Pixel CFD Triggers

Figure 1a shows a typical waveform from a shower as it would appear atthe input to a PCFD. Note that this is a simulated waveform using the arrivaltimes of the shower PE's at the camera and randomly generated noisePE's (including afterpulsing). The timing spread of the shower PE's due toshower structure, mirror aberrations and the path length differences of thephotons to the camera focal plane have been carefully included.

Figure 1b shows the same pulse, plus that pulsedelayed by 4 ns and amplified by 1/PCFDFRAC , where PCFDFRAC is the trigger fraction that the PCFD is set at. In our case 67%, we think. 4 nsis the delay used in the PCFD's. In these figures, 1 bin is .25 ns. The timing of the PCFD output is set by where this second curve exceeds the firstanytime after the threshold is reached (a simple threshold discriminator) on the first curve. This is how CFD's work.

The parameters used in the model to generate these single pixeltriggers and their timing follows.

• Pixel Spacing: .243 deg.
• Noise Rate: .744 PE's/ns/pixel
• Expected Pedestal width: 3.86 PE's
• Noise Afterpulse Fraction: $3.\cdot 10^{-4}$
• PCFD Threshold: 11.58 PE's
• PCFD Delay: 4 ns.
• PCFD Trigger Fraction (PCFDFRAC): 67%