Observation of diffraction-like effects in Multiple Spin Echoe (MSE) experiments in structured samples

A single sequence of two r.f. pulses applied at times 0 and tau is capable of producing a train of echoes (multiple spin echoes, MSE) occurring at multiples of tau. This has been interpreted as a non-linear effect of the 'dipolar field" generated by the bulk nuclear magnetization acting on the spins as an additional component of the magnetic field. This effect, originally observed in solid and liquid He-3, has also been observed in ordinary samples such as water, at room temperature. It has been proposed that MSE could be used to investigate structured samples, such as porous media. Bowtell and Robyr (Phys. Rev. Lett. 76 (1996) 497 1) have deduced a direct relationship between the sample structure and the echo amplitude in MSE experiments. Simple Fourier transform arguments show that diffraction-like effects in the k-space are expected if the dipolar field is modulated by a sample with a periodic structure. Our theoretical analysis and experimental results show that such effects can be detected as additional anomalous echoes refocusing at times other than multiples of tau. Without careful setting of timings and pulse cycling, these echoes interfere and corrupt MSE measurements. The effects observed agree well with the theoretical predictions. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.