Comment by tropo
7 years ago
Hydrogen would be better. Size isn't the most important factor. Gasses with low molecular mass have higher particle velocities. The velocity affects mixing, passage through holes (including filters), and the speed of sound. Hydrogen is about 2 AMU, 1 for each atom, while helium is about 4 AMU.
You can find the formulas in a typical college chemistry textbook.
If "hydrogen would be better", I guarantee companies that make highly-sensitive cryostats (like companies that manufacture MRIs) would do it.
They don't - even knowing He is more expensive that H, because helium is better at leak testing than hydrogen is