WHEN a charged particle passes through any medium at a velocity greater than the velocity of light in the medium, it emits a short flash of blue light known as Cherenkov radiation in its wake. This is the electromagnetic equivalent of a sonic boom. This flash is detectable at tens of metres by light sensors in water or ice, which are transparent to light. The velocity of light in clear water or ice is 2.19 × 10 8 m/s, which is about three-fourths of its velocity in vacuum (3 × 10 8 m/s). Muons, in fact, travel with this velocity in ice. An interesting consequence of this is that muons produced by neutrino interactions in ice overtake the main wave front of Cherenkov light from other charged particles produced in the event (by 1.23 ns/m) and deposit Cherenkov pulses in digital optical modules slightly earlier than the other charged particles (Fig. 3). This allows IceCube to unambiguously detect muons.
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