Bright-Pixel Defects in Irradiated CCD Image Sensors
- PDF / 313,893 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 277 Views
0994-F12-06
Bright-Pixel Defects in Irradiated CCD Image Sensors William C. McColgin1, Cristian Tivarus1, Craig C. Swanson2, and Albert J. Filo1 1 Image Sensor Solutions, Eastman Kodak Company, Rochester, NY, 14650 2 Research Laboratories, Eastman Kodak Company, Rochester, NY, 14650 ABSTRACT We have examined environmental radiation sources for digital cameras to find the origins of bright-pixel defects known to accumulate with time. We show that beta and gamma emissions from camera parts and lenses cause image transients, but permanent damage can occur with alpha particles from the CCD cover glass. Our experiments with 14-MeV- and thermal-neutron beams confirm that cosmic rays are the primary cause of new imager bright points.
INTRODUCTION Radiation effects in CCD imager sensors are well known from space applications [1], but occur at sea level as well. They range from transient charge clouds collected by imager pixels as ionizing particles pass through to pixels made permanently bright by displacement damage from collisions. Although image transients may be troublesome in scientific applications, our concern here is for permanent changes that cause bright-pixel defects. These defects have been called dark-current spikes, blemishes, or bright points. Bright points are known to accumulate with time and from shipment by air [2]. Recently, Theuwissen showed how the accrual of new points varied for imager storage at different altitudes. The brightest points correlated with cosmic-ray neutron populations [3]. However, the weaker points seemed to have had other causes. We have investigated how radiation sources in the environment of a consumer digital camera can affect bright-point creation. These include not only cosmic rays, but also the camera itself. Camera parts may contain traces of uranium and thorium, whose decay chains generate alpha and beta particles and gamma-ray photons. Betas and gammas also come from K-40, a naturally occurring isotope of potassium, which may be found in camera lenses, the imager cover glass, and even the camera owner. Lanthanum, used to increase the refractive index of some lens elements, also has a radioactive component. La-138 emits gamma rays. To study the effect of cosmic-ray neutrons, we irradiated imagers with 14-MeV neutrons and with thermal neutrons. We have determined the probability of producing bright points for each of these sources. We compare these to the bright-point accumulation rate found for CCD storage.
EXPERIMENTAL The imagers used in this work were KODAK KAF-8300 Image Sensors. These are 8megapixel, full-frame imagers with 5.4-µm-square pixels and an active area of 1.8 cm x 1.35 cm. Bright points in these imagers were arbitrarily defined as pixels with dark-current generation rates at 60∞C of at least 355,000 electrons/s above the local average. Table I lists the radioactive isotopes found in cameras along with their properties. Also listed are the calibration sources used as models for irradiation. Because we had no enriched K-40 source, we bracketed the K-40
Tab
Data Loading...
