本文介紹一個量測磁性材料內部磁區之方法，磁性材料內部磁區之量測，目前僅有去極化中子測量之方法可以得之。清華水池式反應 W-3 實驗站建立了一部三維去極化中子儀，可以觀察極化中子通過磁性材料過程中受到材料內部磁區的擾動，使得極化中子向量的方向與大小改變，進而可以分析材料內部的平均磁區大小、平均感應磁化量，以及磁區平均的平方方向餘弦等資訊。

We introduce a technique of neutron depolarization measurement to observe interior magnetic domains of magnetic materials. An apparatus of three-dimensional depolarized neutron beam has been setup at W-3 beamport of Tsing Hua Open Pool Reactor. When polarized neutrons pass through a magnetic material, the existence of magnetic domains will alter the direction and magnitude of the polarized neutron vector. This alternation would provide useful information of the average domain size, the averaged strength of the induced field, and the mean square directional cosine of the material.