Gamma-ray irradiation tests on CCD and CMOS sensors used in imaging techniques

Technologically-enhanced electronic devices are used in various fields as space imaging or diagnostic techniques in medicine. The devices can be exposed to intense radiation fluxes over time which may impair the functioning of the same apparatus, in particular in space applications. In this paper we report the results of a gamma irradiation tests on imaging sensors simulating the space radiation over a long time period. Two types of sensors are taken into consideration: CCD (Charge-Coupled Device) sensors and CMOS based on technology MOS (Metal Oxide semiconductor) used for the realization of transistors widely used in consumer electronics. The devices are supplied by Techno System (Italy), a company specializing in design, development and manufacture of electronic equipments. Gamma-ray irradiation test were carried out by means of IGS-3 gamma irradiation facility of Palermo University, based on 60Co sources with different activities. To reduce dose-rate and realize a narrow beam of gamma, a lead-collimation system was purposely realized. It permits to have dose rate values less than 10 mGy/s and to irradiate CCD and CMOS sensors in operation so as to verify operating parameters in extreme conditions. Remote measurements of dose and dose-rate were performed using PTW UNIDOS electrometers connected via shielded cables of about 30 meters to ionization chambers with various sensitive volumes: TM23361-0561, 30 cm3; TM30015-0011, 1 cm3; TM30013-3256, 0.6 cm3. The electrometers measure both the ionization current (dose rate) and the charge (integrated dose). The total irradiation dose to CCD sensors was about 350 Gy and the functioning of each sensor was verified during operation every 50 Gy while CMOS sensors were irradiated with a total dose of 100 Gy and a verify every 25 Gy. At the end of the tests, the images produced by the sensors have been verified and each highlighted a background noise. The brightness of many pixels resulted greater than the expected value, and some pixels were completely saturated. These effects, however, involve isolated pixels and therefore, in principle, should not affect the image quality because the digital image could be filtered by a pre-processing software. The images acquired during irradiation show a widespread and persistent noise, with many pixels in saturation.