  We present here direct evidence for neutrons causing nucleation of
supercooled water. Highly purified water (20 nm filtration) is cooled to well
below freezing (as low as -20 degrees C) with a radioactive calibration source
of neutrons / gamma-rays either present or removed during each of many control
cooling runs for the same volume of water. When it is primarily neutrons
irradiating the sample bulk, the non-equilibrium freezing point (also known as
the "supercooling point") is, on average, +0.7 degrees C warmer than the
control equivalent, with a statistical significance of greater than 5 sigma,
with systematic uncertainty included. This effect is not observed with water in
the presence of gamma-rays instead of neutrons. While these neutrons should
have theoretically had sufficient energy to mount the energy barrier,
corroborating our results, their raising of supercooling temperature has never
been reported experimentally to the best of our knowledge. The potential to use
deeply supercooled solutions, not only water, as metastable detectors for
radiation and perhaps dark matter or neutrino physics presents now a new avenue
for exploration.
