A joint Fermilab/SLAC publication

Precious cargo: Dark matter experiment set to move underground


COUPP-60 experiment in preparation at Fermilab. Image: Fermilab

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.

COUPP bubble chamber. Image: Fermilab

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.

However, there is still a problem. The inner vessels of both COUPP experiments are filled with trifluoroiodomethane (CF3I), a liquid commonly used as a flame retardant. Even though tiny beta particles and Compton scatters don’t interact with CF3I in the bubble chamber, larger particles such as alpha particles and neutrons do. They can leave a very similar signature to dark matter.

“The bubble chamber is a threshold detector,” Lippincott said. “That meant for COUPP events like alphas – which would be clearly identified by a calorimeter because of their energy – we’d only see a bubble, just like dark matter.”

COUPP’s placement in SNOLAB helps minimize the neutron problem, since the underground laboratory is shielded from cosmic rays, which can liberate neutrons from atomic nuclei. And there’s another ingenious solution to the alpha particle problem – listen to the sound of the bubbles forming inside the chamber.

The COUPP team learned from the Picasso experiment in Canada that they could record the sound wave produced by bubble formation. Collisions caused by alpha particles and those caused by dark matter sound markedly different.

“Alpha collisions are four to five times louder than dark matter collisions,” Lippincott said.

The fluorine and iodine atoms in CF3I can also reveal detailed information about the spin of the dark matter particles’ interaction, which is a big advantage of the COUPP detectors over other dark matter detectors. Although CF3I shows enormous promise as a bubble chamber fluid, it does have its problems. The CF3I molecule can become unstable at high temperatures and decompose, and beyond that, it’s mildly toxic in large quantities. Dahl said if problems with CF3I become intractable, there are several other fluids the team can try to use.

The inner vessel will be the last part to arrive sometime this fall. Lippincott is currently in Ontario working to install COUPP-60’s hydraulics, pressure vessel and water tank as parts arrive, with the fluid cart full of CF3I to follow shortly thereafter. Dahl said he expects the detector to be running by the end of the year and data collection to be starting sometime next year.

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