Cerenkov radiation: It’s the blue glow created by (IIRC) neutrons briefly breaking through c
I thought it was due to neutrons exceeding the phase velocity of light in a medium, which is invariably slower than c. The neutron is still going slower than c:
Wikipedia
While electrodynamics holds that the speed of light in a vacuum is a universal constant (c), the speed at which light propagates in a material may be significantly less than c. For example, the speed of the propagation of light in water is only 0.75c. Matter can be accelerated beyond this speed (although still to less than c) during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light propagates in the same medium.
I thought it was due to neutrons exceeding the phase velocity of light in a medium, which is invariably slower than c. The neutron is still going slower than c:
Wikipedia