It uses the camera, which is triggered by the high-energy photons in Gamma radiation. That is why they need to cover the lens with something that only lets radiation through, but no light.
And presumably why it's pretty poor at discriminating Alpha & Beta, since they'll be quite easily stopped by the barrier as well (and probably the lens even if it were exposed)
IIRC actual radiation detectors use very thin mica films to protect the sensor, so they can admit the lesser penetrating particles.
