Mass analyzers leak detection

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

Mass spectrometry is also extremely useful as a process monitor. Less sophisticated residual gas analyzers (RGA) operating on the principles of mass spectrometry are available for these purposes and for end point detection. For the etching of Si 128-130), poly-Si 130), silicon nitride 130), and Si02 (729), SiF (m/e=85) has been shown to be effective for end-point detection. In addition, (m/e=14) is useful for nitride 129,130) in leak tight systems, while O (m/e =16), CO (m/e =44) and Si" " (m/e=29) are useful for oxide (757). Because of the general nature of mass spectrometry as a diagnostic tool, it should be applicable to etching studies of metals and other semiconductor materials. 

The alpha-particle instrument is shown in Fig. 1. The gas, supplied from a conventional gas handling system, is irradiated in the ionization chamber. The radiation is supplied from an enclosed polonium alpha source of a few hundred millicuries. The irradiated gas bleeds through a leak out of the ion source and into the evacuated electrode chamber. There, the ions effusing from the leak are captured by the electric fields while the gas is pumped away. The ions are focused, accelerated, and then subjected to mass analysis and electron multiplier detection in a 90° sector field analyzer tube. 

High-pressure photocatalytic reaction studies can be undertaken by incorporating a sapphire window into the reaction cell. This allows the sample to be illuminated by an external light source (U V or visible), when the sapphire window is aligned with another window in the UHV chamber. Product gases may be detected and analyzed by the use of a gas chromatograph attached to the sample loop, By the use of suitable chromatographic columns and detectors (such as flame ionization, thermal conductivity, or photoionization), most likely product combinations may be separated and analyzed. Moreover, a more direct analysis can be made if the product stream is throttled (by means of a leak valve) into a separately pumped mass spectrometer. 

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