4.11 Radiation Damage and Hot Pixels

In low Earth orbit (LEO) the CCDs are subject to radiation damage from the Earth's radiation belts. The WFPC2 CCDs are shielded from energetic electrons and about half the incident energetic protons. Long term radiation damage to the CCDs from high energy protons leads to an increase in dark count rate (mainly from the creation of hot pixels), baseline shifts in the CCD amplifiers, and long term degradation of Charge Transfer Efficiency (CTE). There has not been a significant degradation in the amplifier baselines. CTE is discussed in the Section 4.12. On the other hand, one of the major radiation damage mechanisms is the creation of new Si-SiO₂ interface states, which cause increased dark current rates in affected pixels. In the MPP CCD these states immediately recombine with holes, reducing the gradual increase in dark noise by factors of about 25, compared to normal three-phase CCDs (Woodgate, et al. 1989, Janesick, et al. 1989b).

Figure 4.12 is a histogram of the dark current distribution (in e^- s^-1) for hot pixels. It contains three curves: solid for the histogram of all hot pixels just before a decontamination (April 7, 1995); dashed only for the pixels that were hot just before the decontamination and were not hot at the beginning of the cycle (March 10); and long-dashed for pixels that were hot at the start of the cycle and were fixed by a decontamination. Thus, the dashed line represents the "new" hot pixels, and the long dashed line represents the fixed hot pixels. The fact that these two curves are very similar shows that the number of hot pixels is roughly in equilibrium. The majority of new hot pixels have low dark current. The hot pixels that constitute the accumulated legacy of previous periods, and thus survived one or more decontaminations, include higher-current pixels. The population of hot pixels increases at a rate of approximately 33 pixels CCD^-1 day^-1 above a threshold of 0.02 e^- pixel^-1 s^-1, while the camera remains at the normal operating temperature.

About 80% of the new hot pixels return to a normal state at decontamination events when the CCDs are warmed to a temperature of +22°C for 6-12 hours. There is no evidence that the fraction of hot pixels that returns to normal is related to the length of the decontamination. Of those pixels that are not fixed, about half will be fixed after two or three additional decontaminations. After that, the rate of correction decreases. It is conceivable that all hot pixels will be repaired eventually. At the moment there is no evidence of a significant secular increase in the number of hot pixels, and we have a firm upper limit of 8% on the fraction of hot pixels that are not repaired after several decontamination cycles.

Figure 4.12: Hot Pixel Histogram.

In order to deal with the hot pixel problem, we provide monthly lists of possible hot pixels via the World Wide Web. Look for hot pixels under WFPC2 Instrument News at:
http://www.stsci.edu/instruments/wfpc2/wfpc2_hotpix.html

These lists are best used to flag hot pixels as bad. While we do provide an estimate of dark current for each hot pixel as a function of time, there are indications that the noise in hot pixels is much higher than the normal shot noise, and thus dark current subtraction is unlikely to give good results.