# Antimatter and bubble chambers?

## Main Question or Discussion Point

I'm confused now... how can be antiparticles be detected in a bubble chamber which is made of ordinary matter? Why does a positron leave its trace interacting with the chamber gas without annihilating immediately?

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Astronuc
Staff Emeritus
I'm confused now... how can be antiparticles be detected in a bubble chamber which is made of ordinary matter? Why does a positron leave its trace interacting with the chamber gas without annihilating immediately?
Antimatter is charged, and charged particles ionize atoms in the bubble chamber, which leads to emission of photons along the track.

Positrons (and other charged particles) slow down by collisions, mainly with electrons, and because the positron has the rest mass of an electron, the slowing down is more rapid than for heavier particles.

Hmmm.... I don't feel this answers the question. Why is there any ionization at all and not an immediate annihilation at the first collision? What is observed are several collisons of a positron with many electrons, and yet no annihilation? That doesen't make sense to me.

Staff Emeritus
2019 Award
Why should there be immediate annihilation? Or more quantitatively, how "immediate" do you think it should be, and why? (Remember, if it lasts a microsecond, it will travel 1000 feet)

Why should there be immediate annihilation? Or more quantitatively, how "immediate" do you think it should be, and why? (Remember, if it lasts a microsecond, it will travel 1000 feet)
In this context "immediate" would mean "at the first collision".

Astronuc
Staff Emeritus
Hmmm.... I don't feel this answers the question. Why is there any ionization at all and not an immediate annihilation at the first collision? What is observed are several collisons of a positron with many electrons, and yet no annihilation? That doesen't make sense to me.
The range of coulomb interaction - attraction or repulsion - is much greater than the range for annihilation. There is a lot of 'distance' between electrons and atoms.

The range of coulomb interaction - attraction or repulsion - is much greater than the range for annihilation. There is a lot of 'distance' between electrons and atoms.
Ok, this makes more sense... but 'googled' and couldn't find a document explaining how it is quantified. Does someone have an idea from what parameters the range of annihilation depends and/or a typical quantitative value?

You are right this does seem difficult to google. I thought about just writing down the cross section for e+e- to two photons, but actually I am not sure that that captures everything since at low energies the e+e- pair usually form the bound state positronium before they annihilate. I also seem to recall there is a thing called the "impact parameter", b, which gives the cross-section a dependence which I think is something like $e^{-b.k}$, so that it goes down as the impact is less "head on" and as the momentum goes up. That doesn't give any hint of the scale that matters though. Also I think it is for two approximately plane wave initial states, which is probably not very reasonable if the target electrons are bound in atoms.

But the overall picture is that the positrons are not very likely to annihilate while they are fast-moving, they need to slow down first by scattering off things.