Spherical Proportional Counters ( SPCs ) are a novel gaseous detector technology employed by the NEWS-G low-mass dark matter search experiment for their high sensitivity to single electrons from ionization .
In this paper , we report on the first characterization of the single electron response of SPCs with unprecedented precision , using a UV-laser calibration system .
The experimental approach and analysis methodology are presented along with various direct applications for the upcoming next phase of the experiment at SNOLAB .
These include the continuous monitoring of the detector response and electron drift properties during dark matter search runs , as well as the experimental measurement of the trigger threshold efficiency .
We measure a mean ionization energy of \mathrm { W } = 27.6 \pm 0.2 ~ { } \mathrm { eV } in Ne + \mathrm { CH _ { 4 } } ( 2 % ) for 2.8 keV X-rays , and demonstrate the feasibility of performing similar precision measurements at sub-keV energies for future gas mixtures to be used for dark matter searches at SNOLAB .