accelerator. When a particle in the beam collides with a target nucleus in a nuclear reaction, there is most often a sizable orbital angular momentum be tween the two particles. The angularmomentum vector is perpendicular to the beam direction, so the reactionproduct nuclei are oriented with their spins perpendicular to the beam, and they radiate anisotropically. Nuclei thus produced in nuclear reactions are candidates for NMR if their lifetimes lie in the range ICH-IO3 sec. This method is abbreviated N M R / N R . Power Requirements for NMR/RD
Turning now to the other side of the N M R / R D methods, we need a descrip tion of the magnetic resonance phe nomenon that will allow us to deter mine the requirements for a successful N M R / R D experiment. Conventional NMR is often discussed using perturba tion theory, but that approach is not suitable for N M R / R D because these experiments require that nearly every metastable nucleus undergo a transition in a very short time. The approach sketched below is ideal for N M R / R D experiments, and it is actually simpler than the perturbation-theory method. A particle with magnetic moment μ in a magnetic field Η will precess about Η according to the equation
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The N M R / R D field started in the early 1950's. It isn't practical to give a complete review of the early work here with a discussion of the value of each contribution, but we can mention a few important papers. A more com plete set of references is available in a recent review article (1). In 1952 Deutsch and Brown reported the radiative detection of magnetic resonance in positronium (2) : this work was the precursor of the N M R / R D field. Later, in 1953, Bloembergen and Temmer pointed out the possibility of NMR/ON (3) and Abragam and Pound suggested NMR/PAC the same year (4). During the next 12 years several experiments were reported that were individually very elegant. Among these were an N M R / O N study of dy namically oriented 76As, by Pipkin and Culvahouse (5) and an N M R / N R ex periment on 8Li in LiF, by Connor (6). Unfortunately none of the experiments done in this period showed promise of being applicable to any but a few nu clear states. As recently as 1965 no NMR/PAC experiment had been re ported, nor had an NMR/ON experi ment been successfully carried out on thermally oriented nuclei. This latter fact was particularly important because thermal nuclear orientation has a very wide range of application. The earlier theoretical papers (3, 4) had discussed the necessary conditions for these N M R / R D experiments, but had not suggested practical ways to achieve these conditions. There was widespread opinion that the experiments were not feasible. Early in 1966 a group at our labora tory in Berkeley tried an NMR/PAC experiment based on "hyperfine en hancement" of the applied field in ferro magnetic nickel. This effect, discovered in 1959 by Gossard and Portis (7), has the effect of amplifying the H1 field by a factor of about 103, thereby making the crucial condition V1 > 1/4τ attain able for even very short-lived nuclear states. The first NMR/PAC result, on 100 Rh ( r = 3 X 10~7 sec) in nickel, was reported by Matthias et al. (8), and
