Particle Accelerators, Detectors and Cross-Sections · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Smashing particles to see the small

Science · Quantum & Modern Physics · ages 20-21
Name ______________________   Date ____________

  1. A conventional cyclotron uses fixed magnets and a timed accelerating gap. Which describes its particle path?

    • A widening spiral with two accelerating gap crossings per lap
    • A fixed-radius ring with two accelerating gap crossings per lap
    • A widening spiral with energy gain supplied by the magnetic field

  2. Why use high-energy accelerator probes to study quark-scale structure rather than improve an optical microscope?

    • Higher beam intensity alone resolves detail even at unchanged wavelength
    • Only larger lenses can resolve quarks without changing the probe wavelength
    • Shorter wavelength needs higher momentum and energy for the same probe

  3. For a visibly curved charged-particle track, known field and travel directions let you infer charge sign from bend direction.

    Circle one:   True   False

  4. A charged track curves in a known uniform magnetic field, but its charge magnitude is unknown. What does its transverse curvature determine?

    • The full momentum, even if its direction along the field is unknown
    • The transverse momentum divided by charge magnitude, p_perp/|q|
    • The transverse momentum alone, without needing the charge magnitude

  5. Protons (charge +e) gain 800 MeV of kinetic energy in a linac with 2000 accelerating gaps. Neglect losses. What average effective accelerating voltage per gap is needed, in MV?

    Answer: ______________

  6. How can detector measurements help identify a passing charged particle?

    • Read momentum alone; every pair of particles with equal momentum must have equal mass
    • Use bend direction alone; every particle species has a different sign of electric charge
    • Compare energy loss, flight time and Cherenkov light with particle predictions

  7. A thin target receives a uniform beam flux. The flux doubles while target count and beam energy stay fixed. Neglect beam attenuation. What happens to the interaction event rate?

    • It stays fixed because the interaction cross-section has not changed at this energy
    • It doubles because rate is proportional to flux under these conditions
    • It halves because doubling the beam flux divides the target nuclei between the particles

  8. A student says detectors film the collision itself. Which description matches how charged-particle tracks are reconstructed?

    • Fit a path through hits recorded in several sensor layers
    • Follow light reflected from each particle with a camera
    • Read a particle's trajectory directly from the beam clock
LightMySky · lightmysky.comW1-mt_euJtxtxXyV-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Smashing particles to see the small W1-mt_euJtxtxXyV-s1

  1. A widening spiral with two accelerating gap crossings per lap · The electric gap adds energy twice per lap. Fixed magnets steer the particle, and its orbit grows as momentum increases.
  2. Shorter wavelength needs higher momentum and energy for the same probe · Finer resolution needs a shorter probe wavelength. For the same probe particle, that means higher momentum and energy.
  3. True · With field and travel directions known, opposite charge signs produce opposite bending directions.
  4. The transverse momentum divided by charge magnitude, p_perp/|q| · The transverse curvature radius gives p_perp/|q| = B r. Charge magnitude is still needed to obtain transverse momentum.
  5. 0.4 · The average gain is 800/2000 = 0.4 MeV per gap. For charge magnitude e, this requires an average effective voltage of 0.4 MV.
  6. Compare energy loss, flight time and Cherenkov light with particle predictions · Several detector measurements can be compared with predictions for candidate species. They help distinguish particles without guaranteeing a unique answer in every case.
  7. It doubles because rate is proportional to flux under these conditions · For a thin target, R = F N sigma. With N and sigma fixed, replacing F by 2F gives twice the interaction rate.
  8. Fit a path through hits recorded in several sensor layers · Tracking sensors record hit positions. A fitted path through those hits reconstructs the outgoing charged-particle track.
Worksheet · LightMySky