AN ION-PUMP BASED LEAK DETECTOR

Some theoretical studies and experimental tests performed to investigate the feasibility of an ion pump based leak detector are reported. A `sniffer' leak detector, composed of a vacuum envelope with a quartz window and with a small ion pump is described. With appropriate design of the window thickness, surface and temperature it is possible to realize a surface acting as a barrier for all atmospheric gases, except helium. Provided the pressure inside the vacuum envelope is maintained in the uhv region by the ion pump, even a small variation in the concentration of helium outside the system causes a detectable increase of pressure inside the system. An increase in the ion pump current of several nA caused by a helium flow in the order of magnitude of 10(-10) mbar.Is-1 has been detected when the outside concentration of helium has been increased by 5 ppm. The most critical items, such as the window heating and cooling, the response and the clean-up times, and the `helium saturation' effect are discussed in detail.