2.3 Cross-section
Figure 1: Deuterium particle moving towards a stationary Tritium particle
No, Figure 1 doesn't consist of planets or suns or moons or stars. Any guesses? The sphere with the letter D is a Deuterium nuclei and the sphere with the letter T is a Tritium nuclei.
In this example, Deuterium is moving towards (going to collide) Tritium so we take Deuterium as the incident particle and Tritium as the target particle. Incident particles move towards the target particles for collision to occur.
Yes, I know, tritium looks like Saturn, but this ’ring’ is actually a sphere-like field created by the electrostatic force. This sphere-like field is termed as the cross-section and is denoted by σ (sigma).
If incident particles (particles that move towards a target), in this case deuterium, pass through this ’cross-section’ there is sufficient force for fusion to occur, which in theory translates to overcoming the Coulomb barrier. So, the larger the cross section, the more particles can pass through, which means there is a higher probability for fusion to occur. Hence the number of fusion reactions is directly proportional to the cross section.
Let me introduce a term that most of you would be unfamiliar with - number density. Like regular density, it is per unit volume. So number density is the number of particles per unit volume. This would mean if the number density is, let’s say 10, there would be 10 particles per unit volume.
Figure 2: Number Density example
