The discussion focuses on simulating gamma ray detectors, particularly the response time of scintillation crystals. Response time refers to the duration required for the crystal to emit detectable scintillation light after photon interaction. While photon traversal time is extremely short, the energy deposition takes longer to convert into a light pulse, influenced by the decay times of crystal excitations. These decay times can vary significantly, with luminescence being around 10^-8 seconds and phosphorescence exceeding 10^-6 seconds. The choice of photomultiplier tube (PMT) also plays a crucial role in determining the overall response time of the detector.