Earlier the Heisenberg Natural Linewidth (HNL) was ignored as being totally negligible compared to $ E_{R}$. However, in recoil-free events $ E_{R}$ is 0 and hence the HNL becomes the major limit on the resolution of the gamma ray energies.

This spread in energies of width $ \Gamma_{s}$ is $ 4.67\times{}10^{-9}$ $ \unskip\,\mathrm{eV}$ in $ ^{57}$Fe. Compared to the Mössbauer gamma ray energy of 14.41 $ \unskip\,\mathrm{keV}$ for this isotope this gives a resolution of $ \sim$1 in $ 10^{12}$. This is an incredibly high level of energy resolution and is of the order of nuclear hyperfine interactions. Hence, the Mössbauer effect can be used to probe the electronic environment of a sample via the hyperfine interactions.

Dr John Bland, 15/03/2003