POISSON
X-ray production is a random process, where each repeated measurement represents a sample of the same specimen volume (for a homogeneous specimen). The expected distribution can be described by Poisson statistics, which for large number of counts is closely approximated by the "normal" Gaussian distribution ("bell curve"). For these distributions, 1 sigma = square root of the counts, and 68.3% of the sampled counts should fall within ±1 sigma, 95.4% within ±2 sigma, and 99.7 within ±3 sigma.