In the context of nuclear physics, "Wu" commonly refers to Weisskopf units, a standard used to compare experimental gamma-ray transition rates with theoretical predictions based on the single-particle (shell) model of the nucleus.
Understanding Weisskopf Units (W.u.)
The shell model provides a framework for understanding the energy levels and properties of atomic nuclei. When a nucleus transitions from an excited state to a lower energy state, it often emits a gamma ray. The rate at which this transition occurs is an important characteristic.
Theoretical calculations of these transition rates can be made using the shell model. However, these calculations are approximations. To assess how well the shell model predicts these rates, experimental measurements are compared to theoretical "Weisskopf estimates."
- Weisskopf Estimate: A theoretical estimate of the gamma-ray transition rate based on a simplified single-particle model. These estimates were formulated by Victor Weisskopf.
- Weisskopf Unit (W.u.): The unit of measurement resulting from the Weisskopf estimate. The experimental transition rate is divided by the Weisskopf estimate, and the resulting dimensionless ratio is expressed "in Weisskopf units".
Significance of Comparing to Weisskopf Units
Comparing experimental rates to Weisskopf units provides valuable information about the structure of the nucleus:
- Agreement with the Shell Model: If the experimental rate is close to 1 W.u., it suggests that the transition is well described by the single-particle shell model.
- Deviations from the Shell Model: If the experimental rate deviates significantly from 1 W.u. (either enhanced or hindered), it indicates that more complex nuclear structure effects, such as collective motion or configuration mixing, are playing a significant role in the transition.
Example
Imagine an experiment measures the gamma-ray transition rate of a specific nucleus. The theoretical Weisskopf estimate for that transition is calculated to be 1010 s-1. If the experimental rate is also close to 1010 s-1, the rate would be approximately 1 W.u., suggesting a single-particle nature of the transition. If, however, the experimental rate is found to be 1012 s-1, it would be reported as 100 W.u., suggesting that the transition rate is enhanced by some collective effect. Conversely, a measured rate of 108 s-1 would be 0.01 W.u., indicating a hindrance.
In summary:
Weisskopf units (W.u.) are a standardized way to compare experimental gamma-ray transition rates with theoretical predictions based on the nuclear shell model. They provide a benchmark for understanding nuclear structure effects.
