Acquisition of clean ultrahigh vacuum using chemical treatment

Abstract

The purpose of this study is to attain a state of ultrahigh vacuum (UHV) under a mild baking condition using chemical treatment. The vacuum test involved the study of an aluminum alloy chamber 8.5 m long, 2 m2 in surface area and 27 l in volume with an elliptical aperture for the Recycler Ring at Fermilab. After the chamber was exposed to enough air of 95% relative humidity and evacuated to 10−5 Torr, dichloropropane (DCP) gas of 1 Torr was injected into the mildly heated chamber (at 80 °C) and evacuated. This process was repeated between three and four times, depending on the type of test performed. After the first chemical treatment (test-1 DCP), the ultimate pressures were 7×10−10 Torr at the left end and 9×10−10 Torr at the right end, an only slight difference. When test-1 DCP reached the ultimate lowest pressure, the chemical retreatment (test-2 DCP) was applied under the mild baking condition on the entire inner surface of all components: chamber, Bayard-Alpert gauges, a quadrupole mass spectrometer, an ion pump, and a turbomolecular pump. Test-2 DCP showed that the ultimate pressure at the left end near the main pump was less than 1×10−11, which was under the detectable limit of the gauge. On the other hand, in the case of a reference test (test-1 ref) conducted under the same condition as test-1 DCP but without the chemical treatment, the ultimate pressures were twice as high at the left end and 80 times higher at the right end than the pressures of test-1 DCP. Therefore, it was assumed that the lower outgassing rate resulted from the chemical treatment. In addition, it was suggested that DCP has a potential to promote water evacuation by breaking a weak intermolecular hydrogen bond of adsorbed multilayer water which is preferentially dissociated rather than a strong intramolecular oxygen-hydrogen bond of water.