XIA UltraLo-1800 User Manual page 64

classified as Midairs). See Figure VII-7 for an example of a run with significant risetime decrease due to
excess moisture. In this case alphas will fail the midair/alpha cut and be classified as midairs. An
inspection of your data's Anode Energy vs. Anode Risetime plot should tell you quickly if this is the case.
If your runs do have too much moisture, what could be causing it? There are a few things. If it's
transitory you probably just need to purge for longer. To check for transitory moisture, you can use the
ROI Timeline to filter out the early hours of a run and see if the risetimes as shown in the
are increasing with time. You can also check the moisture values in the
they are changing with time. The moisture information is contained on a line that finishes "Moisture
meter value = XXX ppm". Interpreting the value here is a bit tricky as every counter has its own baseline,
and that baseline changes with time. Typically, values around 60 ppm are the highest consistent with
normal operation. Significantly higher values (especially above 100 ppm) indicate the presence of excess
moisture.
If the low risetimes and high moisture values persist throughout a run then it's possible that a
source of moisture is in the counter or the gas line. Plastics absorb significant amounts of moisture and
then outgas into environments with lower moisture (this is often observed in vacuum experiments),
including the counting environment in the UltraLo-1800. If you're attempting to measure a plastic
sample it may need a significant amount of time in a dry environment in order to measure (there are
other tricky issues with measuring plastics, see Dielectrics). Likewise, if there are significant amounts of
plastic tubing in the gas lines leading to the UltraLo-1800, that may cause increases in moisture.
Another cause for increased moisture levels is that the constituent plastics of the UltraLo-1800 may
be outgassing. All UltraLo-1800's are outgassed prior to shipping, but if your counter goes for an
extended period without gas flow it's possible for moisture to resorb. In this case keep the flow of clean
gas undisturbed until risetimes return to normal, which should take only a few days.
Finally, it's possible, although quite unlikely, that the silicone rubber seal around the door to the
sample tray may be leaking. The simplest test is a visual inspection of the seal. If there is nothing visually
wrong, then ensure that the tray is closed, put your finger over the gas outlet on the rear of the counter
(see Counting Module), and watch the pressure gauge. If the pressure rises smoothly and steadily up to
2 inches of water, the seal is performing adequately. If it doesn't, especially if it doesn't rise at all, there
is some issue with the seal or door mechanism, and you should contact
Figure VII-7: A run with high moisture. Note the large number of midairs.
support@xia.com
UltraLo-1800 Alpha Particle Counter
XIA
XIA LLC
Analysis Panel
CounterMeasure Log to see if
support.
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