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Ultra-pure copper for ultra-sensitive dark matter detectors



Ultra-pure copper for ultra-sensitive dark matter detectors

SNOBOX (a device for detecting dark matter particles used in the SuperCDMS experiment) will use a nested copper tank similar to the one used in Soudan̵

7;s ancestor CDMS experiment. Image courtesy: Dan Bauer, Fermilab

In February and March, three batches of copper plates arrived at Fermilab and were washed into a warehouse 100 meters underground. Copper has been mined in Finland, rolled into plates in Germany, and transported to the laboratory by land and sea within 120 days. In order to detect dark matter, the copper spent on the ground every day is important. This mysterious matter constitutes 85% of the matter in the universe.


Dan Bauer, a scientist at Fermilab, said: “On the surface of the earth, we are bathing in a burst of cosmic rays.”

When these high-energy particles from space hit copper atoms, they can knock out protons and neutrons, creating another atom called cobalt 60. Cobalt 60 is radioactive, which means it is unstable and spontaneously decomposes into other particles. The trace amounts of copper atoms converted into cobalt have no effect on the daily use of copper. But Bauer and others engaged in the “ultra-low temperature dark matter search” work must take drastic measures to ensure that the copper they use is as pure as possible.

SuperCDMS is the latest result of similar experiments. It will search for dark matter in the underground laboratory SNOLAB near Sudbury, Ontario, Canada. The copper plate will eventually take the shape of six oversized soda cans, which are arranged in a manner similar to nesting dolls. The innermost tank will contain germanium and silicon devices designed to detect hypothetical weakly interacting large particles or WIMPs, especially those with masses less than 10 times the mass of protons. The outermost diameter of the vacuum seal will exceed one meter. The entire device is called SNOBOX and will be connected to a special refrigerator by a set of copper rods, which cools the detector to a level that is a fraction of absolute zero.

At such a cold temperature, the thermal vibration is so small that WIMP may leave a detectable signal when it collides with atoms.

Bauer said: “However, you are looking for dark matter in a haystack.” “The best result you get may be several events a year.”

Ultra-pure copper for ultra-sensitive dark matter detectors

As shown in the section view designed by SNOBOX, the ultra-pure copper plate will be formed into a nested tank. The hexagonal hole in the center will house the dark matter detector.Image source: SuperCDMS cooperation

At the same time, ordinary matter particles flying over the SuperCDMS detector may produce irrelevant signals, that is, background signals, which drown out the signals of dark matter interactions.

Buried SuperCDMS two kilometers underground, and wrapped SNOBOX in a layer of lead, plastic and water, can screen out almost all unwanted particles in the environment. But there is nothing between the copper tank and the detector. Although copper’s excellent heat transfer capabilities make it ideal for cooling detectors, any radioactive impurities in the metal will emit background particles.

This brings us back to Cobalt 60.

Fermilab’s SuperCDMS cryogenic system leader Matthew Hollister explained: “The most important thing is that the longer the copper is exposed to the surface exposed to cosmic rays, the more cobalt 60 is produced.” “So, the background of this experiment Part of the budget includes time limits for surface exposure.”

Cobalt 60 is not the only impurity to worry about. Radioisotopes of uranium, or and potassium are naturally present in the earth’s crust, so the SuperCDMS team must purchase copper with as little of these metals as possible from the mine. Non-radioactive impurities are also important, as they reduce the thermal conductivity of copper, making it difficult for the detector to maintain low temperatures. Overall, the copper purity of SuperCDMS must exceed 99.99%, and the radioactive impurities must be less than 0.1 parts per billion.

Between the inherent impurities and the impurities introduced by cutting, rolling, and transporting copper, the current plate under Fermilab is not very primitive.

Ultra-pure copper for ultra-sensitive dark matter detectors

After crossing the Atlantic, the copper plates used for SuperCDMS have been delivered to a factory in South Bend, Indiana, and then transported to Fermilab for underground storage.Image courtesy: Luke Martin of Fermilab

Hollister said: “Many processes are not directly controlled by us.” “Some of them are really dark shots of what we will eventually end.”

After receiving the plate, the researchers sent the samples to the Pacific Northwest National Laboratory of the US Department of Energy for detailed testing to quantify residual impurities. Soon, these plates will leave Fermilab for manufacturing, and the cobalt clock will ticking again until the cans arrive at SNOLAB’s home.

Bauer said: “Before bringing them underground, the last step is to spray them with an etching agent, which will remove tens of microns from the surface.”

A solution of hydrogen peroxide and dilute hydrochloric acid will remove any surface impurities accumulated during the manufacturing process. The citric acid solution is weak and can maintain the high thermal conductivity of copper by protecting it from oxidation during the experiment.

The SuperCDMS cooperation plans to start collecting data in 2022. All in all, the goal of this experiment is to make the background level 100 times lower than its predecessor, thanks in large part to the purity of copper. As the sensitivity increases, researchers hope to discover any low-quality WIMPs that may exist nearby.

Hollister said: “The program has been in development for a long time, so it’s great to see it start to be put together.” “SNOBOX is indeed the last major component, so we look forward to installing this component and putting it into operation as soon as possible.”

The Office of Science of the US Department of Energy, the National Science Foundation, the Canadian Innovation Foundation and SNOLAB support SuperCDMS’ research on dark matter.


Keep the dark matter detector clean and accurate


Provided by Fermi National Accelerator Laboratory

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