On the suppression of the $n\bar{n}$ transitions in medium

The new model of $n\bar{n}$ transitions in the medium based on unitary $S$-matrix is considered. The time-dependence and corrections to the model are studied. The lower limit on the free-space $n\bar{n}$ oscillation time $\tau$ is in the range ${10}^{16}\mathrm{y}\mathrm{r}>\tau >1.2\cdot {10}^9\mathrm{s}$