Crystal Catches Traces of Invisible Particles express
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.
Original abstract
as written by the authors arXiv:2608.14959v1
The third-forbidden non-unique beta-decay of $$^{87}$$Rb to $$^{87}$$Sr (Q$$_\beta = 282.275(6)$$ keV) has long served as an important benchmark for understanding forbidden beta-decay. To investigate this, we have developed a novel CsI scintillator with a 10 mol% Rb concentration using the Bridgman method. The incorporated $$^{87}$$Rb serves as an intrinsic radioactive source, enabling a source-in-detector configuration with high detection efficiency and minimal energy loss for low-energy electrons from beta-decay. We investigated both Rb doped and Tl co-doped CsI crystals and characterized their scintillation properties, including light yield, energy resolution, and non-linear response. We report distinct scintillation characteristics for the CsI:Rb and CsI:Tl,Rb crystals, with light yields of $$1.38\pm0.01$$ and $$4.73\pm0.13$$ PE/keV, respectively. Using the measured $$^{87}$$Rb beta-spectrum, we search for a keV-scale sterile neutrino admixture through the characteristic kink-like distortion induced by a heavy neutrino mass eigenstate. This study provides a basis for future sterile neutrino searches using rubidium doped CsI crystal.
Rubidium is named after the bright red lines in its spectrum — from the Latin rubidus, meaning "dark red." These lines are visible even through a simple spectroscope and helped discover the element in 1861.