There are helium stars, and discussions refer to "the much larger transparency of helium-rich atmospheres," but the ones I've so far found don't give a number.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760012836_1976012836.pdf tells how NASA had some measurements done, and they showed much attenuation over 60 cm. That seems inconsistent with the astronomical observations.

This bears on some nuclear rocket schemes, and also on how much of a fool I made of myself when I wrote, in my boron paper, "Surrounding this fountain and reflecting neutrons to it, would be a region filled with cool flowing helium. The high transparency of helium to thermal radiation would allow it to fill the space between the fountain and the sheath of iron oxide particles while remaining cooler than either."

So, do you know of any good helium UV-transparency documents on the web? In libraries?

(How fire can be domesticated)