Experiment 61: Single-Transistor Ion Chamber for Detecting Radiation

Radiation fascinates me, but it isn't very interesting unless you can detect it somehow.  Geiger counters cost a lot, so I built something a radiation detector using a soup can and a single transistor.

The ion chamber I built uses a thin whisker wire inside a metal can to collect charge from ions made by passing radiation.  The transistor (I used a BC547B, but any small NPN signal transistor should work) amplifies the difference in charge between the can and the wire and sends this out as a voltage read out on a multimeter.  The other components of this simple radiation detector are a 4.7kOhm resistor, a 9V battery clip, and a 9V battery for power.  My whisker wire was simply some bare wire that held its shape when straightened.  I got my design from this YouTube video, and the video's instructions seemed fairly clear.

I learned some important things through researching this ion chamber.  When picking a can, it is important to pick one without a coating on the inside (or sand it off).  Any coating interferes with picking up charge from the air, which hampers the detector's performance.  I sanded my can's inside to be sure it would work.  Also, the transistor gets epoxied to can.  The epoxy shouldn't touch any of the transistor leads (only the plastic), and the leads shouldn't touch the can.  Either of those situations would cause unwanted electrical conductivity.  The only electrical connection to the can is made through the 4.7kOhm resistor.  If there is a coating on the outside of the can, it should be sanded off to help make a good electrical connection with the resistor.  When attaching the whisker wire, it is important to make sure it doesn't touch the can as it goes from the transistor's base through the hole in the can bottom (see picture at right).

One problem with this design is its sensitivity to external electromagnetic fields.  Simply moving sometimes causes the measured voltage to fluctuate.  To help prevent this, the detector may be closed off with an aluminum foil "lid" with the radiation source inside.  I also made an electronics cover using the bottom of another soup can and taped it over the electronics with some foil tape (as seen in the two pictures below).

Using the detector is simple.  With a 9V battery connected, exposing the detector chamber to radiation creates an increased voltage readout on a connected multimeter.  I have tested this detector with an americium source and with uranium ore, and while both work, the americium definitely has a greater effect.  Sadly, I do not have any other radioactive items to test; if I did, I would check whether this ion chamber can sense beta and gamma radiation.  Still, I am truly amazed at what can be accomplished with just a soup can and a single transistor.