Strong natural suppression of the monopole broadening of Mössbauer lines

The study of natural strong narrowing of Mössbauer lines on long-lived isomers is continued. This phenomenon was first correctly detected in [Yu. D. Bayukov, A. V. Davydov, Yu. N. Isaev, G. R. Kartashov, M. M. Korotkov, and V. V. Migachev, JETP Lett. $\mathbf{90}$, 499 (2009)] and was consistently explained in [S. V. Karyagin, JETP Lett. $\mathbf{98}$, 174 (2013); $\mathbf{98}$, 695 (2013)]. Thus, natural strong narrowing is a new effect in spite of its 36-yr prehistory. Since natural strong narrowing is based on the collapse of the hyperfine structure owing to chaos in motion of the nuclear spin, “criteria of chaos” have been introduced for the exchange and virtual collapse mechanisms. Types of nuclei and media with these mechanisms have been indicated. It has been found that the lifetime $\tau$ of isomers appropriate for natural strong narrowing is limited by diffusion. The strong natural suppression (below $1/\tau\sim 10^{-2}$ s$^{-1}$) of monopole (isomer, chemical) broadening has been revealed and explained.