The 35-ton phase II run at Fermilab

A shower event induced by a cosmic ray muon observed in the TPC of the DUNE 35-ton prototype on Feb. 18, 2016. (Credit: Tingjun Yang)

A shower event induced by a cosmic ray muon observed in the TPC of the DUNE 35-ton prototype on Feb. 18, 2016. (Credit: Tingjun Yang)

After cool-down and fill in late January, the 35-ton prototype detector began its commissioning run on February 2. Liquid argon purification and recirculation started a few days later and the whole system has been running continuously since then — modulo the effects of a Fermilab-wide power outage on March 4. The prototype is outfitted with small-scale APA modules of the single-phase FD design, a field cage, cold digital electronics and waveguide-style photon detectors — all immersed in the liquid argon — and a triggerless-capable DAQ.

Alan Hahn led an effort to exercise all the hardware during both the cool-down and filling operations to look for any issues that might occur.

“Everything survived,” he said. “After pumping and filtering for about 12 hours, our first purity monitor signal showed an electron lifetime of about 80 microseconds and the first track was seen with a purity in the 100 microsecond range. Now it is in the 2.5 to 3.5 millisecond range — this is faster than we saw during the Phase I run.”

The prototype has been collecting data 16 hours per day with the drift field ramped up to as much as 60 kV at the cathode, half the target field strength. The team can associate tracks in the event displays with hits in the external scintillation counters. However, coherent noise — the source of which is still under investigation — is severely hampering the performance. The tracks are visible qualitatively, but the signal-to-noise is very poor.

Despite the problems, team leaders Hahn and Michelle Stancari agree that it is useful to test the new field cage design at full voltage.

“We’ll take data at this cathode voltage, an intermediate voltage and full voltage,” said Stancari. “How much at each remains to be seen.”

The team is discussing plans to run a few dedicated expert studies after that.

“These studies will benefit future LArTPCs, if not the 35-ton,” said Hahn. “But testing the design was one of our goals.”

— Anne Heavey