Biomedical Engineering Reference
In-Depth Information
Fig. 10
Trajectory simulations of the ions inside the energy filter with a focal point in the center
of the outlet aperture (
a
) and with a focal point before the outlet aperture (
b
)
the ion beam expands again and a wider outlet aperture is necessary and the selec-
tivity is inferior. Nevertheless this 90 ° layout was chosen because it allows for a
better utilization of the total system area.
3.7
Microchannel Plate Ampli fi er
To intensify the ion current, an amplification via electron multiplication is advanta-
geous. For this purpose a MCP is introduced into the PIMMS. An MCP rectangular
cut in shape from a standard MCP [
25,
26
] works as a secondary electron multiplier
(SEM) as shown in Fig.
11
. The MCP is hybrid-integrated into the spectrometer by
clamping with flexible silicon springs 20 mm in thickness. They serve both as
mechanical fixture and electrical contact. The MCP is operated between 400 and
1,300 V and amplifies the current by more than 1,000, which allows for measure-
ments into the <100 ppm region.
3.8
Detector
The detector of the PIMMS is a U-shaped Faraday cup surrounded by guard
electrodes.
Figure
12
shows the detector structure and the surrounding guard electrodes. The
dark regions depict metalized structures on the bottom glass substrate; the light gray
regions are silicon structures. The guard assures that no electrical field can build up
in front of the detector, which would deteriorate the ion measurement. The detect-
able currents range from 0.5 to 200 pA.
