Caution: This device uses a high voltage power supply and radioactive materials. Please follow the proper precautions.
I was introduced to alpha particle spark detectors when I watched a Youtube Video  by Carl Willis of his home made spark detector. I like the detector so much I wanted to build one for myself. So I researched the topic and read the seminal work from 1944 by W.Y. Chang and S. Rosenblum . Overview: The device is pretty simple. Take a thin tungsten wire, charge it to about 3000 volts with respect to ground. Position the wire approximately 1/16" (1.5 mm) above a smooth brass ground plate. The air acts like an insulator between the highly charged wire and ground plate. When an alpha particle passes in between the wire and the electrode it ionizes the air between. The high voltage potential between the wires and the electrode, is triggered by the ionization of the alpha particle, which causes an electrical avalanche that we see as an electrical discharge (spark) between the wires and the electrode. This detector is only sensitive to alpha (α) particles and will Not detect beta(β), x-ray or gamma(γ) radiation. Our device operates on this same principle but there are a few changes. First I am using thinner copper wires (.003 diameter) that are held at ground potential. It is the metal plate that is charged to approximately 8000 volts. The wire is positioned approximately 0.1" (3mm) away from the metal plate.
Step 1: Mark and Cut Prototyping Board
We begin by creating the holder for the .003 diameter copper wire. We start with a basic proto-typing board, shown below.
We mark the top side of the bard with a rectangle.
Cut out the rectangle.
Step 2: Soldering .003 Ground Wires.
After the board has the rectangle cut into it. You can solder the .003 copper wire across the opening.
I soldered five wires across the opening, approximately equally spaced.. One one side I soldered a 22 gauge insulated wire that would connect the assembly to ground.
Step 3: High Voltage Plate
Next I obtained a 1/8" thick x 1" x 3/5" metal plate. To this I attached copper foil tape to make an electrical connection. Just about any metal can be used for this plate. You can also use copper clad PC board. The main criteria for the plate is that it is perfectly flat and doesn't flex or bend.
Step 4: Construction
You can purchase an off-the-shelf adjustable high voltage power supply suitable for this project, see parts list. Using the power supply we wire up our components as detailed in the schematic, photo of set-up is shown below.
For my test I left all the components exposed, and its easy to show the wiring. However this is not a good idea. Since these are live high voltage components I recommend putting all the components in a plastic enclosure.
The high voltage metal plate has two 1/8" plastic squares next to it. The prototyping board rests on the plastic squares as shown. The wires are positioned approximately 0.1" above the metal plate.
Step 5: Try Out
Power is applied to the detector. There should not be any electrical discharge until an alpha particle shown is brought near the detector. If your detector immediately begins to arc, (have a discharge between the wires and plate) use the potentiometer on the adjustable power supply to reduce power. If that doesn't work, you need to increase the space between the wires and the plate. Also note, the .003 copper wires should be soldered taunt across the opening so that they do not droop down close to the plate.
If on the other hand you do not get any sparks flying between the wires and the plate, the first thing to check is your alpha particle source. Make sure it is a reasonable strong alpha source. If you have a Geiger counter capable of detecting alpha radiation use that to check your source.
If the alpha source checks out okay, then you may need to reduce the spacing between the wires and the plate. However, this is usually not the case.
Step 6: Components
Alpha Particle Spark Detector Components
5-8Kv Adjustable Negative High Voltage Power Supply
.003 Copper or Tungsten Wire (36")
HV Diode 6KV
5 Watt 510 ohm resistor
Components may be purchased from Images SI Inc.
Misc. Strong Alpha Particle Source, Proto-typing PC boards, copper foil tape, metal base plate, 12VDC power supply