Rotary spin echoes

Torrey has observed the free precession of nuclear spins around an r-f field H₁, fixed in a frame rotating at the Larmor frequency ω₀ = γH₀ around a large d-c magnetic field H₀. He showed that for an H₁, much larger than inhomogeneity of H₀, the latter has a negligible effect on the decay of the spin magnetization which is mainly due to the inhomogeneity of H₁. We report here on a method of overcoming the inhomogeneity of H₁, by production of echoes in the rotating frame ('rotary echoes'). These echoes are obtained by a 180 deg. phase shift at t = τ on the r-f field so that H₁, is suddenly reversed, producing a re-focussing of the magnetization vectors at the time t = 2 τ. The rotary echoes so obtained are very similar to the usual spin-echoes with, however some specific features that make them particularly suitable for the measurement of long relaxation times. Reprint of a paper published in Physical Review Letters, vol. 2, no. 7, Apr 1959, p. 301-302