Organic-inorganic halide perovskites (OIHPs) bring an unprecedented opportunity for radiation detection with their defecttolerance 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 -ray spectrum collection at room temperature. CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃ are found to be p-type and n-type doped, respectively, whereas dopant-compensated CH₃NH₃PbBr_2.94Cl_0.06 alloy has over tenfold improved bulk resistivity of 3.6×10⁹ φcm. 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 1.8Vmm^-1 at room temperature.