Inquiring Minds

Physics Questions People Ask Fermilab

Dear Sir
I would suspect you have been ask this before. But please give me a simple explanation of the wave phenomenon itself. When you read it on your detectors you get a spectrum reading but, be it particle or wave, what causes the wave that the detectors read as far as energy or vibrations. Waves in air are the percussion of molecules, and in the wave you see when you drop a stone in the lake it's medium is water. Please help me on this, I am not sure if I made myself clear most likely not. But then I know how much I don't know........

Thanks so much

Ron

Dear Ron:
You asked: When a particle (wave?) flies through a detector, what causes the signal (wave?) that the detector reads?

Modern particle detectors usually record electric charges that have been set free somewhere inside the detector. The charges (electrons flowing through a wire) can have been either caused directly or indirectly by the particles to be detected. The standard technique is to identify a charged particle crossing the detector by the ionization that it causes in the detectors material, either silicon strips, massive steel plates or gas chambers.

Particles, as you may know, can also display wave properties (and vice versa. See remark at the end.). But there have to be certain conditions (experimental setups) to test the wave properties of particles. Fermilab's particle detectors are not constructed to do that. They rely on ionization processes which are usually considered to be scattering processes of particles. (Some may consider it a scattering of waves... they would be right, too.)

The ionization process happens like this. The particle entering the detector has (a lot of) energy. As it traverses the material of the detector, it interacts with the electrons of the atoms inside that material. Giving some of its (kinetic) energy to the electrons, the particle ionizes the material, knocking off some electrons from the atoms. (In a quantum mechanical calculation of this process, physicists describe the electrons surrounding an atom as a quantum mechanical wave.) Experimentalists put a voltage across the material to detect the free electrons: A current flows through their meter, evidence that a charged particle just crossed that particular part of the detector.

I hope this answered your question. You see: the particle/wave aspects enter at various points.

I also would like to point out the following, since it is related to your question: Light is also an (electromagnetic) wave and a particle. We use antennae to detect electromagnetic waves (radio), but we need to use other means to detect particle properties of an electromagnetic wave. You can find more information on the particle-wave dualism of light at

http://www.fnal.gov/pub/light/light_page29.html

Please let me know if you have further questions.

Kurt Riesselmann
Fermilab, Office of Public Affairs

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