用于木材水分检测的单边核磁共振传感器设计

为了实现对木材水分的无损检测，设计了一种单边核磁共振（unilateral nuclear magnetic resonance，UMR）传感器，该传感器由单边磁体、抗涡流板、射频线圈、阻抗匹配和调谐电路构成．在距离传感器表面上方75 mm处的50 mm×50 mm的平面内，建立了71.1 mT（共振频率为3.027 MHz）的静态磁场．本文详细介绍了该传感器的设计思路和实现方法，并开展了初步的木材水分无损测量实验．对圆柱形树桩的径向水分分布进行了一维扫描测量，观察了水分由树皮向树芯逐步深入过程中横向弛豫时间（T₂）的变化规律，还使用UMR仪对木材干燥过程中水分的挥发进行了测量.实验结果表明，随着干燥的加剧，被测样品T₂谱的长T₂波峰明显左移、积分面积渐减，而且积分面积与木材样品含水率呈正比．本文为木材研究提供了一种便携的NMR测量设备设计方案，且可以实现野外活体树木的无损测量．

A Unilateral Nuclear Magnetic Resonance Sensor for Nondestructive Wood Moisture Measurements

An unilateral nuclear magnetic resonance (UMR) sensor was designed to measure wood moisture nondestructively. The sensor consisted of a unilateral magnet, an anti-eddy current module, a radiofrequency (RF) coil and an impedance matching and tuning circuit. The sensor produced a static magnetic field of 71.1 mT (resonant frequency:3.027 MHz) in a 50 mm×50 mm plane locating 75 mm above the sensor's surface. Preliminary nondestructive measurement of wood moisture was carried out with the sensor. The moisture distribution in the radical direction of a cylindrical wood sample was scanned. Variations in transverse relaxation time (T2) from the bark to core were obtained. Evaporation of moisture during wood drying was also measured with the UMR sensor. Experimental results showed that: the peak of long T2 component in the T2 spectrum moved to left and the peak integral area decreased gradually during drying. The integral area was proportional to the moisture content of the sample. The work presents a portable UMR device for wood research which may potentially be used for nondestructive moisture measurement on living trees in situ.