Electrons, positrons and photons

The knock-on electron (bottom left) and the lone Compton electrons show that negative particles turn to the left.

There are three linked highlighted examples of high energy photons materialising into pairs in the field of a nucleus. In the order in which it have happened:

• the first photon materialises (nearest the bottom of the picture);
• the second is most likely a bremsstrahlung photon from the of the first pair;
• the Compton electron (on the right of the picture) is caused by a bremsstrahlung photon from the of the second pair;
• the third pair (on the left of the picture) is caused by a bremsstrahlung photon from the of the second pair.

The thick track coming in from the top of the picture (one can tell which way it is going by noticing the knock-on electron) is a cosmic ray, probably a muon. This is a reminder of the link between cosmology and particle physics.

Exercise:

Can you find another clear example of a bremsstrahlung photon materializing into an pair? (Often referred to as pair-production.)