Fano noise


Fano noise is a fluctuation of an electric charge obtained in a detector, arising from processes in the detector.
It was first described by Ugo Fano in 1947, as a fluctuation of amount of ion pairs produced by a charged particle of high energy in a gas. The amount of the ion pairs is proportional to the energy the particle loses in the gas, but with some error - due to the Fano noise. Surprisingly, the noise is usually smaller than a Poisson distribution noise, showing there is an interaction between ionization acts. A Fano factor was introduced to describe it, and the factor is almost independent of the energy measured. Fano expected it to be between 1/3 and 1/2 for gases, for moderate energies of ionizing particles.
The Fano noise applies as well to other processes in which an energy is converted to an electric charge - solid state detectors of charged particles and gamma radiation, and even semiconductor light detectors like image sensors. E.g. it is a limiting factor in the noise characteristics of CCDs and CMOS image sensors. The Fano factor achieveable is an important parameter of the detector material - the smaller it is the better.