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Representative exposure results for the U-eLektron.
Radiochromic film was exposed by varying
acceleration potential (keV) and cathode current
(uA) using as exposure conditions 10 mm from window,
6 sec exposure, 2 minutes He purge.
kGys results are calculated from the densitometer
readings. Densitometer readings were taken using a
reflection densitometer calibrated with white,
yellow and magenta calibration standards.
These are representative results for one U-eLektron
system. Each system will require testing to confirm
output performance as a function of distance,
atmosphere and exposure time.
Densitometer readings as a function of distance from
window at constant output.
The densitometer readings below show the variation
in the exposure of the sample as the distance from
the window is varied. The optimal distance as seen
by the curves at 50 keV, 200 uA and 40 keV, 200 uA
appears to be 20 mm from the window. We believe that
this result is due to solarization of the film by
the electron beam at 5 and 10 mm. As can be seen as
the distance from the window is increase from 20 mm
to 50 mm the density readings indicate a falloff in
the exposure with 40 keV having the more dramatic
falloff than 50 keV. This is consistent with
absorption of the electrons by the He gas. This
result coupled with the result above support the
conclusion that 50 keV, 200 uA is closer to the
optimal window transparency. The difference in the
exposure is not noticeable at the closer distance
and higher output settings due to the saturation
effects of the film, but as the distance is increase
and the exposure settings are lowered the move away
from the optimal settings will have to be considered
when thinking about window lifetime.
The 6 second exposures were done in a He atmosphere
with a 2 minute He purge before exposure. All
voltage variations were done at 200 uA.
The 30 keV, 200 uA curve indicates that the falloff
with distance is closer to the expected performance,
that is, a nearly linear dropoff as the distance is
increased from 5 mm to 20 mm. A density plateau is
reached at 20 mm due to near total absorption of the
electrons by the He atmosphere and only minor
deviations from linearity are seen from 20-50 mm. At
30 keV the gas transparency at 20-50 mm from the
window is more important to the exposure than the
distance from the window. The electrons that are
getting through at 30 keV, 200 uA are exposing the
film at a constant dosage.
Densitometer readings at constant amperage as a
function of output voltage.
The graph below shows the change in the color
density of the exposed sample film as the output
voltage is changed from 30-50 keV. The output
current for the test was 200 uA, 150 uA, 100 uA and
50 uA. The results for the exposure test indicate
that the window of the U-eLektron has the best
transparency to electrons at the higher acceleration
voltage and higher current. The 40 and 50 keV
density readings are almost identical at 150 and 200
uA. When the output is adjusted to the lower voltage
and current, the readings decrease as the window
transparency is reduced. At all four currents the
falloff is most pronounced at 30 keV, but
indications of movement away from highest
transparency can be seen at 40 keV and 100 uAs as
well. At 50 uA, the plot is almost linear form 50-30
keV indicating that at these output settings the
system is already outside the saturation region of
the film. As the output is moved away from the
optimal settings the window lifetime is reduced. The
window lifetime reduction will be a function of
setting, usage and chamber atmosphere.
We also believe that the very constant densitometer
readings at 40 and 50 keV at 150 and 200 uA, may
well be a saturation effect as we have seen in the
distance analysis done above. The result is that we
do not know for certain the actual output. Our
findings would be more of a lower level at these
settings for the output of the system.
The 6 second exposures were done in a He atmosphere
at 10 mm from the window with a 2 minute He purge
before exposure.
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