Precious cargo: Dark matter experiment set to move underground
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The COUPP bubble chamber. Photo: Fermilab
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For the past two years, COUPP-4, a 4-kilogram bubble chamber experiment, has searched for signs of dark matter a mile underground at SNOLAB in Sudbury, Ontario. Now that experiment is about to get company – its big brother is moving in.
COUPP-60, a bubble chamber 10 times the size of the current COUPP, was developed at Fermilab and is currently being transported for assembly at SNOLAB. Since it is a larger detector, it is expected to detect particles at a better sensitivity than its predecessor.
Getting a device as large as COUPP-60 underground is difficult, and it's a long trek to enter SNOLAB. Scientists working there take a mile-long elevator ride down, get out, walk a mile through an old mine, take a shower to remove any uranium or thorium dust from the rocks underground and dress in gowns and hairnets. Only then can they can finally enter SNOLAB.
"It looks like science fiction," said Hugh Lippincott, the operations manager for COUPP-60.
Almost all of the parts of COUPP-60 have been designed to fit into the elevator car, but some will need creative transportation measures. Like the scientists, the parts will need to be cleaned before they enter the lab. For the most part, this process doesn't give Lippincott pause, until you mention the bubble chamber's inner vessel.
The inner vessel is the most fragile part of the COUPP-60 apparatus. Made of radioclean quartz, the transparent tank took a year to manufacture.
"If that broke, we'd be in trouble," Lippincott said.
The inner vessel is the crucial component of COUPP-60, or any other bubble chamber. As bubble chambers, the COUPP experiments contain superheated fluid that will not boil until there is a particle interaction. Manipulating the pressure of the fluid leaves a single bubble that indicates a single interaction. Particles of dark matter, like neutrinos, rarely interact with the matter they pass through, but when they hit a molecule in a bubble chamber, the recoil of that molecule creates a bubble.
As difficult as that sounds, the real sticking point in finding dark matter particles is distinguishing between collisions caused by dark matter and those caused by background radiation. Most particle detectors will pick up signals from all particles flying around. By nature, bubble chambers don't pick up a great deal of the background radiation present.
"The big advantage of COUPP as a dark matter detector is that we're completely insensitive to beta decays or Compton scatters from gamma rays," common types of background in dark matter experiments, said COUPP-4 operations manager Eric Dahl.
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—Joseph Piergrossi
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