# Distinguish between positron and proton

• blue_leaf77
In summary, the conversation discusses how to tell apart protons and positrons in a cloud chamber based on their momentum and energy loss. It is suggested that a uniform magnetic field can be used to distinguish the particles, as they will bend differently. The conversation also mentions the use of the Bethe formula to describe energy loss per distance for slower nonrelativistic particles. Additionally, the energy scale of the particles and their track shapes inside the chamber are discussed as factors for identification.
blue_leaf77
Homework Helper
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?

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blue_leaf77 said:
Suppose the two particles undergo the same amount of energy loss upon traversing the lead plate inside the chamber
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?

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.

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

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

mfb said:
At the same momentum, they will bend exactly in the same way. Which is probably the way blue_leaf77 measures their momentum.
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?
ChrisVer said:
the proton will lose energy [and momentum] much faster than the positron ?
Not the other way around?

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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?

mfb said:
What is the rough energy scale of your particles?
This problem simply popped up in my mind during the class though, it has no relation to any real problem I'm facing.
mfb said:
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.
So, what's the difference in their track shape inside the chamber?

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.

blue_leaf77

## What is a positron?

A positron is a subatomic particle with the same mass as an electron but with a positive charge. It is the antiparticle of an electron, meaning that it has the opposite charge.

## What is a proton?

A proton is a subatomic particle with a positive charge that is found in the nucleus of an atom. It is one of the building blocks of an atom, along with neutrons and electrons.

## How are positrons and protons different?

The main difference between positrons and protons is their charge. Positrons have a positive charge while protons have a negative charge. Additionally, positrons are much less common than protons and are usually only found in high-energy environments.

## Can positrons and protons interact with each other?

Yes, positrons and protons can interact with each other through a process called annihilation. When a positron and a proton meet, they can combine to form two gamma rays, which are high-energy photons.

## What is the significance of distinguishing between positrons and protons?

Distinguishing between positrons and protons is important in understanding the behavior and interactions of particles at the subatomic level. It also has practical applications in fields such as nuclear physics and medical imaging.

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