Ionizing Dose Effects

The X-ray ionizing dose test was made by placing an Fe-55 source in proximity to an engineering front illuminated CCID-17 with a geometrical baffle that limited exposure to a small section of the image array. No part of the frame store or serial register output node was exposed. The exposed region experienced a flux of 7 counts per pixel per second for 10 hours for a total equivalent dose of 900 rads. This exposure was made while the CCD was at room temperature in vacuum. The standard calibration timed exposure readout method was employed for all testing.

The most noticeable result of this test was the production of an area of enhanced dark current directly coincident with the exposed region (see figure 4.120). The dark current was uniform on small distance scales and no exceptionally hot pixels were seen. The enhanced dark current could only be measured for operating temperatures above -80 C. At the standard -120 C operating temperature no enhanced dark current could be measured even for exceptionally long integration times. Furthermore no increase in CTI was seen at any temperature. Long periods of high temperature bakeout (20 hours at +80 C) had no affect upon the damaged region, which appears to be a permanent feature. Experience with overexposure to X-rays at BESSY-PTB, XRCF-2CACIS and SES confirms the enduring nature of this type of radiation damage.

A similar X-ray ionizing dose experiment was made with a back illuminated CCD which yielded qualitatively similar results. A special overexposure of focussed bare carbon anode radiation on the backside w134c4 S3 was made as part of the XRCF testing. At -40 C an apparent spot of increased dark current can be seen but at lower temperatures this feature is not evident.