Xenon delivered, ‘charcoal mellowed, drop by drop’
The Large Underground Xenon (LUX) dark matter detector obviously could not operate without its main ingredient—xenon—and last Wednesday 400 kilograms were delivered. “As of 10:05 a.m. today, the full inventory of LUX xenon was in the clean space in the Davis Campus,” Science Director Jaret Heise reported in an email. “Congrats to everyone involved!” Laboratory Director Mike Headley responded, “Thanks for getting this done safely!”
Heise credits the safe delivery to Science Integration Engineer Wendy Zawada and Facilities Technician Oren Loken, who worked with Infrastructure Technician Calob O’Grady and Facilities Technician Shane Heydon to move the xenon 4,850 feet underground.
LUX Science Coordinator Simon Fiorucci of Brown University also credited the team at Case Western Reserve University that provided an extremely pure batch of xenon. Off-the-shelf xenon gas might contain as much as 100 parts per billion of krypton, an impurity that would stymie the sensitive LUX detector. Case Western physicist Dan Akerib said his colleagues, including graduate student Chang Lee, built a system that removed krypton to less than 5 parts per trillion.
How did they do it? “You remember Jack Daniel’s whiskey, ‘charcoal mellowed drop by drop?’” Akerib asked. The Case Western team used about 60 kilograms of charcoal in a column just a little smaller than the 6½-foot-tall LUX detector itself. Using a “breeze” of helium gas, the team wafted impure xenon through the charcoal. Xenon is “stickier” than krypton, so it lagged behind the impurity, which was removed at the end of the column.
How does the team know xenon is pure? A team at the University of Maryland, led by physicist Carter Hall, developed a sampling mechanism. “They kind of rewrote the book on how you detect trace gases in xenon,” Akerib said. The mechanism is a modified residual gas analyzer. Maryland graduate students Attila Dobi and Richard Knoche used it in the Davis Campus last week to sample the delivered xenon, which Fiorucci pronounced pure enough for dark matter detection. “It was a very large achievement,” Fiorucci said.
Today the LUX detector itself has been cooled to its operating temperature of minus 145 degrees F. Researchers will soon begin adding ultra-pure xenon to the detector. A pair of liquid nitrogen thermosyphons will convert the xenon gas to a liquid state. Once filled with liquid xenon, later this month, the detector will be leveled to sub-millimeter accuracy. The liquid xenon also will begin circulating through yet another purifier in a process that will take several weeks.
Once that final purification is complete, the hunt for dark matter will begin.