Found something, they don’t mention these physicist sticking their head in the beam but this probably points to the origin of the story.

I’m not misremembering what my professor said, I even remembered the years (1930s) correctly. To quote him more accurately he said “a lot of early particle physicist damaged their vision because they would stick their head on a particle accelerator to check if they had a beam going” and then he proceeded to mimick the motion of bobbing a head back and forth pretending to intercept a beam with his temples.