FIG. 4. NMR signals. a) shows the decay following a spinecho pulse sequence. b) is the spectrum corresponding to a), while c) shows the variation in signal observed as the duration of the initial pulse is varied. The numerical labels indicate the number of full audio-frequency cycles applied. d) shows the result of a Carr-Purcell Meiboom-Gill multi-echo sequence with 16 echoes. a) and b) are the result of four transients, each spectrum in c) was acquired with two transients, while d) was 256 transients. Most of the noise spikes in b) correspond to odd harmonics of the 60 Hz power-line frequency.