Imaging $^{55}$Fe Electron Tracks in a GEM-based TPC Using a CCD Readout

The first optically resolved electron tracks from an $^{55}$Fe X-ray source are presented together with the resulting energy spectrum. These tracks were produced in a TPC operating in low pressure carbon tetrafluoride (CF$_4$) gas, and imaged using a fast lens and low noise CCD optical system. Using GEM/THGEM amplification devices, effective gas gains of $\gtrsim 2 \times 10^5$ were obtained in pure CF$_4$ at low pressures in the 25-100 Torr range. The detector's very high signal-to-noise and the low gas pressures allowed individual ionization tracks from 5.9 keV X-ray interactions to be imaged and resolved. The ability to resolve tracks of such low energies has important applications in dark matter and other rare event searches, as well as in X-ray polarimetry. A practical application of the optical signal from $^{55}$Fe is that it provides a tool for mapping the detector spatial uniformity.