Crystal Detector Based on Rubidium-87: Hunting for Traces of Sterile Neutrinos express

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Ordinary table salt on the kitchen counter seems inert, but inside its crystal lattice, events invisible to the eye can take place. If the crystal contains atoms of rubidium-87, they occasionally undergo beta decay — a type of radioactive transformation in which a neutron in the nucleus turns into a proton, emitting an electron and an antineutrino. As a result, rubidium-87 becomes strontium-87, and the ejected electron carries away part of the decay energy.

Traditionally, such decays are studied by placing a radioactive source next to a detector that registers the emitted particles. However, in the experiment described here, a more elegant approach was used: radioactive rubidium-87 was embedded directly into the crystal structure of the detector itself. The crystal was grown from a melt with a rubidium additive — a method similar to growing large optical salt crystals. Thanks to this, an electron born from a decay inside the crystal almost immediately enters the detector material and transfers all its kinetic energy to it. This eliminates energy losses on the path from source to instrument and increases measurement accuracy. In effect, the source and the receiver are combined in a single piece of crystal.

The key goal of the experiment is the search for so-called sterile neutrinos. Ordinary neutrinos are the lightest known particles, interacting extremely weakly with matter; they are called "ghost particles." A sterile neutrino is a hypothetical particle that interacts even more weakly, only through gravity, and thus does not participate in known types of interactions except oscillations. If such a neutrino exists and has a certain mass, it could be produced in beta decay instead of an ordinary neutrino. Since the energy carried away by the neutrino depends on its mass, the electron spectrum, the distribution of emitted electrons by their energies, would show a characteristic kink or "knee" at an energy corresponding to the mass of the sterile neutrino. It is precisely such a kink that was sought in the measured spectrum.

An acoustic analogy helps grasp the essence: imagine the noise of a large crowd, where many voices merge into a general hum. If one voice sounds noticeably louder than the rest, a prominent peak appears in that hum. Here, instead of sound, we have electron energy, and instead of a loud voice, we have the contribution of an additional massive particle — the sterile neutrino.

So far, the characteristic kink in the spectrum has not been found, which imposes constraints on possible parameters of sterile neutrinos. However, the method itself has demonstrated its viability. The crystal with embedded rubidium-87 proved to be a convenient experimental platform: it eliminates the need for an external radioactive source, and sensitivity can be increased simply by growing a larger crystal. This paves the way for more extensive searches in the future.