Yep that's a full 12in cathode. I nudged it in so that it hangs diagonal accross my optical drive.

I guess I really should really give a full run down of what I have there. Currently I have 6 12" blue cathode tubes in my G5. Two are in the front panel, one is in the back, one is hanging just above my optical drive, one sits just under the heat sink on the bottom of my case and the last one is hanging from the plate that the optical and hard disks sit on. All of the inverters are hanging just behind that last cathod tube. I was really surprised that I was able to place all of them without have to cut any plastic away.