All pixels in the picture are caused by radiation hitting the camera sensor

I don't think so. I rather think it's water droplets acting as little prisms. Radiation would hit the camera sensor in a random pattern and the effect would decay over time. A bit like how you determine pi with the monte calro method by throwing points at a circle and then checking whether they are inside or not.

I found a video of the robot on YouTube [1] and it seems that the rainbow patterns don't follow the "monte carlo throwing" pattern, but their location depends on the scene.

I don't know how radiation would affect a CMOS sensor, but I would guess it to be similar to what strong light is doing, e.g. when you point your camera into the sun. What then happens is an effect called "blooming": CMOS sensors work by photodiodes converting light to electric potential. If the radiation that reaches one of the photodiodes in the sensor plane is strong enough, the potential would jump to the other sensors next to it, causing a large bright field in the digital image.

Edit: Oh and it seems that the material itself is reflecting light in a rainbow pattern, but that's not the same as "ionizing radiation hitting the sensor".

Edit2: it could just be oil. Oil on water causes rainbow like artifacts.

[1]: https://www.youtube.com/watch?v=cyKEVD4oC4s

yes