Metal halide perovskites are promising candidates for room temperature gamma-ray spectrum detectors, however, it is challenging to grow high-quality single crystals under operation conditions to make them economically competitive to existing materials. Here, we report the growth of centimeter-size formamidinium-cesium lead bromide (FACsPbBr3) single crystals at high yield from low purity (98%) and thus low-cost precursors using a relatively low-temperature solution method. The introduction of FA into CsPbBr3 reduces defect density in crystals by eliminating the phase transition during the cooling from the growth temperature to room temperature, which significantly improves the yield of crack-free and low trap density crystals. As-grown FACsPbBr3 exhibits a high resistivity of 9.5×109 Ω cm, balanced hole and electron mobility-lifetime products of (2.2–3.2) ×10− 3 cm2 V− 1, a long charge recombination lifetime of 182 ns, and a record low deep trap density of 5.6 × 1010 cm− 3, which eventually results in a high charge collection efficiency of 84% under gamma-ray. The FACsPbBr3 spectrum detectors with an asymmetrical metal electrode configuration have achieved an energy resolution of 2.9% for 662 keV 137Cs γ-rays. The improved spectrum capability allows them to resolve Pb Kα,β X-rays (75–85 keV) for the first time in 137Cs spectrum. FACsPbBr3 single crystals show excellent stability under large biases up to 1000 volts, and no degradation of spectrum performance after 7 months.