The preceding chapters have shown that the CCD has achieved unprecedented levels of performance in read noise and charge transfer efficiency. Optimizing the sensor to near perfection in these areas has produced a device that is extremely vulnerable to damage induced by high-energy radiation sources. CCDs are also more vulnerable because past performance was not as prominent as today. Now, ultralow dark currents and ultrahigh charge transfer efficiency (< 0.9999995) make the presence of a single defect introduced by radiation important.
The CCD is being used more often in hostile environments where high levels of radiation are encountered (e.g., outer space imaging applications, particle detectors used in beam colliders, nuclear weapon use, plasma physics). The emphasis of this concern reflects upon the author's own experience, because the radiation problem has surfaced on every flight CCD mentioned in this book. Many surprises were encountered, including some show stoppers. For example, the Galileo CCD would have arrived at Jupiter in a saturated state if neutron testing was not carried out before launch. Here, neutrons created very large dark spikes in the array. To circumvent the dilemma, the operating temperature for the CCD was reduced from â40Â°C to â120Â°C. The SXT CCD would have also failed prematurely because of a serious x-ray-induced ionizing radiation problem. The solution for SXT was to employ a UV flood that neutralized radiation-induced charge in the gate dielectric. The operating temperature for Cassini and Hubble was optimally selected (â90Â°C) to ameliorate a serious trapping problem induced by protons. The Cassini Star Tracker CCD experienced a reverse annealing problem that forced engineers to a new cooling temperature (â40Â°C) and clocking sequence. Unfortunately, the new Chandra mission was not as lucky as these missions. As we will discuss below, this flight camera experienced a radiation problem that seriously degraded CTE performance shortly after being launched into space. The experience gained from these missions clearly shows that the radiation problem cannot be taken lightly and must be thoroughly analyzed to forestall potential crises.
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