|CRPG Noble Gas Laboratory||
Mapping changes in helium sensitivity and peak shape for varying parameters of a Nier-type noble gas ion source
Jennifer Mabry, Peter Burnard, Pierre-Henri Blard and Laurent Zimmermann
In this study we characterized a Nier-type electron impact source, a commonly used ionization source in noble gas mass spectrometers (e.g. MAP 215-50, VG Helix SFT, and VG 5400).
A Nier-type source ionizes neutral gas molecules by producing a stream of electrons perpendicular to the path of the ion beam. The ionizing chamber (‘ion box’) is held at high voltage (typically 3000 - 5000 V). Magnets are placed on either side of the electron beam to cause the electrons to move in a helical path. This increases the total path length between the filament and the electron trap, and thus the chance of a collision with a neutral gas molecule. The energy of the source electrons must be high enough to strip an electron from the neutral noble gas atom. Then a repeller plate which has a slight bias voltage relative to the ion box, compensates for space charge effects and helps to repel the positive ion beam out of the ion box. This beam is then accelerated by two sets of plates, the half-plates and the zero plate. The half-plates are held at an average voltage, (V1+V2)/2, which is typically 80 – 90% of the ion box. A bias voltage, V1-V2, can be applied (generally less than 100 V) to provide some steering of the beam. Then the beam passes through the grounded exit slit plate (zero plates).
Tuning a Nier-type ion source involves adjusting many different parameters which affect the resulting signal in complicated ways. We mapped the sensitivity of 4He and the peak shape while varying the total extraction voltage, the half-plate bias, the repeller voltage, and the electron energy.
With the particular source settings that we used, we see an asymmetric rise and fall in the sensitivity as the extraction is raised, but a symmetric rise and fall as the half-plate bias voltage is varied. The best peak shape is found generally at the same extraction value of the maximum sensitivity, but at a distinctly different half-plate bias than the maximum sensitivity, thus it is necessary to monitor the peak shape while tuning the half-plate bias.The extraction and the repeller values of the maximum sensitivity and the best peak shape are strongly correlated, and therefore these two parameters must be tuned together. And finally, we see a double-peak in the sensitivity as the electron voltage is increased, so it may be worthwhile to check a wide range of electron voltages when tuning.