Development of a New Pulsed Polarized Neutron Spectroscopy Using Crosscorrelation Method

利用脉冲极化白色中子束，发展了一种新型的极化中子能谱。本方法的原理如下：脉冲入射中子束的偏振调制通过使用自旋翻转线圈，其驱动电流根据伪随机二进制序列（PRBS）调制。控制偏振调制，使得PRBS图案在由中子爆发触发的每个顺序计数帧处循环地移位。该过程继续进行，直到PRBS的一个循环移位完成，然后它重复一个新的散射中子计数序列。以这种方式，在二维时间分析器中对散射中子事件进行计数，其中一个时间标签是总TOF，而另一个时间标签对应于表示PRBS的移位数的计数帧数。计数率沿着帧编号轴在每个总TOF信道处与适当PRBS模式的互相关 ...更多信息 恢复二维TOF谱，其特征在于样品处的入射时间和总TOF。由于只对入射中子的极化进行调制，因此恢复的TOF谱由一个与自旋翻转开关谱之差成正比的谱和一个平坦的背景组成。翻转器线圈的驱动电流强度被控制为与1/t关系成比例，其中t是从每个中子爆发开始的时间，使得对于任何入射中子能量确保中子自旋的180度翻转。然后根据PRBS接通和关断电流。切换的定时由生成PRBS模式的模块给出，PRBS模式在中子爆发的每一帧移位一步。散射中子事件在二维时间分析仪的相应存储器中按其总飞行时间和帧数计数，在KEK的散射中子源KENS上用PEN谱仪进行了自旋波散射测量。样品是铁磁性Heusler合金Cu 2 MnAl的单晶。我们可以在一次测量中观察到广泛的（Q，ω）空间中的自旋波散射。本实验结果表明，脉冲极化白色中子束与极化调制互相关方法相结合，对于研究各种磁性材料中的磁激发或磁涨落是很有前途的。为了获得具有良好分辨率的结果，当然需要强脉冲中子源。少

We developed a new type of polarized neutron spectroscopy utilizing a pulsed polarized white neutron beam. The principle of the present method is as follows ; The polarization of the pulsed incident neutron beam is modulated by using a spin flipper coil whose driving current is modulated according to a pseudorandom binary sequence (PRBS). The polarization modulation is controlled so that the PRBS pattern shifts cyclically at each sequential counting frame triggered by the neutron burst. The procedure continues until one cycle shift of the PRBS is finished, and then it repeats a new sequence of the counting of scattered neutrons. In this way, the scattered neutron events are counted in a two-dimensional time analyzer, one time label of which is the total TOF, and the other corresponding to the counting frame number which represents the shift number of the PRBS. The crosscorrelation of theacount rate along the frame number axis at each total TOF channel with the appropriate PRBS pattern … More recovers the two dimensional TOF spectrum which is characterized by the incident time at the sample and the total TOF. Since only the polarization of the incident neutrons is modulated, the recovered TOF spectrum consists of a spectrum which is proportional to the difference between the spin flipper-on and -off spectra and an otherwise flat background.A neutron spin flipper system and a two dimensional time analyzer were constructed for the present work. The driving current intensity for the flipper coil is controlled to be proportional to the 1/t relation, where is the time from each neutron burst, so that a 180 degree flip of the neutron spin is ensured for any incident neutron energy. The current is then switched on and off according to the PRBS. The timing of the switching is given by a module which generates the PRBS pattern which shifts by one step at each frame of the neutron burst. The scattered neutron events are counted in the appropriate memories in the two dimensional time analyzer according to their total flight time and frame number.The measurement of spin wave scattering were carried out on the PEN spectrometer at the spallation neutron source KENS at KEK. The sample was a single crystal of ferromagnetic Heusler alloy Cu2MnAl. We could observed spin wave scattering in a wide range of (Q, omega) space in a single measurement. The results of the present experiment show that the combination of the pulsed polarized white neutron beam with the crosscorrelation method with polarization modulation is quite promising for the study of magnetic excitations or fluctuations in various magnetic materials. In order to obtain results with good resolution an intense pulsed neutron source is required of course. Less