1. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 1 ACD Pedestals ACD Team at Goddard: Alex Moiseev Dave Thomspon Bob Hartman

2. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 2 ACD Pedestals Pedestals monitoring: we want them to be within ±5 ADC counts of reference value (< ±1.25% of the mip, which is at minimum 400 ADC counts); otherwise reference values have to be changed respectively. Another variable is a temperature; pedestals vary with the temperature. Our tests at Goddard demonstrated that pedestals variation is acceptable within ± 5C temperature variation; thus pedestals are calibrated for 10C temperature bins. All results presented here assume ambient temperature (18C – 28C) Summary: we found that pedestals stay stable within needed range for extended period of time if there was no (known) external configuration changes made we found that some pedestals change by up to 40 ADC counts when: - other GASU is used - HV is applied or not applied to ACD (? – see below)

3. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 3 ACD Pedestals (cont.) Tests Compared Mean of 3 runs GSFC, May 23, 2005 GASU 1, HV On Pre- thermal GSFC, July 21, 2005 GASU 1, HV Off Thermal 1, GSFC, July 24, 2005 GASU 1, HV Off Thermal 10, GSFC, August 4, 2005 GASU 1, HV Off Post- delivery, SLAC, August, 2005 GASU 1, HV Off ACD CPT 1, SLAC, 12/28, 2005 GASU 2, HV Off ACD CPT 2, SLAC, 12/28, 2005 GASU 2, HV Off B13 ZS Off SLAC, January 14, 2006 GASU 2, HV Off B13 ZS Off SLAC, January 14, 2006 GASU 2, HV On 57 62 11 8 39 104 1 Red frame and plot numbers – compared with “Mean of 3 runs” Blue frame and plot numbers – compared with ACD CPT 1 (new GASU)

4. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 4 ACD Pedestals ( cont.) During a number of test runs at Goddard: Before and after Vibration, Acoustic, and Thermal tests, as well as after delivery of ACD to SLAC (in total over 3 months) – pedestals stay stable within ± 5 ADC counts (plots 1 - 4) Pedestals shifted significantly – by 5 and more ADC counts – when GASU was changed (ACD CPT, December 2005), 39 channels in total – plots 5 – 8. Channel-to-channel comparison with data from Margin Test (performed at Goddard) is well consistent with the assumption that FREE voltage changed to 3.25. This is very marginal to ACD Performance. Needs attention from Electronics Team! Pedestals again stay stable during CPT and full LAT B13 runs (with HV Off by mistake) – plots 9 and 10 But some channels shifted when HV was brought On (plot 11)! – see next slide – new puzzle!

5. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 5 Sudden pedestals change during B13 Found a fact that in two B13 runs, separated by a few minutes, the first one (135005319) was running with ACD High Voltage turned Off by mistake. ACD High Voltage was turned On in all next B13 runs. ChannelPedestal Position difference, ADC counts 0-002-19 0-042-15 1-124-11 0-130-23 1-222-11 1-320-20 0-230-9 1-230-18 0-330-16 1-422-17 0-430-12 1-430-18 It appeared to be a luck for us to find a strange thing: there are 12 channels where pedestals shifted by more than 10 ADC counts between runs with and without HV (Pedestal HV On – Pedestal HV Off ) All these changes have negative sign! All other channels stay stable, but with slight negative difference (about 1 ADC count) We have previously compared pedestals positions with and without HV applied and FOUND NO DIFFERENCE – “Mean of 3 runs” was taken with HV On, and “Prethermal”, “Thermal 1, 10”, “Post- Delivery” were taken with HV Off (ACD Long Functional test, pedestal runs). This is a new problem!

6. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 6 New Puzzle Puzzle: All that 12 shifted pedestals are from GAFEs with number 17 (last on the board) – one per each (!) ACD FREE board! – discovered by Dave Thompson All pedestals shifted in one direction – cannot be explained by voltage change because it causes both direction change All other pedestals stay stable within 1-2 ADC counts. There was small shift of all pedestals in the same direction by ~1 ADC bin. It is negligible, but the fact itself can contain a hint. This pedestal shift happened during a few minutes – the only change made was applying High Voltage – and pedestals changed! Pedestal change is significant to cause high rate – more than Zero Suppression threshold Need to repeat this test to confirm the effect So far NO IDEA how to explain it SHIFTED CHANNELS NOTHING!

7. GLAST LAT Project IA weekly meeting, February 3, 2006 ACD subsystem Alex Moiseev 7 Conclusions Conclusions: 1.ACD Pedestals seem to be stable over time if no changes are made 2.We believe that pedestal change, when other GASU was installed, was caused by slight change of low voltage (3.3 V) supplied by GASU to ACD FREE board 3.Our previous tests demonstrated that pedestals behave by different way with variation of FREE voltage – some stay unchanged, some have changes in both direction. The fact that ALL changes in B13 runs have negative sign, does not agree with this 4.Our previous tests also demonstrated that pedestals do not depend on HV (On or Off) 5.We request for additional pedestal runs, with HV On and Off 6.Until the behavior of pedestals is completely understood, pedestals runs have to be run periodically 7.Would it be possible to run periodical trigger with switching to ZS Off mode? 8.The puzzle with “end” GAFE’s pedestal shift – needs explanation