Inorganic phosphor material, gadolinium oxysulphide active with terbium (Gd 2 O 2 S:Tb 3+ ) has been previously investigated as a scintillator used in the dosimeter to detect the X-Ray dose due to its high light yield efficiency of converting the X-Ray into the visible green lights. Apart from the properties of light yield efficiency, fast scintillation and lower energy resolution matters for the clinic applications. Therefore, this work is focusing on the difference between the long and short decay time of the scintillator when the sensors are exposed to radiation source delivered by a modern LINAC (Linear accelerator) machine. Herein, the output signal with up to 40 ns temporal resolution has been measured. The final results show that inorganic phosphor material used in this study with long decay time (Gd 2 O 2 S:Tb 3+ ) has a heavy after-glow phenomena compared with the organic scintillator with short decay time (BCF-12 scintillating fibre). This work is to show the magnitude of difference of the photon yield efficiency and decay time between the Gd 2 O 2 S:Tb 3+ and the BCF-12. The output intensity of Gd 2 O 2 S:Tb 3+ is approximately 180 times higher than the BCF-12's, however the BCF-12 can effectively profile the duration of the radiation source against the beam on time.