Final states produced by a decay have a much broader energy spectrum than the natural line width at times much shorter than the decay lifetime. This tends to render impossible the use for neutrino detection of the high value of the resonance absorption cross section at the peak of the resonance.
@article{arxiv.0904.4913,
title = {Difficulties in using the sharp neutrino spectrum at short times},
author = {Harry J. Lipkin},
journal= {arXiv preprint arXiv:0904.4913},
year = {2009}
}