A comparative study of the excitation of soft X-Rays from single crystal surfaces and from Polycrystalline surfaces Graphite and Aluminium

Abstract

These experiments originated in an attempt to improve and extend those of Richardson and Chalklinf on the soft X-rays generated by a tungsten surface deposited on carbon in a vacuum. The results obtained with the carbon alone, however, were so very much more complicated than those found by Richardson and Chalkin that an attempt had first to be made to arrive at some understanding of these results before the effect of depositing tungsten on the carbon could be investigated. The method consisted in bombarding the carbon target with electrons from a glowing filament and measuring the photoelectric current produced when the soft X-rays generated by this bombardment fell on a nickel plate. The photoelectric current per unit thermionic current is then plotted against the bombarding voltage as this is increased in small steps and discontinuous changes of slope in the resulting curves are observed. 2. Apparatus and Method. The experimental tube was of quartz and similar in design to that used by Richardson and Chalklin. It was connected through three liquid air traps, one containing coconut charcoal, to a three-stage Gaede diffusion pump backed by a box oil pump and a Geryk oil pump. With this arrangement no difficulty was found in quickly obtaining and maintaining pressures which were certainly less than 10 -6 mm. and probably of the order of 10 -8 mm. The cylindrical shield, condenser plates, photoelectric plate and box were made of nickel and the cathode consisted of about three turns of 2 amp. tungsten wire. In Richardson and Chalklin’s experiments a revolving type of target was used so that one face might be bombarded during the degassing process and the other used for the measurements, but it was found impossible to restrict the deposition of matter during bombardment to the side of the target facing the cathode and hence, in this apparatus, the target used was of another form in order to avoid this difficulty. The first type of target used is shown in fig. 1. It is made in two halves, one of which can be rotated, by means of an external magnet operating on the soft iron studs a , so that it completely covers the other half during the degassing bombardment. After this is complete the target is opened out and the experiments made on the clean inner surface. This target was made from gas carbon which, though rather brittle, is fairly hard. It was thought that some of the effects obtained with this target, although it was known to be fairly pure, might be due to impurities, so another target was made from spectroscopically pure carbon kindly supplied by Dr. R. C. Johnson. This carbon was so very much softer than the other that it was found impossible to make it in the previous hinged form and another device had to be, adopted. This second target is shown in fig. 2. The whole tube was rotated through 90° so that the target arm was horizontal. The target was again made in two halves and one half could be slid over the other to cover it during the degassing process.