Why are electron guns and accelerators evacuated?

So the particles do not collide with air molecules and the hot filament does not oxidise.

What does a magnetic field do to a moving charged particle?

It exerts a force perpendicular to the velocity, changing its direction but not its speed.

BQv = mv²/r, so r = mv/BQ = p/BQ.

Why is there no electric field inside a drift tube?

The metal tube shields the inside, so the particles drift at constant speed.

Why do the drift tubes in a linac get longer?

The particles move faster, so in the same time (half a period) they travel further.

What accelerates the particles in a cyclotron?

The alternating electric field in the gap between the dees.

Why can a cyclotron use a fixed frequency?

The time per semicircle, πm/BQ, does not depend on speed.

How do particle tracks show the sign of a charge?

The direction of curvature in a known magnetic field, found with Fleming's left-hand rule.

Why does a charged particle's track spiral inwards in a detector?

It loses energy by ionisation, so p decreases and the radius r = p/BQ decreases.

Why do neutral particles leave no track?

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

Particle accelerators and detectorsEdexcel International 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 because they gain enough kinetic energy to escape.
Why are electron guns and accelerators evacuated?: So the particles do not collide with air molecules and the hot filament does not oxidise.
What is the kinetic energy gained by a charge Q accelerated through p.d. V?: QV, so ½mv² = QV.
What does a magnetic field do to a moving charged particle?: It exerts a force perpendicular to the velocity, changing its direction but not its speed.
Derive r = p/BQ.: BQv = mv²/r, so r = mv/BQ = p/BQ.
What happens to the radius if the charge is doubled at the same momentum?: It halves, since r = p/BQ.
Why is there no electric field inside a drift tube?: The metal tube shields the inside, so the particles drift at constant speed.
Why do the drift tubes in a linac get longer?: The particles move faster, so in the same time (half a period) they travel further.
What accelerates the particles in a cyclotron?: The alternating electric field in the gap between the dees.
Why can a cyclotron use a fixed frequency?: The time per semicircle, πm/BQ, does not depend on speed.
Give the frequency of the supply needed in a cyclotron.: f = BQ / 2πm.
How do particle tracks show the sign of a charge?: The direction of curvature in a known magnetic field, found with Fleming's left-hand rule.
Why does a charged particle's track spiral inwards in a detector?: It loses energy by ionisation, so p decreases and the radius r = p/BQ decreases.
Why do neutral particles leave no track?: They do not ionise the material directly and are not deflected by the magnetic field.

Exam questions on Particle accelerators and detectors

An electron gun in an evacuated tube has a heated tungsten filament. Electrons released from the filament are accelerated through a potential difference of 1500 V between the filament and a metal anode.
Explain why the electron gun must be evacuated.2 marks
A cyclotron accelerates protons using two hollow D-shaped metal electrodes (dees) separated by a small gap. A uniform magnetic field of flux density 1.2 T acts perpendicular to the plane of the dees, and an alternating potential difference is applied across the gap.
Explain why the radius of the path increases as the protons gain speed, but the time taken for each semicircle stays the same.2 marks
A proton moves in a uniform magnetic field of flux density 1.5 T that is perpendicular to its velocity, in a bubble chamber. It leaves a circular track of radius 0.085 m. The proton charge is 1.60 × 10⁻¹⁹ C.
Calculate the momentum of the proton.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).