Episode 519: Particle detectors


  • Discussion: The idea of particle detectors (10 minutes)
  • Demonstration: Cloud chamber (and spark detector) (15 minutes)
  • Discussion: Explaining tracks (10 minutes)
  • Student questions: Interpreting tracks (20 minutes)
  • Particle tracks: Try analyzing some particle tracks (time permitting)
Miniature cloud chamber

Discussion: The idea of particle detectors
What particle detectors do you know of? (Spark Counters, Geiger counters.) What do these tell us? (They count particles of ionizing radiation.)

What else might we want to know about particles? How could we tell which particle we have detected? (Need to know a range of properties: e.g. mass/energy, electric charge, momentum, lifetime, etc.)

In general, detectors work by analyzing particle collisions using conservation laws (momentum, energy, charge).

Demonstration: Cloud chamber (and spark detector)
Cloud and bubble chambers make visible the invisible: alpha diameter ~ 10-14 m gives a visible track 0.1 mm wide, a factor of 1010 increase in size!

If you have access to a spark detector, you could also demonstrate this at this point.

You may have demonstrated a spark counter before?

Episode 509-4: Rays make ions (Word, 71 KB)

Episode 518-2: Electrical breakdown (Word, 61 KB)

Episode 519-1: Range of alpha particles with a cloud chamber (Word, 43 KB)

Discussion: Explaining tracks
Students may be familiar with the patterns made by particles in detectors, and so you could discuss the basic ideas behind analysing the tracks. The length of track is related to the energy of the particle, and also to its lifetime if it decays with a very short half life. Magnetic fields deflect charged particles and so bend their tracks. The curvature depends on momentum, charge and the strength of the field.

How could you tell whether a particle had positive or negative charge? (Curving to left or right; Fleming’s rule.) If a track is a spiral, what does this tell you about the particle’s motion? (It is slowing down; charged particles radiate as they are accelerated, so they slow down.

Episode 519-2: Measuring the momentum of moving charged particles (Word, 42 KB)

Student questions: Interpreting tracks
Students can apply their knowledge to the interpretation of tracks from a bubble chamber.

Episode 413-6: Charged particles moving in a magnetic field (Word, 65 KB)

Particle tracks
If time permits try analysing some particle tracks on the Theoretical Physics Group, University of Manchester website.

Episode 519-3: Particle tracks (Word, 612 KB)

Download this episode
Episode 519: Particle detectors (Word, 690 KB)