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

## TL;DR

This paper demonstrates the first optical imaging of 5.9 keV electron tracks from $^{55}$Fe in a GEM-based TPC, highlighting its potential for dark matter detection and detector calibration.

## Contribution

First-time imaging of $^{55}$Fe electron tracks using an optical readout TPC with detailed analysis of energy resolution and gain in CF$_4$ gas.

## Key findings

- Achieved effective gas gains of over 2×10^5 in CF$_4$ at low pressures.
- Resolved low energy electron tracks with 30-40% energy resolution.
- Showcased potential applications in dark matter searches and detector calibration.

## Abstract

Images of resolved 5.9 keV electron tracks produced from $^{55}$Fe 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 (CF$_4$) 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 $\gtrsim 2 \times 10^5$ in CF$_4$ 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 $^{55}$Fe is that it provides a tool for mapping the detector gain spatial uniformity.