PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN99060/1388
Radiation Detection at 50 km
Randy Hansen, Anthony Peurrung, Theodore Bowyer
This project seeks to begin development of a long-range radiation detection technology that exploits regions of elevated
ionization in air. A variety of recent results published in the literature has indicated that detection of radioactivity from as
far as 50 km may be possible via three fundamentally different approaches. This project studied the underlying premises
of the three approaches and assessed their capability for long-range radiation detection.
Project Description
The background level of ionization in the lower
atmosphere arises primarily from natural radiation.
Consequently, any radioactivity release or strong
radioactive source would be expected to lead to elevated
levels of ionization in the atmosphere. Possible longrange
radiation detection techniques were divided into
three categories. The first approach, using microwave
reflection to locate regions where the index of refraction
is substantially altered by high concentrations of free ions,
and the second approach that looks for changes in the
lower ionosphere caused by local alteration of the earth’s
circuit were shown to be physically impractical. The third
basic approach looks directly for altered ionization levels,
space charge levels, or electric fields in the downwind
plume. Analysis of this approach indicated radiation
sources should be detectable from more than 100 meters.
Results and Accomplishments
With relatively inexpensive instruments, experiments
were conducted to demonstrate the feasibility of detecting
radiation via altered ionization levels downwind from a
source. In the first phase, the instrumentation was
selected and purchased. Learning the intricacies of the
instruments and their ability to measure changes in
atmospheric electrostatics composed the second phase.
The third phase was the proof-of-principle experiment
where we attempted to detect a real radiation source using
the suite of commercially available instruments.
Due to budgetary constraints, most of the testing was
conducted using an ion generator. We were able to detect
ions from the generator at 100 meters with a handheld
instrument that cost less than $600. The final test we
conducted was to detect an Ir-192 gamma-ray source.
Although we clearly saw a change in the ion flux at
40 meters and arguably at 80 meters, a number of
research issues were identified. A radiation source can be
detected using atmospheric electrostatic measurements,
but a number of questions need to be answered before the
viability of this technique is determined.
Suite of Instruments
The suite of instruments was composed of two ion meters
used to measure the concentration of both positive and
negative ions, an electric field meter used to measure the
earth’s electric field, and a set of weather monitoring
instruments. A data acquisition system was engineered to
facilitate in the acquisition, analyses, and display of realtime
data.
The commercial ion meters are constructed of a stack of
three parallel plates with small air gaps between them.
The two outside plates are the opposite polarity of the
middle plate. A fan on the end draws air (200 cc/s)
between the plates. The ions of the opposite polarity of
the center plate are repelled from the outer plates and
attracted to the inner plate. The charge collected on the
inner plate is measured and converted to the number of
ions per cubic centimeter contained in the incoming air.
The units have a collection efficiency of 65%, and an
accuracy of ± 25%. They were modified so the data
acquisition system could directly read the value of the
charge collected on the inner plate. One meter measured
the concentration of positive ions, and the other meter
measured the concentration of negative ions. The space
charge was calculated from the difference between these
values.
The electric field meter (electric field mill) is composed
of a motor and two vanes. One vane is stationary, the
stator, and the other rotates at a given frequency, the
rotor. The vanes look like a pie cut into six pieces with
every other piece removed (a trifoil). When the three
lobes of the rotor are out of phase with the three lobes of
the stator, the earth’s electric field produces a potential
that causes a current to flow. As the lobes come into
Sensors and Electronics 409