Kitchen salt sits quietly, but inside it, rubidium-87 atoms occasionally transform into strontium and eject an electron. This process, beta decay, is usually studied by placing a radioactive sample next to a detector. Here, they did things differently: the radioactive rubidium was embedded directly into the crystal, which itself catches the emitted electrons.

Such a crystal was grown from a melt with a rubidium additive — much like growing large salt crystals for optics. An electron born inside immediately enters the detector material and deposits all its energy. No losses on the way from source to instrument. The result is a source and detector in one piece.

The shape of the electron spectrum, how many electrons fly out with each energy, is sensitive to a heavy neutrino admixture. If, in addition to the usual light neutrinos, a more massive sterile neutrino exists, it should leave a characteristic kink in the graph. Such a kink was sought in the measured spectrum.

This is like listening to the noise of a crowd: individual voices blend together, but if one voice is louder than the others, a noticeable peak appears in the general hum. Here, instead of sound, it's electron energy; instead of a loud voice, it's an extra particle.

No kink has been found yet, but the method itself works. The rubidium crystal proved to be a convenient platform for future searches: no need to fuss with an external source, and sensitivity can be increased by growing larger crystals.