On the Role of Polyatomic Gases in Fast Counters

The polyatomic gas serves a twofold purpose: (1) to quench ultraviolet photons and (2) to quench secondary emission by positive ions reaching the cathode. The characteristic property of a polyatomic molecule which is of importance for the operation of a fast counter is the large probability of predissociation from excited electronic states. The quenching of ultraviolet photons in the initial avalanche occurs through photo-decomposition of the polyatomic gas. In a fast counter containing a self-quenching mixture the polyatomic gas has invariably a lower ionization potential than the monatomic constituent. The positive ions make ${10}^5$ collisions in crossing the counter and the electron transfer probability ensures that the positive ion sheath, when it reaches the cathode, is composed entirely of polyatomic ions. These are neutralized at about ${10}^{-7\mathrm{}}$ cm from the wall and the excited neutral molecules predissociate in ${10}^{-13\mathrm{}}$ sec. before they can liberate a secondary electron by an inelastic collision with the wall. With no supply of secondary electrons the discharge terminates. Other features of fast counter behavior are discussed and some of the conclusions reached have been checked experimentally.