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Nuclear and Particle Physics

Particle tracks, conservation laws and why high energies are needed

  • 3 things to know
  • Papers 1 and 3

Overview

Recall questions on reading momentum and charge from a track, why an uncharged particle leaves none, applying conservation at a vertex, and why high energies are required.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Process

Why does a track in a detector often spiral inwards rather than staying a circle?

Process

How is conservation of energy applied to an interaction between particles?

Process

Why does a head-on collision between two beams make more energy available than firing a beam at a stationary target?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How is a particle's momentum determined from its track in a detector?

    Model answer

    The radius of curvature of the track is measured and used in p = BQr with the known magnetic flux density and the particle's charge.

  2. 2

    How can the sign of a particle's charge be identified from its track?

    Model answer

    Particles of opposite charge experience magnetic forces in opposite directions in the same field, so their tracks curve in opposite senses.

  3. 3

    Why does a track in a detector often spiral inwards rather than staying a circle?

    Model answer

    The particle loses energy and momentum by ionising the material it passes through, so the radius r = p/(BQ) shrinks steadily along the track.

  4. 4

    On what general principle does a detector register the passage of a charged particle?

    Model answer

    The particle ionises atoms of the detecting material along its path, and the ions and electrons released produce a visible trail, a pulse of current or a flash of light.

  5. 5

    Why does an uncharged particle leave no track in a detector?

    Model answer

    It does not ionise the material it passes through, so its presence must be deduced from the tracks of charged particles produced when it decays or interacts.

  6. 6

    What does a short, thick track tell you about the particle that made it?

    Model answer

    It was strongly ionising, producing many ion pairs per unit length, so it lost energy quickly and came to rest after a short distance.

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