# Why does positron not annihilate sooner in cloud chamber experiment

1. ### WillietheKid

2
Specifically in Carl D Anderson's cloud chamber experiment, the first experiment to prove the existence of the positron, a positron travels through all of these mediums: glass, charged liquid particles, water vapor, and lastly passes through a lead plate. If antimatter is suppose to anihilate upon contact with ordinary matter how did this positron make it through all of this?

Is it that the positron is so small that it was able to pass all of this without contact?

Last edited: Jun 18, 2014
2. ### Bill_K

4,159
Because, just as with all other particle interactions, there is a probability/sec for it to occur. It's perfectly possible for a positron and electron to collide without annihilating. The average lifetime of a positron in ordinary matter is of the order of 10-10 sec.

No, all elementary particles including the positron are the same size, namely they are pointlike.

Last edited: Jun 18, 2014
3. ### Matterwave

3,853
I wonder if the fact that most of the electrons in everyday space are not free electrons, but are bound to atoms also makes somewhat of a difference. Sadly, I have never seen any calculation of electron-positron annihilation from a bound state, so I can't talk with any authority on the matter.

4. ### Bill_K

4,159
The interaction of positrons with solid materials is apparently a very interesting and fruitful subject! Positron lifetimes range from a few hundred picoseconds to a few nanoseconds depending on the solid. And positron spectroscopy is used to study the properties of the material.

For metals, positrons tend to surround themselves with conduction electrons, and their lifetime gives information on the concentration of different types of metallic defects present. For an ionic crystal, positrons may form positronium-like complexes with the negative ions.

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5. ### Matterwave

3,853
Ah, thanks for the information. :)

6. ### kurros

387
I'm not familiar with the experiment, but on top of what others have said, it sounds like they must be fairly high energy positrons if they are going through lead. This makes them less likely to interact with things; they can kind of just "punch through".

If you take some radioactive material that decays by beta+ decay, then usually the positron is not such a high energy and will only travel a millimetre or so in something like water. This is why you can do positron emission tomography for medical imaging (PET scans); the positrons annihilate near where the radioactive dye they inject into you travels, and you image the gammas that come flying out. Still, there are gazillions of electrons over that millimetre, so it does still scatter off lots of them before annihilating.