Mechanism of Secondary Emission and Single-Particle Statistics from Low-Density Films of Alkali Halides

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Abstract
The mechanism of field‐enhanced secondary emission has been studied experimentally in detail for KCl low‐density transmission dynodes transferred in a dry nitrogen atmosphere. By means of a Kelvin probe located at the center of a collector, dynode surface potentials and their relationship with collector potential, secondary yield, primary energy, and current density have been studied. A unique relationship between yield and surface potential has been found, independent of all other parameters for current densities between 1 and 10 nA/cm2. This study clearly shows that an avalanche process, together with an increase in escape length, is responsible for the high yields obtainable. A study of transient effects reveals that the high surface potential needed for high yield decays rapidly after the primary beam is turned off. A smaller surface potential, with a long decay time constant, can be expected to provide slight field enhancement for single particle bombardment at low rates. The statistical distributions of the number of emitted secondary electrons per primary incident at high (1‐MeV) and low (9.5‐keV) energies have been measured for low‐density films of CsI and KCl at low count rates. The results do not differ very much from those obtained previously from normal density films except that the yields are somewhat higher, particularly after the film has been charged with a dense primary beam. The distributions obtained are non‐Poissonian in nature (more nearly exponential), and this is explained by their similarity to normal density films.