The green giant


From left: Matt Toups, Jarrett Moon, Alberto Marchionni, Kanika Sachdev and Evan Niner next to the dipping apparatus for the photon detector light guides. All are from Fermilab except Moon, who is a grad student at MIT.

A production facility for the single-wavelength-shifting light guide for ProtoDUNE-SP is turning on at Fermilab. Matt Toups and his team, with their Operational Readiness Clearance in hand, have just set up their first production light guide bar for dip-coating in a solution of wavelength-shifting tetraphenyl butadiene, or TPB.

Janet Conrad’s laboratory at MIT developed the bars and the bar-dipping technique. The Fermilab system is an enlarged version of the one at MIT.

“Our tech, Len Bugel, was instrumental in this system’s design and spent several weeks at Fermilab to help set up the system,” said Conrad. The joint Fermilab-MIT team has been working day and night to get it up and running.

An impressive four-meter-plus tall cabinet suffused with low-humidity nitrogen gas stands in Fermilab’s Lab 6 – virtually the only place on site with a ceiling high enough to accommodate it. Barely.

Kanika Sachdev, a Fermilab postdoc, points up to a crane bridge.  “See the bridge? The cabinet only clears it by a few inches,” she said. “We would really like to work in a clean room since the light guide bars tend to collect dust, but none of the clean rooms are big enough.”

The first production acrylic light guide, 2.1 m long, inspected for dimensions and clarity, and carefully cleaned, hangs above a bath of TPB solution, waiting to be dip-coated.

“The solution is just slightly more viscous than water,” said Sachdev. “We have to lower it slowly, and pull it out at about 2 inches per second. That speed yields the most uniform and smooth coating.”

The Fermilab-MIT team has been honing the process with a series of 20-inch prototype bars, while engineers figured out how to maneuver the giant dipping cabinet into place.

Interior of scanning box, showing a prototype bar on the right attached to SiPMs and electronics.

In their “clean enough” lab, Toups and his team prepare the bars for dipping, dip them, let them dry, then place them in a black scanning box to test the attenuation length using LEDs and silicon photomultipliers (SiPMs). Some (shorter) bars will be further tested in a dewar of liquid argon while the rest will be shipped to Colorado State University (CSU) for assembly. (See related article.)

Evan Niner, another Fermilab postdoc, will accompany the bars to CSU later this month and then to CERN for installation. Over the next few months the team will dip, test and ship a total of 36 bars – five plus a spare for each of ProtoDUNE-SP’s six APAs.