Abstract In this study, polymer-gel dosimeters containing N-(Hydroxymethyl) acrylamide (NHMA) with different concentrations of potassium chloride (KCl) were developed and introduced for use in radiotherapy. The gels were irradiated with a megavolt X-ray photon beam up to 10 Gy using a medical linear accelerator. The irradiated NHMA_KCl dosimeters were evaluated by nuclear magnetic resonance (NMR), in terms of relaxation rate (R2). The results show that the dose sensitivity of polymer gel was improved by 33% when KCl concentration was increased from 0 to 1.9 wt%. The decrease in R2 values with increasing scanning temperature was 1.6% per °C at 6 Gy. The dosimeter was found stable within a period of 2–120 h after irradiation and is independent of dose-rate in the range of 50–600 cGy/min and independent of photon beam energy between 6 and 15 MV within 7.5% overall uncertainty.