SPIN ECHOES
SPIN ECHOES
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DOI：
10.1103/physrev.80.580
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发表时间：
1950-01-01
期刊：
影响因子：
--
通讯作者：
HAHN, EL
中科院分区：
文献类型：
--
作者：
HAHN, EL
脉冲形式的强射频功率被施加到置于大静磁场H 0中的液体中的自旋系综。脉冲rf功率的频率满足核磁共振的条件，并且脉冲持续的时间与整个自旋系综的章动宏观磁矩可以衰减的时间相比是短的。在去除脉冲后，等色宏观矩的非平衡结构仍然存在，其中矩矢量自由进动。每个力矩矢量在给定的进动频率下具有幅度，该进动频率由H 0中的不均匀性施加在系综上的拉莫尔频率的分布确定。在由过去施加的脉冲序列确定的时间，这些矩矢量的相长干涉产生可观察到的自发核感应信号。利用布洛赫理论讨论了这些自旋回波信号的性质和基本原理。弛豫时间是直接和准确地测量回波幅度的测量。分析包括对含有共振核的液体分子的自扩散的弛豫测量的影响。初步研究了与自旋回波相关的几种效应，包括由于分子中所含原子核的磁屏蔽而观察到的自旋磁共振频率的变化。
Intense radiofrequency power in the form of pulses is applied to an ensemble of spins in a liquid placed in a large static magnetic field H 0. The frequency of the pulsed rf power satisfies the condition for nuclear magnetic resonance, and the pulses last for times which are short compared with the time in which the nutating macroscopic magnetic moment of the entire spin ensemble can decay. After removal of the pulses a non-equilibrium configuration of isochromatic macroscopic moments remains in which the moment vectors precess freely. Each moment vector has a magnitude at a given precession frequency which is determined by the distribution of Larmor frequencies imposed upon the ensemble by inhomogeneities in H 0. At times determined by pulse sequences applied in the past the constructive interference of these moment vectors gives rise to observable spontaneous nuclear induction signals. The properties and underlying principles of these spin echo signals are discussed with use of the Bloch theory. Relaxation times are measured directly and accurately from the measurement of echo amplitudes. An analysis includes the effect on relaxation measurements of the self-diffusion of liquid molecules which contain resonant nuclei. Preliminary studies are made of several effects associated with spin echoes, including the observed shifts in magnetic resonance frequency of spins due to magnetic shielding of nuclei contained in molecules.