IEEE Spectrum June, 2014 - 22

RadCaM: copper foil and mesh applied to a funnel [top] form the electrodes for an
electrostatic concentrator that boosts the count rate of the radon detector. the high
voltages required are provided by a voltage multiplier [bottom left]. the detector itself
is a cMoS-based webcam with its image sensor exposed [bottom right].

capture one of radon's alpha-particle-emitting
daughter products.
When radon decays into polonium-218, the
polonium is usually left with a positive charge,
so it can be swept toward the webcam's image
sensor with a suitably shaped electric field.
Some copper tape (VentureTape 1626) and a
powder funnel (I happened to have a Nalgene
4252-0100 powder funnel on hand) is all you
need to make a two-electrode electrostatic concentrator that creates a field of the right shape.
I applied the copper tape to the interior of the
funnel, making sure to preserve electrical con22

Jun 2014

norTH aMerIcan

tact for both electrodes. I used a bottom electrode size and spacing of 15 millimeters. I then
stretched copper mesh over the top, soldered a
few spots into position, and I was done.
To get an idea of how the concentrator would
behave and how strong a field I would need to
generate, I modeled it using Comsol's multiphysics simulation software. For a concentrator
10 centimeters high, I estimated that an electric field strength of 50 volts per meter would
suffice for collecting charged polonium-218.
Although high voltages are required at the concentrator's electrodes to create this field, they

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aren't that hard to generate because, effectively,
no current will be drawn between the electrodes.
I put together a 12-stage Cockcroft-Walton
voltage multiplier driven by a bipolar 555 timer
feeding a Triad Magnetics SP-4 audio transformer.
The multiplier converted a 15-V supply to the
1370 V and 990 V I needed for the upper and lower
concentrator electrodes, respectively. Although
you should be careful dealing with voltages as
high as these, the voltage multiplier supplies so little current that an accidental contact shouldn't be
lethal. I also used small (1 nanofarad) capacitors
to avoid a dangerous buildup of charge. But again,
be careful, and proceed at your own risk. Turning
on the high-voltage concentrator improved the
count rate of the detector by 25 times or more.
Not bad for an old funnel and copper tape!
I used Matlab software to control the webcam
and also analyze the data. As the image sensor
can potentially also detect less strongly interacting beta particles, I set the detection threshold, over which a pixel is considered to have been
struck by an alpha, high enough to avoid noise
and beta impacts. Also, when the sensor is struck
by an alpha particle, an entire cluster of adjacent
pixels will often register the impact, so it's necessary to identify any clusters to avoid overcounting
strikes. I used an 8-nearest-neighbor algorithm to
find clusters. In all, it took about 100 lines of code.
Calibrated with a Safety Siren Pro Series3
radon detector (about $130), my system counted
5.2 alpha strikes per hour in an ambient radon
concentration of 159 becquerels per cubic
meter. At this level of sensitivity it would take the
detector about 20 hours to determine whether
radon was present at levels recommended as
actionable by the U.S. Environmental Protection
Agency with about 10 percent accuracy.
(For the future of this detector, I am thinking of using electrostatics-modeling software
to optimize the concentrator shape, electrode
size, and voltage to make an even more efficient
detector. It would also be interesting to rigorously calibrate the detector and observe activity dependencies on temperature, humidity, and
the presence of airborne particulates.)
If you build a similar detector yourself and
start reading high counts, contact a professional
(or buy a calibrated detector). It might be
time for radon mitigation in your own home.
-Ryan H. GRIffIn

clockwise froM top: ryan Griffin (2); Jon holMes

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Table of Contents for the Digital Edition of IEEE Spectrum June, 2014