The secondary electron emission from metals in the low primary energy region

Abstract
This paper describes a new method and apparatus for measuring and analysing the secondary electron emission from non-gaseous materials. It has been devised especially for dealing with secondary emissions caused by primary electrons of very small energy. The essential features include a beam of primary electrons defined and controlled by an electrostatic electron lens system, which directs it towards the centre of a sphere A of the material investigated. A is surrounded by a concentric conducting sphere which collects the secondary electrons emitted from the surface of A . With the arrangement as set up it is possible to measure the contact potential difference between essential elements of the apparatus during the course of the experiments, without moving them or doing anything which would change the nature or composition of their surfaces. The method is applied to the case of pure gets-free copper with primary electrons having energies down to the lowest practicable with a tungsten thermionic source (about 0.35 eV at 2000° K). The distribution of energy is analysed both for the primary and the secondary electrons. It is found to be practically the same for both groups for all energies below a few volts. From this we deduce (1) that for these low-energy electrons the secondary electrons are just reflected electrons, and (2) that the coefficient of reflexion r varies very little with the energy of the electrons. Numerous direct determinations of r have been made. It is shown that no manipulation with fields can ever reduce the mean energy of electrons from a thermionic source below 2 kT , where T is the temperature of the source and k is Boltzmann’s constant. Many determinations of r have been made from a few volts down to 2 kT , and the average value of r is about 0.24. No variation of r has been established with certainty, but there are indications that it drops a little from the value at 2 kT to a minimum at about 2 kT + 0.5 eV, then increases slightly.