UltraLo-1800 Alpha Particle Counter
XIA LLC
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b)
Electric Field Strength
If the electric field inside of the detector is not at the correct value then it won’t be possible to
observe any alphas. This shouldn’t happen, but if it does CounterMeasure should indicate it with a red
HV status icon, and the run will terminate with an error message. For more information on debugging
red status icons, see
c)
Oxygen
Oxygen is electronegative, and if it is present it will capture electrons in the active volume. This
means that induced signals will be smaller than expected. By itself this effect will only decrease apparent
alpha energy, but Oxygen is very rarely present in the active volume alone; it is usually accompanied by
water vapor (see above). Getting rid of oxygen in the active volume follows the same procedure as
water, but is even easier because oxygen isn’t readily absorbed by plastics.
6.
No Events
If there are no events present at all after several hours of running, there are several possibilities. The
first thing to check is if the baseline voltage value in the active volume has drifted out of the range
accepted by the electronics. This can happen if the high voltage was ramped immediately before starting
a run without a purge. To test for this simply stop the run and start another one. Other than baseline
drift, the remaining possibilities to check are the same as in the “No Alphas” case above. If the counter is
run filled with room air, for instance, it will likely see no events.
7.
Excess Events
There are a number of causes of extra events. The following sections include reasons for excesses of
all event types, as well as excesses of specific event types. Excess event types don’t always indicate a
problem, many times they are simply the result of the sample, or conditions in the counter that are
easily rectified. But they can inhibit proper operation, so keeping an eye on typical event rates can help
you notice changes and spot a problem early.
a)
All Types
The most likely cause of a large number of events is excess noise in the environment, particularly
from mechanical vibrations, which cause the inherent noise level in the system to increase dramatically.
(The mechanism is that vibrations produce slight changes in the detector’s capacitance, which generate
changes in the output current of a similar size to our signals. See
information.) Mechanical noise is usually very easy to spot, for an example see Figure VII-8. Additionally,
mechanical noise will often cause many events to occur in rapid succession, and in that case scrolling
through events sequentially in the Analysis panel will show streaks of these traces. Eliminating this noise
will depend on the environment, in some cases it may be straightforward, in others it could be quite
difficult. But the UltraLo-1800 needs a quiet environment to function properly, so eliminating sources of
vibrations or physical noise is paramount. Finally, if these vibrations are strong enough they can even
trigger a built-in accelerometer that will cause the run to stop.
