A series of very high-vacuum valves with electric heaters for nominal apertures of 10, 20, 30, 50, 100, 150 and 200 nm: I R Kirichek et al, Physico-Technnical Institute, Academy of Sciences of the Ukrainian SSR, Questions of Atomic Science and Technology, Series High-Vacuum Physics and Technology, No. 1 (1), Khar'kov (1973)