Optimization of spherical proportional counter backgrounds and response for low mass dark matter search

Abstract

The NEWS-G collaboration uses Spherical Proportional Counters to search for Weakly Interacting Massive Particles (WIMP). The first detector developed for this goal is a 60 cm diameter sphere installed at the Laboratoire Souterrain de Modane in France. In 2015, the collaboration took a run with neon as the target material for an exposure of 9.7 $\mathrm{kg\cdot days}$. This run allowed new limits to be set on the spin-independent WIMP-nucleon cross-sections with $\mathrm{90\%}$ confidence upper limit of $\mathrm{\sigma_{SI} < 4.4 \times 10^{-37} cm^{2}}$ for a $\mathrm{0.5\, GeV/c^{2}}$ WIMP. The study of the background events observed during this run shows that it is dominated by the presence of the $\mathrm{^{210}Pb}$ decay chain in the different materials composing the detector, its shielding, and on the inner surface of the sphere. The experiences acquired during the utilization of SEDINE and the analysis of its data allowed a procedure to be developed to avoid radioactive contaminations and minimize the background of the experiment. The background of the next detector was estimated by a stringent selection of the materials, the measurements of their radioactive contaminations and the simulation of the different components. The development of new sensors allows a better homogeneity of the detector response and good data acquisition in large detector. The new detector is a 140 cm diameter sphere, to be installed at SNOLAB in Canada in 2020. Its performance will be also enhanced by the development of methods of signal characterisation and calibration.