High-Purity Copper Used for Dark Matter Study

In an effort to study dark matter, three batches of copper plates from Finland are going to be used by Super Cryogenic Dark Matter Search (SuperCDMS). The material has a high-purity quality that will increase the material’s sensitivity and allow SNOLAB scientists to detect dark matter interactions. The material is currently in the US and will be transported to Canada, where the investigations will be carried out, reports Fermilab.

These batches of copper were 99.9 percent pure, containing less than .01 percent of radioactive impurities. These samples were rolled into plates in Germany and shipped to Fermilab, a US physics and acceleration laboratory, to be stored 100m underground. According to Fermilab scientist Ban Bauer, this copper could detect dark matter.

This SuperCMDS SNOLAB project aims to look for dark matter at SNOLAB, an underground laboratory in Canada. Bauer says the plates need to be underground because the copper must maintain its purity and there is risk of losing it by exposing the material to the daylight. The cosmic rays could strike a copper atom that subsequently could produce cobalt-60 that is radioactive. Therefore, the instability would increase and affect other particles of the project, reported Fermilab.

In a media statement, Bauer said that the copper plates will have the shape of six soda cans and will be arrange like nesting dolls. The inner can will have germanium and silicon devices that will detect weakly interactive massive particles (WIMPs). The device will be connected to a refrigerator by copper stems that will allow it to cool the detectors to near 0K temperatures.

In cold temperatures vibrations are limited but still WIMPs can be detected. In addition, ordinary matter particles that will be in the SuperCDMS detectors could produce extraneous signals that could indicate dark matter interactions, says Fermilab.

Cobalt-60 is not the only impurity that could affect the project. Other adulterations like nonradioactive impurities, radioactive isotopes of uranium, thorium and potassium can decrease the copper’s ability to conduct heat and to keep detectors cold, said Bauer.

The copper was analyzed by the US Department of Energy’s Pacific Northwest National Laboratory that quantified its impurities. There are plans to send the plates to SNOLAB and subsequently acid etching them to eliminate tens of microns of their surface.

According to SNOLAB, the hydrogen peroxide and diluted hydrochloric acid will remove the impurities that accumulated during manufacturing and transportation. In addition, Bauer said the acid will preserve the metal’s conductivity and protect it from oxidizing during the investigations.

Due to the purity of the copper, the project will have a higher sensitivity to WIMPs. SuperCDMS is looking forward to disclosing the project’s results and expects future to collect data in 2022.