Photoluminescence (PL) and radioluminescence (RL) measurements were made on NaMgF3:Sm before, during and after exposure to high doses of ionising radiation. Magnetic measurements prior to irradiation showed that approximately 10% of the total Sm concentration was in the divalent state. The RL from Sm3+ was found to increase while the Sm2+ RL decreased with increasing x-ray dose before reaching steady-state values for high doses. This behaviour is opposite to that previously reported for Sm3+ and Sm2+ PL. We show that this apparent discrepancy can be accounted for by a RL model where there is a hole trap, an electron trap, and direct x-ray induced carrier recombination at Sm2+ and Sm3+. Furthermore, a good fit to the dose-dependence of all of the Sm RL emissions can be obtained by assuming that the relevant electron and hole traps are close to Sm3+. Our model accounts for F3-centre production during irradiation that affects some of the Sm3+ RL emissions via reabsorption of the RL by the F3-centres. Thus, the rate of F3-centre production can be conveniently monitored by the RL intensity ratio, I RL(567 nm)/I RL(650 nm). Additionally, the Sm2+ RL emissions may be expressed as [1.94 × I RL(721 nm)] - I RL(695 nm) to determine the real-time dose rate, independent of dose history.