Shinku

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Residual gases in a vacuum system made of stainless steel were analysed by a mass spectrometer having a partial pressure sensitivity of 5×10^-13 mmHg and having a resolving power of about 150. The pumping system consists of a 4″ oil diffusion pump (Apezon oil C) followed by dry ice and liquid nitrogen cold traps. The mass spectrometer is 90° sector type and its mean ion radius is 200 mm. Its main parts are made of stainless steel and gaskets of aluminium are used for vacuum shielding. After about 20 hours evacuation and baking at 150°200°C, the final pressure of 12× 10^-8 mmHg was obtained. Decreasing manner of peak heights during degassing of the ion source was observed. At the final state of the evacuating process, predominant components of the residual gases were air (N₂, O₂, Ar etc.), H₂, H₂O, CO, and CO₂. Partial pressures of these gases were of the order of 10^-9 mmHg. As for hydrocarbon gases, the partial pressures were less than 10^-10 mmHg and it was observed that heavier components had smaller partial pressures. From these results it was supposed that hydrogen and carbon monoxide would be left as main components of residual gases in a well baked, leakless vacuum system.

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