Gamma-ray Source

$^{57}$Fe Mössbauer spectroscopy uses a $^{57}$Co source. The decay scheme of this isotope is shown in Figure 4.1. The half-life of $^{57}$Co is $271.7\ensuremath{\unskip\,\mathrm{days}}$ and decays by electron capture to the $I=\nicefrac{5}{2}$ excited state of $^{57}$Fe. This excited state decays to the $I=\nicefrac{3}{2}$ excited state ( $14.41\ensuremath{\unskip\,\mathrm{keV}}$) or to the $I=\nicefrac{1}{2}$ ground state by gamma-ray emission. The $14.41\ensuremath{\unskip\,\mathrm{keV}}$ state decays in turn to the ground state by gamma-ray emmission or internal conversion. The ratio of these two decay rates are given by Equation 2.28, where $\alpha=8.21$ for $^{57}$Fe. The half-life of the $14.41\ensuremath{\unskip\,\mathrm{keV}}$ excited state is $97.8\ensuremath{\unskip\,\mathrm{ns}}$, giving a Mössbauer gamma-ray with a linewidth of $0.097\ensuremath{\unskip\,\mathrm{\nicefrac{mm}{s}}}$. The linewidth of a resonant emission and absorption event is thus $0.194\ensuremath{\unskip\,\mathrm{\nicefrac{mm}{s}}}$ in perfect conditions.[7]

Figure 4.1: Decay scheme for a $^{57}$Co source leading to gamma-ray emission. Internal conversion accounts for the remaining $91\%$ of $14.41\ensuremath{\unskip\,\mathrm{keV}}$ events.

The sources used in this thesis are fabricated by diffusing $^{57}$Co atoms in a rhodium foil matrix: the rhodium matrix provides a solid environment for the $^{57}$Co atoms with a high recoil-free fraction and a cubic, non-magnetic site environment to produce mono-energetic gamma-rays. The initial source activities are $\sim 100\ensuremath{\unskip\,\mathrm{mCi}}$ with a linewidth of $0.22\ensuremath{\unskip\,\mathrm{\nicefrac{mm}{s}}}$ measured with a thin absorber.