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To get reasonable count statistics approximately one particle per
second should be registered. The overall efficiency is mainly influenced
by the efficiency of the neutralizer and by the reflection properties
of the conversion surface. Additional losses are due to the small
size of the conversion surface samples and because the extraction
lens does not focus in a direction perpendicular to the plane by the
conversion surface normal and the impinging neutral beam. The number
of counts per second 
to be expected is given
by
denotes the neutral flux from the neutralizer, 
the reflection efficiency of the CS, 
the ionization efficiency
of the CS, 
the loss due to the non-rectangular shape
of the CS including losses due to the clamps holding the CS in place,
the loss due to the fact that the lens focuses only
in one direction. 
denoted the loss due to the
movable lens. The neutral beam remains fixed while the lens with the
CS is movable to allow for energy selection at the lens exit aperture
with the TOF entry slit. As the lens was moved approximately 25mm
and the typical sample also had a diameter of 25mm the neutral beam
had to be enlarged such that the CS was covered all the time by the
neutral beam footprint. This implied that at least half of the neutral
beam hit some lens structure instead of the CS. 
would account for the geometrical coverage of the exit slit by the
TOF entry slit and 
denotes the detection efficiency of the
CODIF at particle energies around 14keV.
Using Equation 6.13 with the values for the diamond
sample (low energy neutral atomic oxygen, = 0.2,
= 0.15, = 0.5, = 0.25,
= 0.5, = 0.5, and = 0.7) and a neutral
beam intensity of 6000 particles per second produced
by a 110eV CO
primary beam (see Section 6.2.2),
about 4 ions should be detected per second. The observed value for
was 0.3 to 1 ions per second. According to [39]
CO-molecules are very likely to survive the collision with the CS
without dissociation. The oxygen in the CO molecule would then be
lost for this calculation. Thus the neutral oxygen atom beam intensity
in the above example is only half the value (3000 oxygen
atoms per second) yielding 2 oxygen ions per second to be detected
with the TOF. Given the large uncertainties in the above calculations
the estimated and the measured count rates are identical.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland
