8.6 Interpreting Particle Tracks

Interpreting Particle Tracks

Particle detectors that count particles, like Geiger-Muller tubes, are useful but they cannot distinguish different types of particle
Modern detectors can show the paths of charged particles, from which physicists are able to interpret the characteristics of the particle

The curvature of the particle tracks gives an indication of its momentum
- A smaller radius means the particle has a smaller momentum
- A larger radius means the particle has a larger momentum

This is due to the equation for the radius of a charged particle in a magnetic field:

$r = \frac{m v}{B Q} = \frac{p}{B Q}$

Where:
- r = orbital radius of charged particle (m)
- p = momentum of charged particle (kg m s^–1)
- B = magnetic field strength (T)
- Q = charge of particle (C)
- m = mass of the particle (kg)
- v = velocity of the particle (m s^-1)

If the radius of a track is decreasing (i.e., it is spiralling closer inwards)
- This means the particle's momentum is decreasing
- This is because r ∝ p
- Therefore, the velocity of the particle is decreasing
- Hence, the kinetic energy of the particle is also decreasing, due to ionising other particles in its path

The direction of a track's curvature gives an indication of the particle's charge
- Fleming's Left Hand Rule can be used to determine the sign of the particle's charge

Sometimes, particle tracks appear to start out of 'nowhere'
- This indicates particle-antiparticle creation
- These paths are in opposite directions because the particle-antiparticle pair is oppositely charged
- Therefore, the magnetic force on them is oppositely directed
- However they have the same radius because they each have the same mass (and hence, momentum)
Therefore charge, energy and momentum are always conserved in interactions between particles

Exam Tip

Sometimes, examiners will ask you to explain whether particles are simply moving upward or downward across an image. The first thing to consider should be the radius of the particle track: you expect the radius to be decreasing, because charged particles will tend to continue ionising other particles around them - hence losing kinetic energy.

As their kinetic energy decreases, so does their momentum - and hence, track radius will also decrease. This should be enough to determine which direction the particle is coming from and heading towards!

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Edexcel A Level Physics

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8.6 Interpreting Particle Tracks

Interpreting Particle Tracks

Exam Tip

Author: Deva

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