PHY 525
Revised Particle Track Answer Key
Analysis of Particle Tracks
I. Gamma Ray Pair Production (Figure 73-1):
Eugene Van Dette
10/01/02
1. In the figure we can see a neutral photon particle creating matter when it decays. Two new electrons can be seen after photon decay spiraling in opposite directions. (Both mass-energy and momentum is conserved).
2. The spiral curved electron paths indicate the effect of a magnetic field on the electrons in the bubble chamber.
3. The spiraling curves of the electrons in opposite directions indicate that each was of opposite charge in the same magnetic field. (Charge was conserved and their directions of motion conform to the right hand rule).
4. The gamma ray is not seen it is deduced from the direction of motion of the electron pair which was created from the gamma ray photon. (Conservation of momentum).
5. The solid lines of the electron pair indicate that the pair was traveling at sub-light speed immediately after creation. (Conservation of momentum).
6. The inward spiraling curved paths of the electrons in the magnetic field of the bubble chamber show that the electrons lost kinetic energy and were slowing down because of continual mini-collisions in the chamber. (Conservation of momentum).
7. The larger radii of curvature indicate that the e- electron had more momentum because of greater speed mass or both; than the e+ electron produced at the same time.
II. Pions or Pi-Meson Decay (Figure 73-2): There are two types of Pi-Meson decays. Type 1: Pi-Meson = Negative Muon + Neutrino. Type 2: Muon = Electron
+ 2 Neutrinos.
8. The direction of motion is toward the created particles and the electron.
(Conservation of momentum).
9. We know that the electron and the pi-meson both have the same negative charge because both paths are curved in the same direction.
(Conservation of electric charge).

10. The brief direction change which we can see indicates that another unseen neutral particle (a neutrino) was produced in the first decay in addition to the muon which can be seen. (Conservation of momentum).
11. The smaller spiral of decay of the electron shows us that it has less momentum (speed, mass or both) than the muon from which it came. The only way we can account for the larger arc of curvature in the opposite direction for the incoming muon, is to say that other unseen particles must have been liberated in the opposite direction causing the direction change (Conservation of momentum).
12. The curvature of the spiraling indicates a negatively charged muon.
We can see that the electron and the incoming muon both have a clockwise spin in the same magnetic field (Conservation of charge).
13. The muon has a longer half-life than the pi-meson which can be observed by the longer trail it leaves on film. It is also a more stable lower energy particle because it was produced in the second decay.
III. K-Lambda Associated Production (Figure 73-3): Seen as tracks left by a beam of protons in a liquid Hydrogen bubble chamber.
14. The incoming proton collided with a hydrogen molecule/ hydrogen nucleolus.
15. The straight proton paths are due to the high velocities and /or large masses of the protons. (Large radii of curvature).
16. We know that the K and Lambda particles have a neutral charge because they did not leave tracks in the bubble chamber. (Conservation of charge).
17. The products produced indicate the presence of the unseen K and
Lambda particles from which they were produced. (Conservation of momentum).
18. The proton and the pi-minus meson produced in decay must have opposite but equal charges because charge is always conserved.
19. Both the K and Lambda particles have short lifetimes. This is seen by the relatively short gap between the proton collision which produced the K and
Lambda particles, and the trails of the new particles which they created (A proton-negative pi-meson pair and a positive-negative pi-meson pair).
Lehrman (1968). Analysis of particle tracks. Scientific Experiments in Physics,
New York, Holt, Rinehart and Wilson.