Loading...
Particle physics runs on two pieces of equipment: an to give charged particles enormous energies, and a to work out what emerges when they collide. Both rely entirely on the electric and magnetic field physics you already know.
What you'll be able to do
A is a long line of hollow metal drift tubes connected alternately to a high-frequency alternating supply. Inside a tube there is no field, so a particle drifts at constant speed. In the gaps between tubes it is accelerated by the p.d.
The supply reverses each time the particle is inside a tube, so every gap it reaches has the right polarity to push it forward. A particle of charge gains energy at each gap.
Because the particle speeds up but the alternating supply has a fixed frequency, it must spend the same time in each tube. Successive tubes therefore get .
Tip — An electron-volt is the energy gained by one electron charge through 1 V. A charge of through volts gains exactly eV, which saves conversions.
A cyclotron has two hollow D-shaped electrodes, the , between the poles of a large magnet. The magnetic field bends particles into semicircles inside the dees, and an alternating p.d. across the gap accelerates them each time they cross.
With each crossing the particle speeds up and its radius increases, since . It spirals outwards until it leaves at the edge.
The key insight is that the time for each semicircle does not depend on speed. A faster particle travels a bigger circle in the same time, so a fixed-frequency supply stays in step. This holds only while the particle is non-relativistic; at very high speeds its effective mass increases and it falls out of step.
Detectors reveal charged particles through the ionisation they cause along their path. Neutral particles leave no track, but they can be inferred from gaps — a track that starts from nowhere, or a "V" of two opposite tracks appearing where a neutral particle decayed.
A magnetic field is applied across the detector. From the tracks you can deduce: the from the direction of curvature, using Fleming’s left-hand rule; the from the radius, since ; and a loss of energy from a track that spirals inwards as the particle slows.
Tighter curvature means lower momentum. Two particles with equal and opposite charges created at the same point curve in opposite directions — the signature of pair production.
Tip — For track diagrams, state the rule you used for direction and the relationship between radius and momentum explicitly.
In a high-energy collision, kinetic energy can become mass: new particles appear that were not there before. The conversion follows .
Particle masses are often quoted in or — the energy equivalent divided by . A proton has mass , so creating a proton and an antiproton needs at least 1876 MeV.
In the reverse happens: a particle meets its antiparticle and both masses become energy, usually as two photons travelling in opposite directions so momentum is conserved.
In every interaction, charge and momentum are conserved. Momentum conservation is why collisions between two beams moving in opposite directions release far more usable energy than firing a beam at a fixed target.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
Ready to lock in Particle Accelerators and Detectors? Pick a mode and earn XP & Dobloons.
Real past-paper questions on Particle Accelerators and Detectors, marked mark-by-mark. How you do feeds straight into your weak-topic list, so your revision keeps targeting what actually needs work.