Purpose: Recent work has shown that the storage phosphor, KCl:Eu2+, has great potential to become the physical foundation of a reusable, highresolution planar dosimeter. The activator ion (Eu2+) plays an important role in storage phosphor performance. The effects of activator concentration on dosimetric properties must be understood to design a robust dosimeter with a linear dose response, high sensitivity, and is easily integrated into clinical application. In this work, we investigate the effects of adjusting the activator concentration in KCl:Eu2+ on dose response, sensitivity, and temporal stability. Methods: Six mm diameter, 1mm thick KCl:Eu2+ pellets were fabricated using a hydraulic press. Europium concentration was varied between 0.01–5.0 mole percent. Pellets were irradiated with a 6MV photon beam. Signal stability and dose response linearity were studied using a laboratory photostimulated luminescence (PSL) readout system. PSL emission and stimulation measurements were obtained at room temperature to determine if concentration‐induced effects occurred in the electronic structure. Results: The highest PSL intensity was achieved for samples with approximately 1 mole % europium. Significant degradation of PSL intensity was observed for samples with europium concentration greater than 1 mole %. PSL stimulation data showed that no additional storage centers were created with increased europium concentrations. Fading curve measurements showed a thermal treatment at 70 °C for 15 minutes was able to eliminate the fast fading component, regardless of concentration. Conclusions: Concentration quenching of CsBr:Eu2+ (commercially available computed radiography storage phosphor panel) occurs at much lower activator concentrations (∼0.05 mole % Eu2+), compared to KCl:Eu2+. The higher quenching concentration of KCl:Eu2+ may indicate that precipitate formation has an insignificant effect on PSL properties. Further, KCl:Eu2+ dosimeters can be manufactured with an activator concentration as low as 0.01 mole percent while maintaining a linear dose response and reasonable PSL sensitivity. This work was supported in part by NIH Grant No. R01CA148853.