Energy resolution: Full width at half maximum of graph of detected counts vs. energy, expressed as a percentage of the energy.
Poisson statistics: Expresses probability in situations of equal probability for an event per unit of time, such as radioactive decay or cosmic-ray appearance. The standard deviation of a mean number of counts is the square root of the mean number of counts, which is a decreasing fraction of the number of counts when expressed as a fraction of the number of counts.
rad: The unit of radiation energy absorption (dose) in matter, defined as the absorption of 100 ergs per gram of irradiated material. The unit is being replaced by the gray, an SI unit, where 1 gray (Gy) = 100 rad.
