An apparatus is for a detector assembly configured to receive a signal from a scintillation detector. The apparatus includes a controller assembly including a controller analysis section. The controller analysis section includes a microprocessor assembly, and a memory assembly operatively coupled to the microprocessor assembly, the memory assembly tangibly storing microprocessor-executable code. The microprocessor-executable code to be read by the microprocessor assembly. The microprocessor-executable code is configured to urge the microprocessor assembly to execute an operation for: (A) establishing an energy calibration using features in a spectrum associated with known gamma radiation from radioactive decay or other nuclear processes; and (B) adjusting a gain to be applied by the detector assembly based on the known energy features and thus maintain the energy calibration. With a known energy calibration, the equivalent energy of the single photoelectron peak is inversely related to the light collection efficiency and the detector sensitivity.