Pretty knock-on electron (delta-ray)

Picture from CERN 2-metre hydrogen bubble chamber exposed to a beam of negative kaons ( ) with energy 4.2 GeV. This piece corresponds to about 70 cm in the bubble chamber.

The 12 parallel lines are trails of bubbles – initiated by the ionization of hydrogen by the beam particles, which enter at the bottom of the picture.

Occasionally, by chance, a particle can give an electron more energy than usual. In this case, instead of contributing to the normal string of bubbles, the more energetic electron can itself travel and produces its own trail of bubbles. Electrons (and also anti-electrons, or positrons) are instantly recognizable in the bubble chamber because their tracks spiral; for more details, click here. On this picture there is one spectacular electron spiral and a few smaller ones.

These spiralling knock-on electrons are helpful because they enable us to determine the direction of the magnetic field that is causing the tracks to curve.

As far as the particle physicist is concerned, this picture would not be useful because not one of the beam tracks has collided with a proton.

Questions:

1. How many beam tracks enter the bubble chamber?
2. Try to identify some electrons. What is the direction of the magnetic field?
3. Why do electrons leave curly tracks?
4. Why are all the electron tracks not of the same size?

Answers:

1. 12
2. Into the screen/paper
3. Because accelerated charges lose energy by emitting electromagnetic radiation.
Electrons, being by far the lightest charged particles, respond (accelerate) more to forces exerted on them (both electric from the nuclei, and magnetic from applied field); a=F/m.
4. Chance - it depends on how close the moving charged particle gets, and on how fast it is moving.