In my first class of the new semester, Modern Physics Lab, I finally learned about something that has intrigued me for quite some time: The Geiger Counter.
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I first heard of the Geiger Counter during an interview with Dr. Michio Kaku of CUNY City College. He mentioned it as he was describing the famous thought experiment of Schroedinger’s Cat. This thought experiment involves the decay of a radioactive atom which is detected by the Geiger Counter. Eventually, I learned that Geiger Counters measure radioactivity of radioactive materials.
In my Modern Physics Lab class, I learned how it actually worked though. A Geiger Counter typically consists of a metallic cylinder, known as a Geiger, with an opening at one end of its circular surface, known as the window. The inside of the cylinder has a rigid metallic wire running down the center and the cylinder is filled with a gas of some sort. The window is covered with a thin piece of material to prevent the gas from escaping.
When you point the probe in the direction of a radiation source, the radiation from the source travels via electromagnetic waves (such as gamma rays). These waves pass through the window, into the cylinder, and come in contact with the gas. The energy of these waves is sufficient to knock off electrons from these gas atoms (i.e. they “ionize” the gas). These freed electrons then in turn knock off other electrons of the surrounding gas atoms, creating an avalanche effect. The wire is connected to a high voltage source that makes the wire positively charged. Since it is positively charged, the freed electrons become attracted to the wire, and eventually they come in contact and the electrons travel along the wire to the rest of the device. Usually, the wire is connected to a microphone that turns the traveling electrons, or electrical current, into a sound vibration. That’s why if you’ve seen a Geiger Counter, you hear the crackling/popping noise when it is pointed closely to a radiation source.
If you come in contact with a Geiger Counter one day, you can still use it even if you are not in the presence of any obvious radiation source. Just point the counter towards the sky and you are bound to hear a few clicks. This is because the Earth is constantly hit by radiation all of the time in the form of cosmic rays. These are equally capable of setting off the Geiger counter.
My Professor had exactly one of these types of Geiger Counters yesterday that he showed the class. He played a nice trick on us by borrowing a student’s pack of cookies and placing it on a plate. The counter starting crackling quite loudly and he had us believe that the cookies contained radiation. It turned out though, that the plate he placed them on was actually made in Russia and contained some radioactive material in it.
