam is changed during sample rotation, and part of the inverse hologram is recorded together with the inside-detector hologram, which can cause distortion in the holographic reconstruction. In this paper, we simulate neutron holograms and reconstructions based on three different sample/detector rotations. In the first case, the detector moves on a circle, while the sample rotates about an axis perpendicular to the detector moving surface. In the second case, the detector is fixed, while the sample rotates around two perpendicular axes, the θ axis rotating the sample through πradians is perpendicular to the incident beam-detector plane, while the θ axis rotating the sample through 2πradians moves on a circle parallel to the incident beam-detector plane, this rotation can be carried out on a 3-axis spectrometry. In the third case, the detector is also fixed and the sample rotates around two perpendicular axes, but the θ axis is parallel to the sample-detector direction, while the θ axis moves on a circle perpendicular to the incident beam-detector plane, this rotation can be carried out on a 4-cycle spectrometry. The distortions and corresponding correcting methods of three kinds of rotations are discussed. The result shows that most distortions can be corrected by using special measurement or reconstruction techniques. Furthermore, pure sample rotation based on 3-axis spectrometer can achieve the best reconstruction result, so this rotation approach is preferred if conditions permit.