Methane Update and Run Plan Ben Jones

Configuration 8 inch PMT with TPB plate - 8” from alpha source - Now includes magnetic shield - 2 inch PMT without TPB - Inside metal casing to be mostly blind to indirect light - Gaps at top and open bottom for liquid flow and mixing - Alpha source ~1cm from tube face

Vacuum and Purity We did not reach our usual vacuum requirement We suspect that failure to clean some of the new parts is at fault. In this run we see: Elevated water levels (120 ppb) Reduced light yield (~40 PE alpha peak was ~74 PE with better vacuum)

Vacuum and Purity Since we dumped on July 4 th and started pumping again, seems that we have picked up where we left off Although seemingly we’re doing even better this time (some of the water / dirt already left with the last argon perhaps?) We don’t have time to wait for the plateu, but will not the final vacuum value.

Status of Monitors We have now lost H20 and N2 monitors to MicroBooNE We still have an O2 monitor For the first time we also now have access to a Universal Gas Analyzer (mass spec sensitive to ~0.01% levels of impurity) We calibrated the RGA using methane / argon pre-mix gas injected near Bo at 10psig

After calibration

Methane Injections We had no idea what concentration we were likely to see methane absorption or re-emission Plan was a logarithmic scan, starting at a 1ppm injection, and taking points at 10, 100, 1000, etc 1,10,100 ppm injected using N2-esque gas injection method Higher points injected using flow meter calibrated for methane by Ami and Gabriel

Linear scale Log scale All the alpha scintillation light is GONE at 1ppm CH4!

Maybe some tiny emission feature at 3PE… Maybe just stray light from the outside. But whatever it is, its too tiny to be useful (remember, alpha is 1cm from PMT here) Note : take SPE scale with a pinch of salt, since calibrated with noise only (no pulser or late light)

Whats Next? Our question is answered conclusively – adding methane to argon means no scintillation light There appears to still be Cerenkov light We don’t know what the relevant CH4 concentration for light loss is (just that its < 1ppm) We don’t know whether it is a quenching or absorption process We can find that out both those things by injecting pre- mix calibration gas to get to lower concentrations We have 2% CH4 in argon available – this will allow sensitivity to the PPB scale and up. If that’s not enough, we can order more dilute pre-mixes

Who cares? Not people thinking about a CH4 / LAr neutrino TPC, thats for sure. But, highly radiopure argon extracted from underground for darkside has a known methane contamination at the few % level. They are developing methods to remove it at Fermilab. Best performance so far is “below sensitivity of RGA”, which means >0.01%. This has been assumed to be good enough, based on radiopurity specs Apparently it was not previously known that this argon has no scintillation light. Finding out where the cutoff happens and whether its quenching or absorption is now very important for that project

Run Plan Monday – Fill with LAr, PMT cooldown Tuesday – PMT cooldown Wednesday – ppb range studies Thursday – ppb range studies, dump Friday – Dumping Saturday - Vacuum Sunday – Vacuum Monday – Fill for Nevis If Bo not required for first days of Nevis residency, let us know and we can take more data points.