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

Images of resolved 5.9 keV electron tracks produced from Fe-55 X-ray interactions are presented for the first time using an optical readout time projection chamber (TPC). The corresponding energy spectra are also shown, with the FWHM energy resolution in the 30-40% range depending on gas pressure and gain. These tracks were produced in low pressure carbon tetrafluoride (CF4) gas, and imaged with a fast lens and low noise CCD camera system using the secondary scintillation produced in GEM/THGEM amplification devices. The GEM/THGEMs provided effective gas gains of greater than or similar to 2 x 10(5) in CF4 at low pressures in the 25-100 Torr range. The ability to resolve such low energy particle tracks 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 55Fe is that it provides a tool for mapping the detector gain spatial uniformity.