Single electron detection in proportional gas counters

Abstract

By releasing single electrons with insufficient energy to produce immediate ionization it has proved possible, experimentally, to study individual avalanche distributions in isolation from confusing effects arising from fluctuations in the number of primary ion pairs. The study of the development of avalanches from a single pair of ions is not only of importance for a complete understanding of the detection mechanism in gas-filled proportional counters but also for the detection of soft X-rays, UV radiation and electrons of essentially zero energy. Several experimental studies that have been carried out within the recent years are discussed. Pulse spectra produced by the release of single primary electrons in a proportional counter have been measured for a variety of gas fillings, gas mixtures and anode voltages. A brief survey of different theoretical approaches of avalanche formation in proportional counters is included. For low values of E/ p the pulse spectrum produced by single electrons decreases exponentially with increasing pulse height. At high field and low pressure the distributions have pronounced maxima. Modifications to the theoretical distribution can account for deviations from the exponential form. When secondary processes become significant, avalanche breeding may take place and avalanche chains are detected and the distribution of the electron number in the total charge pulse may be altered.