The ionisation of helium by neutral helium atoms

A method is described for investigating the ionisation produced in helium by collisions between quasi-stationary atoms of helium - that is, atoms possessing only thermal velocities - and a beam of neutral helium atoms with kinetic energy less than one hundred electron volts. Ionisation is shown to begin when the kinetic energy of the impinging particle is twice as great as the minimum energy which an electron must possess in order to ionise helium. This value is lower than any which have been reported previously, and agrees with that predicted by a simple theory of collision, in which the impacting atoms are regarded as perfectly elastic spheres. Electrons resulting from the bombardment of metal surfaces by helium atoms were detected, and methods are described by means of which they were distinguished from those produced as a result of ionisation of the gas by atomic impact. Approximate values are obtained for the coefficient of liberation of electrons from nickel, by normally incident helium ions,and for the variation of the secondary electron emission from the same target, when the kinetic energy of an impacting beam of neutral helium atoms is increased from 42 to 70 electron volts. The theoretical implications of the results are discussed. The sensitivity of the apparatus used in this research iscompared with that of the arrangements employed by Varney* and by Rostagni** in similar investigations.Reasons are suggested for the complete failure of the former author to detect ionisation in helium,with impacting helium atoms of kinetic energy up to 400 electron volts, and for the high value reported by Rostagni for the critical energy for ionisation.