Signal variance in gamma-ray detectors—A review

Signal variance in gamma-ray detector materials is reviewed with an emphasis on intrinsic variance. Phenomenological models of electron cascades are examined and the Fano factor (F) is discussed in detail. In semiconductors, F is much smaller than unity and charge carrier production is nearly proportional to energy. Based on a fit to a number of semiconductors and insulators, a new relationship between the average energy for electron–hole pair production and band gap energy is proposed. In scintillators, the resolution is governed mainly by photoelectron statistics and proportionality of light yield with respect to energy.