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: http://www.stsci.edu/hst/HST_overview/documents/datahandbook/hst/wfc3/documents/handbooks/currentDHB/Chapter6_CTE3.html
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WFC3 Data Handbook V. 4.0 |
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CTE losses arise during the readout process. The charge packet for each pixel is transferred pixel-by-pixel down the detector, in parallel, to the serial register. As the packet moves through the silicon of the detector, it often encounters imperfections (traps) in the lattice that have been caused by radiation damage. These traps can temporarily detain individual electrons. When an electron is trapped, it gets separated from its original charge packet. It is often released some time later and finds itself in an upstream pixel. For this reason, CTE takes charge away from bright downstream pixels and deposits it into fainter upstream pixels. Charge packets that have more electrons tend to occupy a physically larger volume within the pixel. As such, larger packets have larger cross-sections to traps as they are shuffled through the silicon. For example, a pixel cloud with ten electrons will interact with more traps in its trip down the detector than will a pixel cloud with only one electron. For the WFC3/UVIS model we discuss below in Section 6.3, a cloud with ten electrons will see about 3 times more traps than a cloud with only 1 electron. It is worth noting, however that even though losses increase in an absolute sense when we have more electrons in a cloud, the per-electron fractional losses go down with increasing packet size.Figure 6.1: Effect of Electron TrapsFigure 6.1 shows the model constructed by Anderson et al. (2012) (see the document on the CTE-Tools webpage and Section 6.3 below). The model was based on observations taken in August 2012. On the left, we show the cumulative number of traps as a function of packet size. We see that a charge packet that contains just one electron will encounter 20 traps on its 2000-pixel journey to the serial register, if there is no background. Clearly we should not expect such an electron to survive the journey. A charge packet with ten electrons will encounter 60 traps and is also not likely to survive to be detected. A packet with 100 electrons will lose 90, and something should therefore be detected at the readout register.On the right of Figure 6.1, we show the marginal number of traps. By "marginal" we refer to the number of traps that would be seen by the Nth electron in a charge packet. The first electron will encounter almost 20 traps on its ride to the readout register. As such it has a ~1 x 10-12 chance of making the journey without being delayed. The second electron will see 12 traps, the third 4, and the tenth less than 1.