I How Do Positrons Reach Cloud Chambers Without Annihilating?

[Physicsislove30]

I was researching on the Anderson cloud chamber that discovered the positron in 1932. I couldn't help but wonder, if positron annihilates upon contact with an electron, how was it able to get till the cloud chamber without annihilation? Shouldn't the 2 photons be the only thing observed in the cloud chamber which are produced upon annihilation? Or does a cloud chamber detects only the existence of positron and does not mean that the positron is exactly at that point in that time.

 

[Orodruin]

”Upon contact with an electron” is an extremely popularised version of what is going on. It is so inaccurate that no reasoning can be based on it.

 

[Physicsislove30]

”Upon contact with an electron” is an extremely popularised version of what is going on. It is so inaccurate that no reasoning can be based on it.

Could you then tell me what the accurate reasoning is?

 

[Vanadium 50]

The probability of annihilation is very low unless the positron is moving slowly with respect to the electron. While it's slowing down its moving quickly and thus covers a lot of ground.

 

[jtbell]

Both of V50's points are illustrated in the bubble-chamber photograph on this page:

https://hst-archive.web.cern.ch/archiv/HST2005/bubble_chambers/BCwebsite/gallery/gal3_photon1.htm

It shows a positron track that stops abruptly because it annihilates in flight with a stationary electron, a rare process. One of the invisible photons that it produces passes close to a nucleus and converts into an electron-positron pair, with tracks spiraling in opposite directions.

 

[Meir Achuz]

The probability of annihilation is very low until it hits a heavy plate with a large density of electrons.