Development of a New Pulsed Polarized Neutron Spectroscopy Using Crosscorrelation Method

我们开发了一种利用脉冲极化白中子束的新型极化中子能谱。本方法的原理如下：利用自旋翻转线圈调制脉冲入射中子束的偏振，旋转线圈的驱动电流按伪随机二值序列调制。控制极化调制，使PRBS模式在中子爆发触发的每个顺序计数帧上周期性地移位。这个过程一直持续到PRBS的一个周期移动完成，然后它重复一个新的序列，计数散射中子。这样，在二维时间分析仪中对散射中子事件进行计数，其中一个时间标签是总TOF，另一个时间标签对应的计数帧数表示PRBS的移位数。在每个总TOF信道上，记录率沿帧数轴与适当的PRBS模式的相互关系更能恢复由采样处的入射时间和总TOF表征的二维TOF谱。由于只有入射中子的偏振被调制，所以恢复的TOF光谱由一个与自旋翻转开关光谱和平坦背景之间的差异成正比的光谱组成。为此，构建了一个中子自旋鳍波系统和一个二维时间分析仪。翻转线圈的驱动电流强度被控制成正比于1/t关系，其中是每个中子爆发的时间，因此对于任何入射的中子能量，都保证了中子自旋的180度翻转。然后根据PRBS接通和切断电流。开关的时间由产生PRBS模式的模块给出，该模式在中子爆发的每一帧移动一步。根据散射中子事件的总飞行时间和帧数，在二维时间分析仪的适当存储器中进行计数。在KEK的散裂中子源KENS上，用PEN光谱仪进行了自旋波散射的测量。样品为铁磁Heusler合金Cu2MnAl单晶。在单次测量中，我们可以观察到自旋波在（Q, ω）大范围内的散射。实验结果表明，脉冲极化白中子束与偏振调制的互相关方法相结合，对于研究各种磁性材料的磁激励或波动是很有前途的。为了获得高分辨率的结果，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