In a gas chamber where fast electrons are usually detected, slow charge carriers suddenly appeared: negative ions. They move like heavy porters who do not scatter to the sides but march in an even column. That kind of tidiness is exactly what detectors hunting for dark matter need: the less a particle's track is blurred, the more accurately its direction can be reconstructed.

Until now, to obtain negative ions, special substances were added to the working gas: electronegative gases. They captured electrons, but at the same time they reduced the signal gain and demanded caution, since some of them are toxic or flammable. The authors of the new work took a different route. They took pure CF4, a gas usually considered non-electronegative, and raised the electric field above a certain threshold. As the electrons accelerated, they began to break CF4 molecules apart, and an electron attached to one of the fragments. That is how negative ions were born without any additives.

To see these ions, the scientists used fast photomultipliers, devices that turn faint flashes of light into electrical signals. Each negative ion entering the gain region produced a charge avalanche, and its light was recorded as a separate peak. By counting such peaks one by one, the particle energy was measured with an accuracy of 7.9% instead of 8.9% for a conventional electron signal. The difference is small, but it shows that the ion mode is not inferior to the electron mode in quality.

Returning to the porter image: previously, to make electrons march in formation, one had to hire special overseers, the electronegative gases, which slowed and weakened the whole crew. Now it turns out that CF4 molecules themselves, in a sufficiently strong field, can turn fast couriers into neat porters. Switching between the modes is simply a matter of voltage.

This discovery opens the way to large gas detectors for the search for dark matter and solar neutrinos, where millimeter-scale spatial resolution matters. The method could also be useful in medical imaging and radiation monitoring, anywhere fine track details need to be seen without hazardous gas additives.