A-Level · 26 September 2026 · 6 min read
Particle detector questions: tracks, charge, momentum and evidence
A detector image can look like a collection of curved lines, but each feature answers a different question. Curvature may indicate charge sign, radius may constrain momentum, and the start of visible tracks may identify an interaction point. None of these features alone automatically gives a particle’s complete identity.
These original questions practise particle reasoning relevant to Edexcel IAL A2 Unit 4. They explain the assumptions behind each inference rather than asking you to memorise the appearance of one image. Follow the source specification and the apparatus described in your actual question.
Explain how an invisible particle produces visible evidence
A charged particle can ionise material along its path. In a cloud chamber, droplets form along suitable ionisation trails in a supersaturated vapour. In a bubble chamber, bubbles can form along an ionising path in a superheated liquid. The visible trail records interactions in the medium; it is much larger than the particle itself.
Detector technologies differ, so avoid copying a cloud-chamber description into a question about electronic detector signals. Identify what is measured and how the particle produces that signal. A diagram showing a neutral particle as a dashed line may represent an inferred path, not a directly visible ionisation track.
Worked example: infer charge sign from curvature
Imagine a particle moving initially to the right in a magnetic field directed into the page. For a positive charge, the magnetic force points upward, so the path initially curves upward. A negative charge with the same initial direction curves downward. The inference depends on knowing both the field direction and the direction of travel.
If the image does not establish which way the particle moved, the same curve can be consistent with a different charge travelling the opposite way. State that limitation rather than overclaiming. A track’s arrow, interaction vertex or other information may resolve the direction, but curvature by itself does not always do so.
Worked example: calculate momentum from radius
A singly charged particle moves perpendicular to a uniform 0.50 T field with track radius 0.080 m. Use p = |q|Br. With |q| = 1.60 × 10⁻¹⁹ C, momentum is 6.4 × 10⁻²¹ kg m s⁻¹. The radius is measured from the centre of curvature to the track, not across the circle’s diameter.
If the particle is known to be a proton, using p = mv gives approximately 3.8 × 10⁶ m s⁻¹, comfortably below c for this example. Do not automatically use the same non-relativistic speed calculation for a very light, high-momentum particle. The track gives momentum; converting that to speed requires an appropriate relation and knowledge of the particle.
Change the conditions before identifying the particle
For fixed field and charge magnitude, doubling radius doubles momentum. Two particles with equal charge magnitude and equal momentum have equal radii even if their masses differ. Therefore a larger radius does not directly mean a larger mass. If charge magnitude changes as well, radius depends on the ratio p/|q|.
A charged particle losing momentum as it travels through detector material can follow a progressively smaller-radius path in a uniform field. The magnetic force itself is perpendicular to velocity and does not supply that energy loss. Interactions with the material can transfer energy; distinguish the bending mechanism from the slowing mechanism.
Worked example: pair-production energy accounting
A photon of energy 2.00 MeV produces an electron–positron pair near a nucleus. Taking each particle’s rest energy as 0.511 MeV, 1.022 MeV is needed for the pair’s total rest energy. Ignoring the small recoil contribution only for this estimate, the remaining combined kinetic energy is 0.978 MeV. It need not be shared equally.
The nearby nucleus or another interaction partner participates in momentum conservation. Do not describe an isolated photon simply disappearing into two particles without checking the complete system. Oppositely curving outgoing tracks can support an opposite-charge interpretation, while energy and charge conservation provide additional checks on the proposed event.
Write an evidence-based interpretation rather than a guess
Organise the answer as observation → physical relationship → inference → limit. For example: the track bends upward; the incoming direction and field are known; the magnetic-force rule indicates positive charge; curvature alone does not establish mass. This structure makes it clear which conclusion each piece of evidence supports.
Practise one charge-direction diagram, one momentum calculation and one interaction with missing information. Ask what extra measurement would help: field strength, momentum, energy deposited, travel direction or another track. The strongest answer is not the most confident particle name; it is the conclusion justified by the available evidence.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
Is a particle track a photograph of the particle?
No. It records effects of the particle interacting with detector material, such as ionisation followed by droplets, bubbles or electronic signals. Explain the mechanism appropriate to the detector in the question. The visible trail is not the particle’s actual size.
Does a larger track radius mean a heavier particle?
Not by itself. For perpendicular motion in a uniform field, radius is p/(|q|B), so it depends on momentum, charge magnitude and field strength. Particles of different masses can have the same radius if they have the same momentum and charge magnitude.
Why might a neutral particle have no visible track?
A neutral particle does not produce the same direct charged-particle ionisation trail or magnetic curvature. Its presence can sometimes be inferred from interactions, outgoing charged tracks or missing energy and momentum. Absence of a visible trail is not proof that nothing travelled through that region.
Why can a particle track spiral inward?
If a charged particle loses momentum through interactions with the detector material while moving in a uniform magnetic field, its bending radius can decrease. The magnetic force bends the motion but does no work on the particle in this ideal model; the energy loss has another mechanism.
Why does electron–positron pair production need at least 1.022 MeV?
The photon must provide the combined rest energy of the electron and positron, 0.511 MeV each. Any remaining available energy contributes to kinetic energy and recoil, with the full interaction also satisfying momentum conservation. The pair’s kinetic energy does not have to be divided equally.