Note that if you want the mask you must go after you have the recipe, and while the Nuka-Cola Challenge quest is active. Once you complete the quest they will no longer spawn.
Yeah thats right, you do need the receipe before, but you don't need that quest active. I just did it on this play through without that quest being opened. I find the first person shooting a bit rough in FO3 and New Vegas, so I prefer VATS and as much AP as possible.