Experiment 58: Alpha Particle Spark Detector

I wanted to "see" nuclear radiation, so I looked online for experiments similar to making a Geiger counter.  One of the ones I found was an alpha particle spark detector, which shows passing alpha particles as high-voltage sparks.  A wire grid hovers a few millimeters above a plate charged to a high voltage, and when an alpha particle shoots by, it ionizes the air between the grid and the plate, which allows a spark to jump across.  I enjoy this device because it helps me visualize alpha radiation emanating from a source.

I made extensive use of this tutorial, which was the only one at the time.  To generate the high voltages required for this experiment, I powered a CCFL inverter with 12V from my lab power supply.  While the inverter's 900V is high voltage, it isn't high enough for this experiment, so I fed the inverter output into a Cockcroft–Walton voltage multiplier, as per the tutorial.  I used 0.01uF ceramic capacitors and 1N4007 diodes for my multiplier, which had four stages.  When powered with my lab power supply, that increases the voltage about eight-fold, generating the approximately 10kV necessary for a successful spark detector.  I also tried to round off the solder joints to minimize sharp edges that might cause performance-harming corona discharge.

When detecting alpha radiation, it is nice to have frequent small sparks that represent individual alpha particles, rather than one large spark every now and then.  To achieve this affect, I placed a 10 mega-Ohm high voltage resistor (from Vishay) on the high voltage output of my voltage multiplier.  This makes each spark smaller, but it also enables a greater number of sparks at a time.  The resistor also limits the current--larger currents might burn out the extremely thin wire grid.  A normal-voltage resistor might possibly work, but it could fail under 10kV.

Before building the wire grid and plate assembly, I picked out a case for my project.  Previously, I had gotten a sleek enclosure from OKW Enclosures, so I decided to use it in this project.  Because the enclosure was an oval, I picked an oval-shaped piece of aluminum for my high voltage plate.  I needed a good electrical connection for the high voltage, so I tried soldering to the aluminum.  After many failed attempts, I found online instructions that suggested scratching the aluminum under a pool of molten solder.  This eventually made the solder wet the aluminum, and soldering a wire to the plate underside was simple with that done.  After sanding the top to a near-mirror finish, I used four bolts and springs to attach the plate to the enclosure top.  The wire grid sits a few millimeters above the plate, and the bolts and springs let me adjust this distance so that the spark detector does not spark while idle.  I adjust the bolts until the detector only sparks in the presence of alpha radiation.

For my wire grid, I bolted small strips of copper-clad PCB stock to the enclosure top on each side of the aluminum plate.  I used the bolts protruding through the enclosure top's underside as my feedthroughs for the ground connection.  (The wires are ground and the aluminum plate is high voltage.)  I made multiple small cuts in the PCB strips to make individual (but still connected) pads to solder my wire grid to.  I didn't cut all the way through the fiberglass, nor did I separate the pads completely.  They still have copper connecting them in the back, but they are more thermally isolated, which makes soldering easier.  After tinning each pad and using soldering flux, I soldered 10 strands of flexible copper alligator clip lead wire from one pad to its corresponding pad across the aluminum plate.  I tried to make each wire tight so that it would not sag closer to the aluminum plate and create sparks without alpha particles present.  The distance between each wire was 4mm, and the distance from the wires to the aluminum plate was 3mm.  Each wire had a diameter of 0.08mm; such a small diameter is necessary to create the concentrated electric fields which make the spark effect.

As I put everything inside the enclosure, I tried to be sure that none of the high voltage (900V or 10kV) wire were close together or overlapping.  When I finally applied power, I found out that the aluminum plate was too close to the copper pads.  I sanded the plate's sides using 80 grit sandpaper, and eventually, I stopped the continuous sparking that had been I had noticed.  Some of the wires had low spots that sparked even without alpha particles, so I gently pushed those up with a q-tip.

Eventually, I got all the bugs worked out and tested the spark detector using an Am-241 source from a smoke detector.  It worked wonderfully, making a beautiful storm of sparks and small pinging sounds.  If you open the picture on the right and look closely, you can see small sparks on the finished detector as I hold the Am-241 source above it.  Because uranium undergoes primarily alpha decay, this detector also works with uranium ore, although not quite as vigorously, since the source material is more spread out.  This project is one of my favorites because I can "see" radiation with it.  Each spark happens at the actual location of an alpha particle, thus showing me the distribution of the radioactivity, which I think is really neat.