In the context of neutron flux, what does η represent?

In the context of neutron flux, η represents the ratio of fission neutrons to absorbed neutrons. This parameter is crucial for understanding how effectively a nuclear reactor can maintain a self-sustaining chain reaction. The fission process releases neutrons, and those neutrons can either go on to induce further fission events or be absorbed by non-fissile materials (such as reactor components or control rods).

When η is greater than one, it indicates that there are more fission neutrons being produced than are being absorbed, which suggests that the reactor can achieve a sustained chain reaction. Conversely, if η is less than one, it implies that too many neutrons are being absorbed, which could lead to a decrease in the number of fission reactions occurring, potentially resulting in the reactor shutting down.

Understanding this ratio is vital because it directly impacts the efficiency and operational characteristics of a nuclear reactor, influencing factors such as power output and neutron economy. Thus, η is a fundamental concept in reactor physics and criticality safety.