# Distinguish between positron and proton

1. Jun 8, 2015

### blue_leaf77

Probably a basic question in this field but I haven't been able to find the answer upon internet search.
Suppose I send a beam consisting of protons and positrons with a given momentum into a cloud chamber, how can then I tell them apart?

My own rough guess:
Suppose the two particles undergo the same amount of energy loss upon traversing the lead plate inside the chamber then the momentum after the plate will be $p_i' = \sqrt{p_i^2 - 2m_i \Delta E}$ where $i = 1,2$ denoting the indices of each particle and $\Delta E$ is the energy loss. $p$ is the initial momentum which is the same for both particle. Hence the radius of the trajectory is different after the plate despite the are the same before traversing it. Is it true?

Last edited: Jun 8, 2015
2. Jun 8, 2015

### Staff: Mentor

In general they won't. Which is a good way to identify them.

Their energy loss per distance in the cloud chamber is different as well, so their tracks will look different.

What is the rough energy scale of your particles?

3. Jun 8, 2015

### ChrisVer

Why not applying a uniform magnetic field? That way the distinction will become clear, because positrons and protons will bend differently as long as the momenta are not very large.

4. Jun 8, 2015

### Staff: Mentor

At the same momentum, they will bend exactly in the same way. Which is probably the way blue_leaf77 measures their momentum.

5. Jun 8, 2015

### ChrisVer

Well if the momenta are small enough then, the proton will lose energy [and momentum] much faster than the positron ?

6. Jun 8, 2015

### blue_leaf77

Yes that's exactly the thing that confuses me. So, more precisely in which way do they differ in their track due to the difference of energy loss per distance? If this loss is due to collisions with the cloud's molecules then I guess the lighter particle will easily lose its energy as compared to the heavier one? If for the moment I remove the lead plate, then the particle with smaller energy loss (heavy ones) will form bigger spiral while those with bigger energy loss (lighter ones) will form a faster sinking spiral?
Not the other way around?

Last edited: Jun 8, 2015
7. Jun 8, 2015

### Staff: Mentor

Slower nonrelativistic particles lose more energy per distance. This is described with the Bethe formula (and some corrections for very slow particles). At the same momentum, protons are slower.

Faster highly relativistic particles lose more energy per distance, but the momentum-dependence in this region is weaker. Other interactions become more interesting here, like bremsstrahlung of the positron.

What is the rough energy scale of your particles?

8. Jun 9, 2015

### blue_leaf77

This problem simply popped up in my mind during the class though, it has no relation to any real problem I'm facing.
So, what's the difference in their track shape inside the chamber?

9. Jun 9, 2015

### Staff: Mentor

Slower nonrelativistic particles will make thicker tracks. The track length (if the chamber is large enough) will depend on the mass as well, but that relation is a bit more complicated. If the tracking is sensitive enough, you might be able to see the energy loss (=>smaller radius in the magnetic field) which gives an additional indication of the particle type.