Organic-inorganic halide perovskites (OIHPs) bring an unprecedented opportunity for radiation detection with their defect-tolerance nature, large mobility-lifetime product, and simple crystal growth from solution. Here we report a dopant compensation in alloyed OIHP single crystals to overcome limitations of device noise and charge collection, enabling γ-ray spectrum collection at room temperature. CH3NH3PbBr3 and CH3NH3PbCl3 are found to be p-type and n-type doped, respectively, whereas dopant-compensated CH3NH3PbBr2.94Cl0.06 alloy has over tenfold improved bulk resistivity of 3.6 × 109 Ω cm. Alloying also increases the hole mobility to 560 cm2 V-1 s-1, yielding a high mobility-lifetime product of 1.8 × 10-2 cm2 V-1. The use of a guard ring electrode in the detector reduces the crystal surface leakage current and device dark current. A distinguishable 137Cs energy spectrum with comparable or better resolution than standard scintillator detectors is collected under a small electric field of 1.8 V mm-1 at room temperature.