Why must accelerators be evacuated?

So particles do not collide with air molecules and lose energy, scatter or ionise the gas.

Why is there no force on particles inside a linac drift tube?

The hollow metal tube shields them, so there is no electric field inside.

Where are particles accelerated in a linac?

In the gaps between adjacent drift tubes.

Why do the drift tubes in a linac get longer?

The particles speed up, so they travel further in each half cycle of the supply.

Why must the supply to a linac alternate?

So that the next tube always has opposite polarity to the particle as it reaches a gap.

What is the role of the magnetic field in a cyclotron?

It makes the particles move in semicircles inside the dees; it does no work.

What is the role of the electric field in a cyclotron?

It accelerates the particles each time they cross the gap between the dees.

What happens to the radius of the path in a cyclotron as the particles speed up?

It increases, so the particles spiral outwards.

The magnetic force BQv provides the centripetal force mv²/r, so r = mv/BQ = p/BQ.

How do charged particles produce tracks in a detector?

They ionise atoms along their path.

Why do neutral particles leave no track in a detector?

They do not ionise the medium directly and are not deflected by the magnetic field.

What can be found from the curvature of a track in a magnetic field?

The sign of the charge (direction of curve) and the momentum (radius).

Particle accelerators and detectorsEdexcel A-Level Physics: Flashcards

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What is thermionic emission?

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What is thermionic emission?: The release of electrons from the surface of a heated metal.
What kinetic energy does a charge Q gain when accelerated from rest through a p.d. V?: E_k = QV
Why must accelerators be evacuated?: So particles do not collide with air molecules and lose energy, scatter or ionise the gas.
Why is there no force on particles inside a linac drift tube?: The hollow metal tube shields them, so there is no electric field inside.
Where are particles accelerated in a linac?: In the gaps between adjacent drift tubes.
Why do the drift tubes in a linac get longer?: The particles speed up, so they travel further in each half cycle of the supply.
Why must the supply to a linac alternate?: So that the next tube always has opposite polarity to the particle as it reaches a gap.
What is the role of the magnetic field in a cyclotron?: It makes the particles move in semicircles inside the dees; it does no work.
What is the role of the electric field in a cyclotron?: It accelerates the particles each time they cross the gap between the dees.
What happens to the radius of the path in a cyclotron as the particles speed up?: It increases, so the particles spiral outwards.
Derive r = p/BQ.: The magnetic force BQv provides the centripetal force mv²/r, so r = mv/BQ = p/BQ.
How do charged particles produce tracks in a detector?: They ionise atoms along their path.
Why do neutral particles leave no track in a detector?: They do not ionise the medium directly and are not deflected by the magnetic field.
What can be found from the curvature of a track in a magnetic field?: The sign of the charge (direction of curve) and the momentum (radius).

Exam questions on Particle accelerators and detectors

In a particle accelerator, electrons are produced by a heated metal filament (the cathode) in an evacuated tube. They are then accelerated towards an anode through a p.d. of 2.0 kV. The charge on an electron is 1.60 × 10⁻¹⁹ C.
Explain why the accelerator tube must be evacuated.2 marks
A cyclotron consists of two hollow D-shaped electrodes (dees) in an evacuated chamber, placed in a uniform magnetic field of flux density 1.2 T perpendicular to the plane of the dees. An alternating p.d. is applied between the dees, and protons are injected near the centre. The charge on a proton is 1.60 × 10⁻¹⁹ C and its mass is 1.67 × 10⁻²⁷ kg.
A proton is moving at 5.0 × 10⁶ m s⁻¹ inside one of the dees. Calculate the radius of its circular path.2 marks
A linear accelerator (linac) consists of a line of hollow cylindrical drift tubes in an evacuated chamber, connected alternately to the two terminals of an alternating supply of frequency 25 MHz. A beam of protons travels along the axis of the tubes.
Explain why the protons travel at constant speed inside each tube, where they are accelerated, and why the polarity of the supply must alternate.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).