swansont

Your plasma ball is giving off X-rays. Very "soft" ones (i.e. relatively low energy), to be sure, so I'm a little surprised that it sets off the Geiger counter, but I suppose it's possible.

Not quite. C-14 decays to N-14. It's a beta decay, so a neutron changes into a proton, and emits an electron and antineutrino. For C-14 to decay to C-12, it would need to emit two neutrons. Neutron emission can happen for some nuclei, but that requires an large excess of neutrons, and doesn't happen in this case.

In a medium (n>1) the wavelength becomes lambda/n. The frequency stays the same. Light travels at c/n through the medium.