Mössbauer method for studying vibrations in a granular medium excited by ultrasound

Vibrations of a granular medium immersed in epoxy resin without a hardener have been studied by Mössbauer spectroscopy. The granular medium consists of small potassium ferrocyanide particles enriched in the $^{57}$Fe Mössbauer isotope and having a mean size of 1.25 $\mu$m. Vibrations of particles in resin at a frequency of 12.72 MHz have been excited by means of a polymer piezoelectric transducer. The spectrum of $^{57}$Fe nuclei at rest in potassium ferrocyanide consists of a single line. Ultrasonic irradiation results in the splitting of the line into a comb structure with a frequency period equal to the frequency of vibrations. The analysis of the spectrum makes it possible to estimate the amplitude of vibrations of granules and damping of sound in such a medium. The proposed method is unique because it allows measuring subangstrom displacements of particles from their equilibrium positions.