Model of high current breakdown from cathode field emission in aged wire chambers ∗ +

Observing single electron pulses provides insight into the mechanism that leads to sudden high current jumps (breakdown) in aged wire chambers. This single electron activity is found to be consistent with the Fowler-Nordheim equation for field emission of electrons from a cathode surface in a high electric field. The high electric field arises from the positive ion buildup on a very thin insulating layer on the cathode surface. A model is presented to explain the transient behavior of single electron pulses in response to abrupt changes in chamber ionization, as well as the steady state rate during a long term aging run. The model is based on properties of the insulating layer (dielectric constant, conductivity, and hole-mobility) as well as the Fowler-Nordheim equation. I. INTRODUCTION It is commonly known that drift chambers operating in a high ionization environment while running with certain gas mixtures containing for example hydrocarbons will soon begin to draw current or suffer high current jumps [1] or breakdown that trip the chamber power supply. It is generally believed that breakdown is due to charge build up on a thin insulating dielectric layer on the field wires (cathodes) that reaches a sufficient magnitude to cause a breakdown in the dielectric layer and a release of electrons that provide the current jump. In a previous study [2] it was shown that small pulses (i.e. avalanches from single electrons) were observed to increase in rate before a breakdown occurs in an aged drift chamber test cell. This paper extends those measurements and uses the single electron rate as a probe to understand the mechanism for high