Progress on resistor failure diagnosis Ben J ( + Matt, Zander)

Effect of Orientation / dark box tests In our “dark cardboard box”, we ran spare PMT all weekend exploring different orientations of PMT We had already reported some possible small gain shifts with the box upright We saw one possible large gain shift over the weekend with box on its side The frequency of the shifts is not the same as the failure mode we saw in situ, and base impedance appears to be unaffected We checked the effect of stray light in the annex (lights on / off) and know this has no effect But, we are doubting the stability of this setup for performing long term tests, and consider these results to be inconclusive at present.

“left” “right” “HV” Strain relief points and failed resistors highlighted During installation we noticed stray conductance between “right” and ground which could be removed by scraping base with stylus. Physical location of failures hints at this as being relevant to failures.

Paula added strain relief to 3 blank bases for us, prepared and cleaned as with MicroBooNE bases Also added extra strain relief at “HV”, not in uB bases With a voltmeter, we measured resistance between strain relief and each component mounting pad for each resistor on the top face Conductive pathways with O(100 MOhm) are found to resistors near the support pins. Paths to more distant resistors show no conductance To cross check whether this is nominal stray conductance of the base material, checked between pairs of adjacent pads elsehwere on base, no other stray conductances seen Where there is conductance, not always possible to obtain a stable measurement - we record here the lowest measured value. Stray Conductance Tests

Measured conductances

Failed resistors highlighted

Stray conductance found on 1/3 bases Stray conductance found on 2/3 bases Stray conductance found on 3/3 bases From “left” reliefFrom “right” reliefFrom “HV” relief

Effect of Temperature If this effect is related to our failures, it is important to understand temperature dependence Matt prepared a PCB using solder flux to artificially create a conductive path between to PCB pads We monitored the resistance as it was lowered into a bath of LN2 Reliably the resistance rises from initial ~180MOhm to off scale (>500 MOhm) during dunk test, and returns to original value in the warm If these conductive paths are important for the failure mode, we will be safer at LAr temperature than room temp Flux path added Ω

Effect of Temperature on Real Bases Flux paths are intermittent on the real bases The act of soldering a cable onto the base(rosin cored solder) appeared to remove the conductivity on the first 3 bases We had Paula prepare a forth base after cables were attached for measuring paths One path was found, but required moving one of the attached cables. Attaching using organic cored solder, the flux path survived Same behavior observed, 100MOhm resistance increased to 1GOhm (but not open) on cooling to LN2 temperature

Linking flux paths to failures Physical layout of flux and resistors is suggestive We tried to simulate a failure in two ways: – 1. Add flux to a working PMT base over loops and resistors. PMT has been operated stably overnight and shows no sign of failure. First run was about 14 hours with flux added, no problems We then deliberately damaged the cable sheath near the loops and the shield with steadily increasing malevolence, and still see no problems. We accidentally exposed the PMT to a large light leak, and saw no problems – 2. Hold HV across a conductive flux path for some time and check for variations over time. Limited resolution for our ammeter made it impossible to see small changes, but no long- lived changes or large breakdowns were observed. As yet we have not demonstrated a link between stray flux conductance and resistor failure.