Spectrometer
(15)
Figure 11: Magnetic Field of the Trim Coil
For the PHOIBOS 150 analyzer B[μT] = 0.017 x I[mA] and for the PHOIBOS 100 analyzer
B[μT] = 0.026 x I[mA]. In the case of an earth magnetic field of about 50 μT with the
lens axis nearly parallel to the field one needs about 85 mA or -85 mA to compensate
the residual magnetic field within the hemispheres.
To find the optimal value for the current, the signal measured by the analyzer at low
pass energies (< 10 eV) should be maximized by changing the coil current. If the analyz-
er is equipped with a coil (short-circuit of PIN 12 to ground of the analyzer feed-
through) and the power supply HSA 3500 is equipped with a current module, the con-
trol software allows the setting of the coil current. In all programs the coil current set-
ting can be found in the analyzer settings menu as an additional field.
3.4 Slit Orbit Mechanism
The slit orbit mechanism is used to manually change the entrance and exit apertures of
the analyzer. The mechanism positions an entrance and exit opening of the appropriate
size in the analyzer entrance and exit plane. The entrance beam is defined by a pair of
slits which are about 6 mm apart. The posterior slit lies in the entrance plane and
defines the analyzer energy resolution (see equation 5 on page 18), while the anterior
slit serves to match the angular spread for the analyzer. For a given energy resolution
and a given tolerated analysis area and acceptance angle, the largest possible slit
22 PHOIBOS
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