  The 18.6 keV antineutrino line from 2-body decay of 3H in crystals can be
emitted with natural width because of motional averaging by lattice vibrations
despite the very long lifetime of 3H and contrary to commonly held views of
linewidths in such a case. It can be resonantly captured in 3He with
geometrical cross section sigma ~10-17 cm2. Using its hypersharp sensitivity
DELTAE/E ~10-29 and the method of time-filtered resonance, the basic energy
width ~10-24 eV expected of the 3H state can be measured to test if ultimate
nuclear widths are limited by the Planck length rather than time-energy
uncertainty.
