Understanding bubble chamber tracks

[magu1re]

Hey. :)

I am a Year 13 student and do not have to know anything about bubble chambers. However, they are mentioned in my textbook and I am interested in them.

The textbook states: "Measuring the curvature of the tracks allows the charge and mass of the particles to be deduced. The spiral tracks arise from electrons and positrons, which lose energy as they follow a curved path."

I do not understand how you can deduce the mass and charge of a particle from a bubble chamber. It is obvious that you can deduce whether it's charge is positive or negative but surely the electromagnetic force exerted upon it is dependent upon the charge of the particle (as well as it's velocity) and the acceleration of the particle is dependent upon both this force and the mass of the particle. Thus, I do not see how you can deduce these quantities by studying the curvature of the tracks (because they seem to depend on both of these quantities).

Furthermore, why it it that the electrons and positrons spiral? I understand that, when decelerated, these particles lose energy very quickly due to braking radiation and their minute mass. But why are the tracks circular? What causes them to move in such a way? etc.

Any explanations you could offer would be much appreciated. Thank-you.

 

[Creator]

Hey. :)


The textbook states: "Measuring the curvature of the tracks allows the charge and mass of the particles to be deduced. The spiral tracks arise from electrons and positrons, which lose energy as they follow a curved path."

I do not understand how you can deduce the mass and charge of a particle from a bubble chamber. It is obvious that you can deduce whether it's charge is positive or negative but surely the electromagnetic force exerted upon it is dependent upon the charge of the particle (as well as it's velocity) and the acceleration of the particle is dependent upon both this force and the mass of the particle. Thus, I do not see how you can deduce these quantities by studying the curvature of the tracks (because they seem to depend on both of these quantities). ...

But why are the tracks circular? What causes them to move in such a way?

Hi Macquire:
You appear to have forgetten about the applied magnetic field and the formula :
The (initial) momentum can be detemined by measuring the initial curvature in a magnetic field:
The Lorentz force on a charge in a B field is given by:

F = qv X B .

And the direction (of curvature) is given by the type of charge.

That force (classically) must equal the cetripetal force on the particle:
F = mv^2/ R

Setting the two forces equal gives :

qv X B = mv^2/ R

so: mv = qBr = p... (assuming perpendicular field)
You know the charge, radius and magnetic field so you can detemine momentum, p.


Further inward "spiralling" (reduced R) is caused by bremsstrahlung radiation which reduces its momentum as a result of kinetic energy loss.

Creator

 

[magu1re]

Hi Macquire:
You appear to have forgetten about the applied magnetic field and the formula :
The (initial) momentum can be detemined by measuring the initial curvature in a magnetic field:
The Lorentz force on a charge in a B field is given by:

F = qv X B .

And the direction (of curvature) is given by the type of charge.

That force (classically) must equal the cetripetal force on the particle:
F = mv^2/ R

Setting the two forces equal gives :

qv X B = mv^2/ R

so: mv = qBr = p... (assuming perpendicular field)
You know the charge, radius and magnetic field so you can detemine momentum, p.


Further inward "spiralling" (reduced R) is caused by bremsstrahlung radiation which reduces its momentum as a result of kinetic energy loss.

Creator

How do I know the charge of the particle? At this point in time I could only deduce whether a particle had positive, negative or no charge due to the direction of curvature.

Furthermore, why does it spiral? Originally the electromagnetic force accelerates the particle in one direction. Why does this force then change direction (causing the electron/positron to spiral)?