Explanation for the effect of natural strong narrowing of Mцssbauer lines on long-lived isomers. Variety of nuclei and media with this effect

The effect of natural strong narrowing (NSN) of Mцssbauer lines on long-lived isomers has been explained. This effect was doubted for more than thirty years because its mechanism was unknown. The mechanism of NSN has been revealed. Its threshold condition has been established. The theory of inhomogeneous broad-ening of a line in the case of NSN has been developed. Broadenings owing to magnetic, quadrupole, and $2^L$-pole hyperfine interactions are suppressed almost to zero at NSN. The mechanism of NSN provides a width about the natural width. The linewidth at NSN is independent of the magnitude and direction of the magnetic field if it is lower than $\sim 100$ G. The variety of nuclei and media with NSN or with the collapse of the hyperfine interaction (responsible for NSN) is outlined.