Particle detectors and tracks Edexcel International A Level Physics revision
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In plain words
You can't see a proton, but you can see where it has been. A charged particle rips electrons off the atoms it passes, leaving a trail, and a magnetic field bends that trail into a curve. From the curve you can read the particle's charge and its momentum, the way a tracker reads an animal from its footprints.
4 things to know
- Detectors rely on ionisation: a charged particle ionises atoms along its path, leaving a track that can be recorded. Neutral particles leave no track.
- In a magnetic field the tracks curve, with radius r = p ÷ BQ. Opposite charges curve opposite ways. A tighter curve means less momentum.
- A track that spirals inwards shows a particle losing energy, and so momentum, as it goes.
- Wherever particles interact or decay, charge, energy and momentum are conserved. Tracks that start from a point with nothing leading to it show that a neutral particle was there.
Worked example
In a detector with a magnetic field, two tracks start from one point with no track leading to it. They curve in opposite directions with the same radius. Explain what happened.
- No track leads to the point, so whatever arrived was neutral: for example a photon.
- The tracks curve opposite ways, so the two particles have opposite charges. Their total charge is zero, the same as before.
- The equal radii show that they have equal momentum. This is a particle and its antiparticle being created, such as an electron and a positron.
Tips and tricks
- The direction of the curve gives the sign of the charge. The radius of the curve gives the momentum.
- Neutral particles are invisible. Look for a V shape that starts from nothing.
It lands in your notebook with its questions as flashcards.
Particle detectors and tracks: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What must a particle have in order to leave a track in a detector?
It leaves a trail by ionising atoms.
Two tracks curve in opposite directions in the same magnetic field. What does that show?
The magnetic force is reversed for the opposite charge.
A track curves more and more tightly. What is happening to the particle's momentum?
A smaller radius means a smaller momentum.
Two tracks start from a point with no track leading to it. What does this suggest?
Neutral particles leave no track of their own.
Which quantities are conserved in every particle interaction?
Particles can be created or destroyed, but these totals cannot change.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Particle detectors and tracks
Edexcel International A Level Physics WPH · 6 marks · papermunch.org
Name ______________________________ Date ______________
Explain why a neutron leaves no track in a detector.[2]
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It has no charge, so it does not ionise the atoms it passes.
The radius of curvature of a track gets steadily smaller. Explain what this shows.[2]
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The particle is losing momentum (and energy) as it ionises the material, and r = p ÷ BQ, so the radius gets smaller.
Two particles with the same charge move in the same magnetic field. Particle X follows a track of radius 0.20 m and particle Y a track of radius 0.60 m. Compare their momenta.[2]
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Y has three times the momentum of X.
Answers: Particle detectors and tracks
- 1. It has no charge, so it does not ionise the atoms it passes.
- 2. The particle is losing momentum (and energy) as it ionises the material, and r = p ÷ BQ, so the radius gets smaller.
- 3. Y has three times the momentum of X.