摘要
连续波核磁共振是磁共振系列实验的基础项目,深刻理解共振尾波实验原理也是课堂教学难点.以LC谐振频率及调节方法为切入点,由简单数理分析并采用一级近似得知,实验所拾取信号是以LC谐振信号为载波,且其幅值受拉莫频率偏离谐振频率之差频所简谐调制的调幅信号.使用外部可调频率的射频信号模拟拉莫进动感生电动势,通过振荡器(检波)输出分析调制信号的频率特性.实验结果表明:对短弛豫弱调制磁场,共振尾波幅值衰减反映共振横向弛豫;对长弛豫或强调制磁场,共振尾波不能准确地描述弛豫过程.长弛豫左侧信号源于当次激发之前激发态仍有历史激发的“遗留”核磁矩.
Continuous wave NMR is a basic experiment in the series of NMR,and its coda wave is also one of the difficulties in understanding the principle of NMR in experiment teaching.Starting fromLCresonance frequency and its regulation,and also using mathematical analysis in the first order approximation,it could be derived that the amplitude of the pickup signal was almost harmonically modulated by the frequency difference between the Larmor frequency and the resonance frequency.The induced electromotive force(EMF)was simulated by an external radio signal with adjustable frequency,and the frequency characteristics of the modulated signal were analyzed through the output of the oscillator.A series of experimental results on induced EMF produced by the Larmor precession of nuclear magnetic moment in excited state showed that the amplitude attenuation of resonance coda wave reflected the transverse resonance relaxation in the case of short relaxation and weakly modulated magnetic field,but it could not describe the NMR relaxation process accurately for long relaxation or strongly modulated magnetic field.The signal on the left side was generated from the residual magnetic moments in the excited state which had been excited before the current excitation.
作者
李潮锐
LI Chao-rui(National Demonstration Center for Experimental Physics Education,Guangzhou 510275, China;School of Physics, Sun Yat-sen University, Guangzhou 510275, China)
出处
《物理实验》
2019年第8期31-36,44,共7页
Physics Experimentation
基金
中山大学物理学基地能力提高项目(No.J1103211)
中山大学物理学人才培养支撑条件建设项目(No.J1210034)
中山大学本科教学质量工程项目(No.2018WL-ZLJG026,No.2019WL-ZLJG004)
关键词
核磁共振
共振尾波
信号拾取
拉莫频率
弛豫过程
调制磁场
NMR
resonance coda wave
signal pickup
Larmor frequency
relaxation process
modulated magnetic field
