Experimental test of the theory of effect of natural strong narrowing of Mössbauer lines and a more general effect of collapse of the hyperfine structure because of fluctuations of the Fermi contact field. Role of virtual transitions in these effects

To test the theory of the effect of natural strong narrowing (NSN) of Mössbauer lines on long-lived isomers and a more general effect of collapse of the hyperfine structure (CHFS) because of fluctuations of the Fermi contact field (S.V. Karyagin, JETP Lett. 98, 174 (2013)) with the use of the data obtained by the team led by Davydov (Yu.D. Bayukov et al., JETP Lett. 90, 499 (2009)), the yield of $88.034$-keV gamma-ray photons from a silver plate with the $^{\text{109m}}$Ag isomer has been analyzed in two variants: (i) the hyperfine structure is resolved and the yield depends on the angle $\psi$ between the wave vector of the photon and the external field ${\mathbf H}_{ex}$, and (ii) the yield is independent of $\psi$ because of CHFS. It has been shown that variant (ii) better reproduces the experimental data: experimental average count numbers at $\psi = 0 $ and $-\pi/2$ differ from each other only in systematic error the same at $4.2$ and $295$ K and, moreover, the count numbers with the exclusion of resonance should be independent of $\psi$; this condition is strongly violated in variant (i) and is well satisfied in variant (ii). The threshold condition of the CHFS for the Fermi field has been obtained with allowance for virtual transitions. The collapse of the hyperfine structure is possible at any lifetimes of nuclear levels at any transitions not only in the gamma range but also in other radiation ranges. It results in the complete depolarization of nuclei and radiation. To estimate the Fermi field from experiments, the field $|{\mathbf H}_{ex}|\sim10^4\,$G is necessary.