PNNL-13501 - Pacific Northwest National Laboratory

phase, less of the earth’s electric field lines contact the
lobes of the stator and therefore the current flow
decreases. This cycle is repeated hundreds of times per
second and an alternating current signal is produced. The
amplitude of this signal is proportional to the earth’s
electric field. During normal atmospheric conditions, the
earth’s potential is around 100 volts per meter.
Instrument Optimization and Testing
Our approach involved three steps: 1) examination of the
instruments in a controlled atmosphere to ensure a robust
setup, 2) brief examination of the environmental factors
that influence atmospheric electrostatics, and 3) direct
detection of an ion source.
Numerous modifications were made to optimize the
instruments and the data acquisition system to make them
more robust. To minimize the variability of external
factors on the ion detectors during this testing, they were
placed in a cardboard box and the air was ionized by the
radiation from an external Co-60 source. The results in
Figure 1 show the dramatic increase in the negative ion
concentration created by the ionizing radiation.
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
Co-60 s ource
br o ught near
ion detector
-3.5
0 50 100
Time (s)
150
Co-60 s ource
removed
Figure 1. Ion concentration in a box as a function of time as
an ionizing radiation source is brought near the box and
then removed
The second step was to try to determine whether
environmental factors other than radiation would
contribute significantly to the normal ion population.
Most of the ions found in the environment are the direct
result of radiation, but open flames, electrical arcs, and
combustion engines can also produce ions. We verified
that these effects were minimal. We set up the
instruments in interesting locations and found an effect
from passing vehicles, but the effect was minimal. It is
not clear whether the effect of the passing vehicle is due
to the ions produced during combustion or more likely
due to the charge it acquires while gliding through the air.
410 FY 2000 Laboratory Directed Research and Development Annual Report
The net result was that there was an effect that will not
have an important impact on sensitivity.
Once the instruments and the environmental factors were
characterized to some degree, experiments were
conducted to see how well the system would be able to
detect an ion source set some distance upwind. We chose
an ion generator for this set of experiments. The goal was
to become familiar with the dynamics of making
electrostatic measurements so only a small subset of
measurements would have to be made with an actual
radiation source. The experimental setup consisted of
placing two ion detectors, one detecting positive ions and
one detecting negative ions, and an electric field mill
downwind from the ion generator. An anemometer was
placed along with these instruments to record the wind
direction and speed. The data from these instruments
were fed into a data acquisition system where a polar plot
was produced. The angular coordinate represented the
wind direction and the radial coordinate was the
magnitude of the instrument response.
Three separate measurements were made: the negative
ion concentration, the positive ion concentration, and the
electric field. The space charge was calculated from the
difference between the positive and negative ion
concentrations. The negative ion concentration
measurement was the most sensitive for determining the
presence of an ion source. An example of the data for an
ion generator located 50 meters upwind is shown in
Figure 2. Data were collected for 30 minutes and the
magnitude of the response was averaged for each angular
coordinate. This shows that with inexpensive equipment
and without sophisticated techniques, we were able to
easily detect an ion source at 50 meters.
The data in Figure 2 correlates very well with the ideal
case, but at greater distances or lower wind speeds the
variability increases significantly. One solution that was
investigated was the use of sophisticated data analysis
algorithms with the aim of improving the signal to noise
ratio. Although these techniques were partially
successful, the fact that the wind direction can vary
dramatically and the ions can take between 30 to
60 seconds to reach the detector made polar plots a
difficult system to implement. Further testing was
conducted by simply obtaining a background with the ion
generator turned off, then looking for significant
deviations when the generator was operating, and then
correlating this data with the wind direction. Under these
conditions the ion generators were detected from
100 meters away. Figure 3 shows the results of this test.
The peaks occurred when the wind direction was optimal
for transporting the ions to the detectors.