Cold Operation of Chip 17, an Irradiated FE-I4

1. Cold Operation of Chip 17, an Irradiated FE-I4 Eric Hunter - LBNL

2. Eric Hunter - LBNL PrmpVbp: Used to turn Preamp On or Off Vthin_altfine: Adjusts Threshold for Discriminator (0 low, 174 nom., 255 hi) Amp2Vbpf: Feedback for Main Amplifier (0 low, 52 nom., 255 hi) Figure 9 from FE-I4 manual Circuit Schematic for the Readout Electronics on each Pixel

3. Eric Hunter - LBNL Amp2Vbpf = 100 This is a noise-occupancy map of the chip, oriented the same way as the picture on Slide 1. The plot variable (for each cell) is the number of hits in a run where only 1 million hits are collected (for the whole chip). In certain conditions, e.g. here at 240 K and low (1400e) threshold, the radiation-damaged cells accrue more noise-hits than the other cells. Amp2Vbpf = 50

4. Eric Hunter - LBNL Chip Noise Occupancy (# Noisy Cells per Chip) usually increases as we lower the feedback voltage at Amp2 (because this voltage biases the input down, it is like another threshold knob). The noisy “pixels” are, at room temperature, the ones in the Burn Zone (BZ).

5. Eric Hunter - LBNL But when “power” (PrmpVbp) to the pre-amplification stage is removed, the effect of Amp2 feedback depends on temperature in strange ways. In these scans at 265 K, the hits are IN the BZ when the preamp is ON, but when the preamp is OFF, hits are mainly OUTSIDE the BZ and the “number of noisy pixels function” is flipped.

6. Eric Hunter - LBNL Scans at 240 K can be hard to take with Chip 17. Data streams are often garbled. Yesterday we chilled the chip to 230 K and this problem ceased (so far). These scans were then taken at 240 K, before the fix. Regardless of preamp power, the hits arrive in the BZ. But the response curve now has a maximum. Similar data was obtained after the fix with the preamp ON, but with the preamp OFF the number of noisy pixels tends toward 26880, regardless of NOCC event collection cutoff. There is however a visible (but not quantifiable, to 4 digits) trend; see slide 9.

7. Eric Hunter - LBNL A “noise resonance” around nominal Amp2 (50) was seen last week, on the first day of testing (below, left). To the right of the peak, hits arrived in the BZ, and to the left, outside the BZ. Exceptional data points have since been seen in this Amp2Vbpf range at 240 K, but not since the chill-fix yesterday. The largish occupancies at nominal Amp2 increase as the threshold is lowered (below, right). But the full “resonance” graph is not reproducible.

8. Eric Hunter - LBNL Noise Distribution by Threshold← Preamp OFF, 240K, Amp2Vbpf 52 Threshold~1800 The relative contribution from radiation damaged pixels is reduced by raising the threshold. Threshold~1400

9. Eric Hunter - LBNL 240 K, Preamp Off, Threshold 2200. Distribution of Noise Hits Turning down the amp2 feedback current makes the damaged channels less noisy than the undamaged, when the preamp is off. Amp2Vbpf = 200 Amp2Vbpf = 100 Amp2Vbpf = 8 Amp2Vbpf = 30

10. Eric Hunter - LBNL Summary • Turning down Amp2Vbpf usually increases the number of radiation-damaged pixels that will accrue hits in noise occupancy scans • Turning down the threshold (Vthin_altfine) has a similar effect • The chip is a little unpredictable around 240 K; Maurice suggested that it may be the temperature at which the water in the dry air supply condenses.