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Here is a video testing it using a small amount of radioactive Americium found in a smoke detector:
Step 1: How It Works
This geiger counter works on similar principles that any other ordinary geiger counter works on. Essentially there is a small gap between two wires - a high dc voltage is applied to both wires, and when an ionizing particle moves between the wires, an ion channel is created and electrons arc across the wires, creating an audible "click" on a radio. Normally, the gap is inside of what is called a "geiger muller tube," where gases ideal for ionization are held for increased sensitivity. With this geiger counter, the absence of gases ideal to ionization is made up for by placing the wires in closer proximity to one another (so close that to the human eye they appear to be touching). The process of slowly moving the wires closer to one another is called "spark gap quenching".
Step 2: Finding a Radio
Any radio will do, just as long as when you create a "spark" (touching a wire to a battery) you hear a "click" on the radio. I tested my radio on the oscilloscope in this video:
As you can see, there are little valleys when I create sparks. This means that the radio is detecting the disturbances properly. The radios I used were a $1 mini radio and a larger more sensitive radio I salvaged from my grandfather.
Step 3: Making a High Voltage DC Power Supply
A high voltage DC power supply can be created from a disposable camera. First, take out the big black capacitor. Take out the smaller capacitor near it and place it where the bigger capacitor used to be. This will be used as a "filter capacitor." Take off all the unnecessary odds and ends (the xenon flash tube, the battery contacts, etc.) refer to the before and after photographs. I added a potentiometer to adjust the output voltage, but this is not necessary (that's the blue thing, don't worry about it). Make sure that if your camera had a "charge" button, solder the two contacts for the button together so that the unit is always in "charge" mode.
Be careful! The output from this unit will be around 400-600v! Test it out by touching two AA batteries to the + and - spots that were taken out, and the red LED should be on (or whatever other light is there).
Step 4: Spark Gap Quenching
Once you do this correctly, there should be arcs every so often, but not continuously (maybe once every couple of minutes). This means that the unit is detecting ionizing radiation! To hear the "clicks," turn on the radio.
I have attached a picture in which I have produced a spark gap between an alligator clip and a screwdriver, but any type of gap would work (between a wire and a plate, between two plates, between two wires, etc.).
Here are some videos of proper operation (notice and listen carefully for the audible "clicks"):
Step 5: Boxing It Up
Now, all that needs to be done is to box it up and make it all look pretty. With a bit of patience and lots of tape, spark gap quenching is possible in the little box too!
I added a switch and added an LED popping out from the box to replace the LED in the camera HV DC board. After gutting the mini radio, everything fits in a little altoids tin (perfect for pocketing).
The wires may need to periodically be re-calibrated, and the unit needs to be held still for it to work (vibration can cause an arc). Don't blow on the spark gap!
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