Development of Radiation Resistant Monitoring Camera System

In response to the lesson of the Fukushima Daiichi nuclear accident, we started a development of a radiation-resistant CMOS color camera system. In order to develop the desired camera, ensuring the radiation resistance of the camera components, namely optical system, electric parts and image sensor, was addressed. About the optical system, trial radiation-resistant prism and zoom lens composed of quartz and calcium fluoride were fabricated and irradiated. The results showed that the both components had enough performance after 1 MGy irradiation. About the electric parts, the gamma irradiation tests were performed for commercially available devices. The results revealed that only few of the electric parts had less than 100 kGy radiation resistances. According to these results of irradiation test of a ready-made camera, dark current in image sensor increased and the camera didn't work only less than one hundred Gy. Therefore, about the image sensor, several different approaches in order to reduce dark current were considered. Firstly, the number of transistor, one of main source of dark current, was reduced to three from four. Secondly, to suppress irradiation-induced charging, a photo gate type instead of normal photo diode type of photoelectric conversion structure was used. Finally, the configuration of electrodes and the driving voltage of the sensor were optimized. As a result, the sensor was usable after 200 kGy irradiation. On the basis of these results, a trial radiation-resistant camera system equipped with shielding structure around the image sensor and electric parts was fabricated. The gamma irradiation test confirmed the system achieved over 900 kGy radiation resistance.