Nitrogen to Help Find Dark Matter

The hunt for dark matter has been on for many years. To trap the mysterious particles, physicists even go underground with their huge detectors. Researchers from the joint NSU and BINP laboratory have found a way to increase the sensitivity of such a detector manifold. They hope the principle proposed will help create a new, the most sensitive detector for trapping dark matter particles.

 Argon prototype of a dark matter detector

First things first, what do we know about dark matter? Strictly speaking, we have no idea what most of the matter in the Universe is made of. More exactly, stars, planets, air, stones, animals and bacteria are known to consist of ordinary matter, i.e. electrons and neutrons. However, scientific data make us think about the existence of some elusive material. We know it must be around us as without it galaxies would fly apart and we would not exist. But so far what it’s made of remains a mystery; we’ve been unable to detect the stuff. If interactions between dark matter and the ordinary one were common, we would have spotted the effects, but most dark matter particles pass through us unhindered. Every now and again, however, they collide with the particles of ordinary matter, and scientists hope to detect the dark matter particles and study their nature in more details.

Nowadays, experiments on dark matter are conducted with detectors working on different physical principles. They require special pure radio-clean materials with the detectors sheltered from background cosmic radiation, which can be guaranteed only deep underground. The methods proposed, of course, should first be tested in laboratories, on prototype detectors, before constructing the real, huge and expensive ones.

The Laboratory of Cosmology and Elementary Particles launched by NSU and the Budker Institute of Nuclear Physics, SB RAS is busy with creating a more advanced prototype of a dark matter detector. A flow of those elusive dark matter particles rarely colliding with ordinary matter is simulated with the help of a neutron generator developed in BINP.

Andrey Sokolov, one of the researchers, says that the underlying principle is that of an argon detector. To put it simply, the silver-colored cryostat resembles a big pan with a web of wires and pipes connected to detectors and indicators. The cryostat contains liquid argon where dark matter particles collide with the atoms of argon transferring energy to atomic matter, creating free electrons and emitting some light, which is detected by the photomultiplier tube.

 Senior researcher Andrey Sokolov

Researchers from BINP being well aware of the characteristics of gas electron multipliers, which they have been working with for more than 15 years, the common scheme of the experiment was altered by including additional multipliers. They could register signals not only in the liquid argon, but also in the gas vapor. The sensitivity of the detector was expected to rise. However, the results went far beyond expectations. “We saw much more light than we had expected. And first we didn’t know how to interpret it.”

Fortunate impurity

It was necessary to sort the matter out. Careful analysis revealed that the cryostat with argon happened to contain a tiny amount of nitrogen in it. While preparing the equipment for the experiment, it is necessary to evacuate air from the cryostat, and some amount of nitrogen could have precipitated on the walls. “It appeared that the nitrogen, due to its atomic shell, absorbed the light from argon and reemitted it in a softer ultraviolet spectrum, which was easy to register,” remembers Andrey Sokolov. “It was a hit!”

“It is quite difficult to register the scintillation light emitted during collisions of the particles in the cryostat. In fact, scintillation light in argon is emitted in the vacuum ultraviolet (VUV) region that cannot be detected in the atmosphere. Its registration is possible only in the presence of a so-called wavelength shifter, a substance that absorbs the light and reemits it at the different wavelength. It leads to a certain reduction of the scintillation light intensity, though, and the sensitivity of the detector decreases.”

“The sudden impurity of nitrogen in the cryostat turned an ideal shifter. What was a bug happened to be a feature,” the researchers jokingly comment.

The effect discovered in the laboratory was first described by scientists about 30 years ago. The then experiments were carried out at room temperature. “In our case,” says Andrey Sokolov, “the mechanism works even better as at low temperature a much smaller amount of nitrogen is enough. We are going to use about 50 ppm.”

“The idea of reemitting the scintillation light with the aid of a tiny nitrogen addition helps simplify the detector construction considerably. The benefit is a simultaneous increase in sensitivity to the limit beyond that of the equipment available. In other words, we will be able to construct the most sensitive installation in the world based on this feature.”

Lab technician Alexander Chegodaev assembling the research installation

Meanwhile, experiments are carried out on a small installation, but the researchers plan to proceed with a larger argon detector prototype having a 150-liter cryostat. According to their previous calculations, the working model of such a detector could register only one collision per day. Now, through adding certain amounts of nitrogen, the increased sensitivity of the detector will allow them to register up to a few dozens collisions.

Andrey Sokolov reported the results of this research at an international conference in Trieste (Italy). He says that the report arouse much interest with the role of nitrogen being unexpected for the participants. The researchers who had worked with argon detectors could have witnessed the same effect, but it could cave been interpreted incorrectly. They might want to revise their data. What might happen when the methods of creating detectors of high sensitivity are tried and tested on prototypes? Novosibirsk researchers plan to join one of collaborations searching for dark matter particles. The technologies developed by NSU and BINP researchers could be applied in the working detectors, those made of super-pure materials and located deep underground.

The joint Laboratory of Cosmology and Elementary Particles headed by Alexander Dolgov was launched with the support of a mega-grant award from the Russian government, which was won by Novosibirsk State University. The Laboratory is hosted by BINP with the researchers from both NSU and BINP working there.

Photos by Valery Nosov, a senior engineer at BINP Prepared by Dina Golubeva