In February and March, backpacks arrived at Fermilab and were put into storage 100 meters underground. Copper was mined in Finland, rolled into sheets in Germany and transported over land and by sea to the lab – all within 120 days. In the dark matter discovery mission, mysterious substances make up 85% of the matter in the universe, every day that copper uses on the ground is very important.
Fermilab scientist Dan Bauer said: “At the surface of the Earth, we are in a cosmic rain.
When these high-energy particles of space-origin collide with a copper atom, they can dislodge protons and neutrons to create another atom called cobalt-60. Cobalt-60 is radioactive, which means it is unstable and spontaneously decays into other particles. The small amount of copper atoms that are converted to cobalt has no effect on daily copper use. But Bauer and others working in the Super-Cool Dark Matter Search have to take drastic steps to ensure the copper they use is as pure as possible.
The latest in a series of similar experiments, SuperCDMS will look for dark matter at SNOLAB, an underground laboratory near Sudbury, Ontario, Canada. The copper plates will eventually take the shape of six oversized soda cans arranged like interlocking dolls. The innermost box will contain germanium and silicon devices designed to detect the hypothetical weakly interacting large-mass particles, or WIMPs, particularly those with masses 10 times smaller than protons. The vacuum-wrapped outer can will be a little over a meter in diameter. The entire device, dubbed SNOBOX, will be linked via a copper body with a special refrigerator that will cool the detectors to a fraction of absolute zero.
At such cold temperatures, thermal fluctuations are so small that the WIMP can leave a detectable signal when colliding with an atom.
But “you’re looking for a needle in a pile of dark matter straw,” said Bauer. “The best you’ll get is probably a few events per year.”
Meanwhile, ordinary matter particles flying through a SuperCDMS detector can produce foreign cues, called backgrounds, that overshadow signals from dark matter interactions.
Burying SuperCDMS two kilometers underground and enveloping SNOBOX in layers of lead, plastic and water will remove almost all undesirable particles from the environment. But there is no separation between a copper can and a detector. And while copper’s superior heat-transport capacity makes it ideal for cooling detectors, any radioactive impurities in the metal will emit background particles.
That brings us back to cobalt-60.
“The bottom line is that the longer copper stays on the surface exposed to cosmic rays, the more cobalt-60 is produced,” explains Matthew Hollister, Fermilab’s SuperCDMS refrigeration manager. “So part of the background budget for testing includes a surface exposure time limit.”
Cobalt-60 is not the only impurity to worry about. Radioactive isotopes of uranium, thorium and potassium occur naturally in the Earth’s crust, so the SuperCDMS team had to buy copper from a mine with as little of the metal as possible. Inactive impurities are also important – they can reduce the copper’s thermal conductivity, making it difficult to keep the detectors cooler. In total, the copper for SuperCDMS must be over 99.99% pure with less than 0.1 parts per billion radioactive impurities.
Between the internal impurities and those introduced through the cutting, laminating and transporting of copper, the metal sheets currently underground at Fermilab are not entirely pristine.
“A lot of processes are not something we have direct control over,” says Hollister. “Some of it is actually a dark shot of what we’re going to end up with at the end of the day.”
After receiving the plates, the researchers sent the samples to the US Department of Energy’s Pacific Northwest National Laboratory for detailed examination to quantify the remaining impurities. Before long, the plates will leave Fermilab to make and the cobalt watches will tickle again until the cans arrive at their home at SNOLAB.
“The final step before we get them underground will be to spray them with an acid detergent that will flake the surface a few tens of micrometres,” says Bauer.
Diluted hydrogen peroxide and hydrochloric acid will remove any surface impurities that have accumulated during the manufacturing process. And a weak citric acid solution will protect the high thermal conductivity of copper by protecting it from oxidation during the experiment.
The SuperCDMS partnership plans to begin data collection by 2022. Overall, this test iteration aims to achieve a background level 100 times lower than its predecessor, largely thanks to the purity of the copper. With the increased sensitivity, researchers hope to spot any low-mass WIMP that may be in the vicinity.
“This show has been in development for a long time, so it’s good to see it starting to work together,” says Hollister. “SNOBOX is really the main end product, so we look forward to installing this and getting it up and running as soon as possible.”
SuperCDMS research on dark matter is supported by the DOE Science Office and the National Science Foundation, as well as the Innovation Fund and Canadian SNOLAB.
Keep dark matter detectors clean and accurate
Provided by the Fermi National Accelerator Laboratory
Quote: Ultra-pure copper for ultra-sensitive dark matter detector (2020, October 30) retrieved October 30, 2020 from https://phys.org/news/2020-10-ultrapure-copper-ultrasensitive -dark-detector.html
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