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Sensitive X-ray detection is crucial for medical diagnosis, industrial inspection and scientific research. The recently described hybrid lead halide perovskites have demonstrated low-cost fabrication and outstanding performance for direct X-ray detection, but they all contain toxic Pb in a soluble form. Here, we report sensitive X-ray detectors using solution-processed double perovskite Cs₂AgBiBr₆ single crystals. Through thermal annealing and surface treatment, we largely eliminate Ag⁺/Bi³⁺ disordering and improve the crystal resistivity, resulting in a detector with a minimum detectable dose rate as low as 59.7 nGy_air s⁻¹, comparable to the latest record of 0.036 μGy_air s⁻¹ using CH₃NH₃PbBr₃ single crystals. Suppressed ion migration in Cs₂AgBiBr₆ permits relatively large external bias, guaranteeing efficient charge collection without a substantial increase in noise current and thus enabling the low detection …