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A dark matter detector is like a super-sensitive microphone listening for silence. It picks up tiny heat jolts, phonons, that occur when a particle grazes a crystal. But instead of silence, the instruments have been hearing noise for years: a slew of weak signals below a few hundred electronvolts. Where do they come from?

Previously, the blame fell on fast neutrons from cosmic rays. As they zip through the detector, they leave microscopic cracks in the crystal, like a bullet leaving chips in glass. Those damages then slowly relax, and each such 'click' is perceived by the crystal as a signal. To test this, physicists took two identical detectors: one was irradiated with neutrons, the other left pristine. Then they compared.

So neutrons are not the main source of the noise. The mysterious signals come from something else — perhaps the crystal's own nature or particles we can't yet tell apart from the background. Now dark matter hunters will have to search for a new source of 'microphone static' so that one day they can hear a truly important signal.

Original abstract

as written by the authors arXiv:2603.17964

Solid state athermal phonon calorimeters used in the search for low mass dark matter or coherent neutrino-nucleus interactions have long observed a large excess of events below several hundred eV. The relaxation of damage created by the interaction of fast cosmic ray neutrons with the detector has been proposed as a source of these excess events. By comparing neutron exposed detectors to control detectors, we report the first measurement of phonon bursts caused by damage created by fast neutrons. Differences in the spectral shape, the rate dependence on thermal history, and the observed spectral rate scaled to the neutron exposure between irradiated and control detectors suggest that our observed LEE backgrounds are not dominated by neutron damage-induced phonon bursts.