Enhanced Pumping of the Secondary Ion Source

Geological samples frequently consist of grain concentrates or other materials which require the use of embedding materials, such as araldite, in order both to hold the sample and also to provide a good surface for polishing.  However, this necessity causes major problems for the vacuum system because such glues tend to degas significantly in ultra-high vacuum.  This phenomena is particularly damaging to the secondary ion background intensity of hydrogen.  Poor vacuum conditions also result in elevated molecular ion intensities.

In order to suppress our outgassing problem, we needed to improve the pumping capacity of our secondary ion source, with the particular goal of increasing the pumping speed for both H and H2O.  One option would have been returning to the earlier technology of a helium cryo-pump, but such pumps are adversely affected by both mechanical vibration and maintenance problems.  Instead, we elected to take the alternative approach of installing a non-evaporative getter (NEG) pump, a technology designed to sequester our specific target species.  Within the Varian line of ion pumps, such NEG modules are only adaptable to the 500 l/s diode pump.  The original 300 l/s diode pump, which did not provide adequate pumping speed for hydrogen, was removed from the system.  A new 500 l/s diode + NEG combination was installed; however this arrangement did not permit the use of the titanium submlimator, which was also removed from our system.  The original Vairan ion pump control electronics were compatible with the new 500 l/s pump and it was also possible to use the Ti-sublimator's high current power supply to degas the NEG module.  Due to the larger physical size of the 500 l/s ion pump, it was necessary to design and machine a new flange in order to move the TP2 turbo pump several cm further away from the secondary ion source.