A Cold Crystal Catches Dark Matter express
A sodium iodide crystal doped with thallium is like photographic film for invisible particles. When a gamma ray or a hypothetical dark matter particle hits it, it flashes with faint light. Two silicon photomultipliers pressed against the crystal's ends catch the flash from both sides. In the new study, a crystal measuring 6 by 6 by 13 millimeters was cooled to minus 179 degrees Celsius for the first time and its glow was tested at various temperatures.
It turned out that at minus 35 degrees Celsius, the crystal emits 34.5% more light than at room temperature: 17.7 photoelectrons per kiloelectronvolt of energy. The flash becomes brighter and easier to distinguish from random noise. Two detectors working together help filter out thermal noise: a signal is counted only if both see the flash simultaneously. It's like two guards who confirm each other.
This approach lowers the detection threshold — the detector starts noticing weaker events. For dark matter and neutrino searches, this is crucial: the lower the threshold, the better the chances of catching a rare particle.
Original abstract
as written by the authors arXiv:2608.11303v1
We report the first temperature-dependent characterization of a NaI(Tl) crystal readout by two silicon photomultipliers (SiPMs) directly coupled to opposite ends of the crystal for rare-event searches. A $$6 \text{ mm} \times 6 \text{ mm} \times 13 \text{ mm}$$ NaI(Tl) crystal was directly coupled to two SiPMs and characterized in a liquid nitrogen-cooled cryostat over a temperature range of 94$$-$$293 K. The light yield, energy resolution, and scintillation decay time were measured using $$\gamma$$-ray peak from a $$^{241}$$Am source. After correcting for optical crosstalk contributions, the light yield increased, reaching $$17.7 \pm 1.1$$ photoelectrons/keV at 238 K, corresponding to a 34.5% enhancement relative to room temperature (293 K). Furthermore, dual-channel configuration effectively suppresses random thermal noise via coincidence triggers, which together with the observed increase in light yield, provides a critical pathway toward lowering the energy threshold for dark matter and coherent elastic neutrino$$-$$nucleus scattering searches.
When the crystal is cooled, the light yield increases by a third, even though semiconductor detectors usually require deep cooling, while scintillators are almost temperature-independent.