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For the first time, the MicroBooNE detector has measured how accurately it can distinguish signals from particles with energies just one and a half times the electron mass.
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A chamber the size of a small truck is filled with liquid argon at minus 186 degrees Celsius. When a particle flies in, it knocks electrons off argon atoms, and those electrons drift toward the walls, tracing a three-dimensional picture of the event. That's how a liquid argon time projection chamber works — the main tool of modern neutrino observatories.
In the MicroBooNE experiment, this chamber was put to the test for the first time at energies around one and a half megaelectronvolts — roughly one and a half times the energy stored in the rest mass of an electron. The signal source was gamma rays with an energy of 2.614 MeV from the decay of the isotopethorium-208. When such a ray enters the argon, it sometimes converts into an electron-positron pair, and the entire energy of the gamma photon is shared between them. The detector records their tracks, and then special algorithms reconstruct the original energy.
The reconstruction accuracy was (7.52 ± 0.78 stat. ± 0.92 syst.)%. That's like measuring a 100-meter sprint with an error of seven and a half meters — crude for a running track, but quite decent for the world of elementary particles.
The result matched the simulation's prediction (9.70 ± 0.65)% within 1.6 standard deviations — in other words, the discrepancy falls within statistical fluctuations. The most important thing here is the very fact that such a measurement was possible at all. Previously, no one knew how well liquid argon detectors distinguish energies in the few-MeV range, and now physicists have a reliable reference point and a calibration method using monoenergetic gamma sources.
It's like tuning a musical instrument with a tuning fork: the pure tone from thorium-208 decay lets you check whether the detector is off-key and, if needed, tweak the reconstruction algorithms. In the future, such calibrations will help catch elusive neutrinos from supernovae or even from the Sun, where energies lie precisely in the MeV range.
The liquid argon in the detector is so pure that a free electron can drift for several meters without ever sticking to an impurity atom — roughly like swimming a kilometer in perfectly clear water without encountering a single speck of dust.
In Peter Watts's novel Blindsight, alien probes communicate using modulated neutrino beams — a technology for which precise low-energy detectors would be indispensable.
