Gamma-ray Large Area Space Telescope Tower 1 TVAC tests Bad ladders issues

Tower 1 TVAC tests: bad ladders issues Description of the issues Layer Y0 (BTM 005 Front) – ladder 3. –Strange behavior during the charge-injection tests (single strip occupancy always close to 50% - independent of the threshold – as if the ladder was not biased). –Noise level not consistent with missing bias. –Observed during the first CPT after the vibe test (in air, room temperature), then disappeared. Layer X16 (MID 063 Back) – ladder 0. –Left side of ladder 0 noisy. –Right side of ladder 0 completely dead. –First observed during the first hot soak. –Consistently there since then.

Tower 1 TVAC tests: bad ladders issues X16: noise occupancy measurement Bias voltage set to 0 V. Tue Mar 08 16:11: , T=? Channels 238 and completely dead (never showing up in the data stream). All other channels are very noisy due to missing bias (the occupancy plot shape is due to the limited size (64) of the GTRC event buffer. Bias voltage set to 100 V. Mon Mar 07 23:24: , T=45C Channels completely dead (never showing up in the data stream).

Tower 1 TVAC tests: bad ladders issues X16: noise occupancy vs. threshold Threshold=30 (range 0), bias=80V Tue Mar 08 01:06: , T=45C Threshold=35 (range 0), bias=80V Tue Mar 08 01:08: , T=45C Threshold=40 (range 0), bias=80V Tue Mar 08 01:09: , T=45C Threshold=50 (range 0), bias=80V Tue Mar 08 01:11: , T=45C

Tower 1 TVAC tests: bad ladders issues X16: trigger rate measurement Bias voltage set to 0 V. Wed Mar 09 12:29: , T = ? Bias voltage set to 100 V. Wed Mar 09 12:40: , T = ? Bias voltage set to 80 V. Tue Mar 08 01:15: , T = 45 C

Tower 1 TVAC tests: bad ladders issues X16: noise and gain measurement I Bias voltage set to 0 V. Tue Mar 08 16:05: , T=? Channels 238 and completely dead (never showing up in the data stream). Bias voltage set to 80 V. Tue Mar 08 00:08: , T=45C Channels completely dead (never showing up in the data stream).

Tower 1 TVAC tests: bad ladders issues X16: noise and gain measurement II Bias voltage set to 0 V. Wed Mar 09 12:24: , T=? Channels 238 and completely dead (never showing up in the data stream). Bias voltage set to 100 V. Wed Mar 09 12:35: , T=? Channels completely dead (never showing up in the data stream).

Tower 1 TVAC tests: bad ladders issues X16: conclusions Register tests are OK. The left side of ladder 0 is much noisier than normal, as if the ladder was not biased. Noise occupancy – as measured with random trigger – is *NOT* strongly dependent on the threshold on discriminators. The right side of ladder 0 (1/2 roughly) is completely dead – it never shows up in the data stream, not even the electronics noise is observed. *NOT* consistent with a simple short on the HV line. –The number of ‘dead’ channels varies with time and bias voltage (and temperature?). Different runs with the bias voltage set to 0V give identical results. Results from different runs with normal bias voltage slighlty different. –The number of dead channels is *NOT* multiple of 64  no correlation with the GTFEs. It seems like it’s NOT a MCM issue. –Not an effect of the GTRC buffer size (at least 2 GTFEs are ‘dead’ at the trigger rate test, which does *NOT* even involve data readout). –A short to ground of the preamplifiers input (wire bondings in contact with the tray/wafer) could explain this kind of behavior. Changing the HV makes some difference. Probably *NOT* an open on the HV line.

Tower 1 TVAC tests: bad ladders issues Y0: noise and gain measurement Evident problems during the tests with charge injection: the occupancy for all the channels of ladder 3 was close to 50%, no matter what the threshold on the discriminator was. Just like the ladder itself was not biased. The bias line is not shorted to ground as the leakage current measurment gave standard results

Tower 1 TVAC tests: bad ladders issues Y0: noise occupancy Three clusters of noisy channels on ladder 3. If the ladder was not biased, noise would be much higher over the entire ladder.

Tower 1 TVAC tests: bad ladders issues Discussion on possible intervention on Tower1 Minimal intervention to identify root cause/confirm current ideas would require: –removal of 1 or 2 sidewalls –inspection of layers X16, Y0 –reinstallation of the sidewalls  can be done (already done for towerA)  minimum delay 1 week Fix would require (at minimum) –complete tower disassembly –fix bad wire-bonds on layers X16-Y0 or use other trays –tower reassembly  could be decided after results of inspection  minimum delay 2 weeks on tower-1, 1 week on tower-3