Soft X-Rays from Iron

Abstract

Critical potentials of iron to 900 volts, determined photo-electrically._____Soft x-radiation from a polished iron target bombarded by electrons from a filament heated by a current constant to.01 per cent, produced a photo-electric current from a platinum semi-cylinder which was so placed that all the radiation from the target fell on it and which was protected from stray electricity by a system of radially disposed nickel vanes charged alternately positively and negatively. A vacuum of ${10}^{\ensuremath{-}6}$mm pressure or better was maintained. When the ratio of the photo-electric current to the thermionic bombarding current was plotted as a function of the exciting voltage a number of breaks appeared. From the similarity of the curves thus obtained to those obtained with gases, breaks at radiation potentials were distinguished from those at ionization potentials. The fact that radiation excited by 2.3 volt electrons produced a photo-electric current, can be explained by taking Einstein's complete equation $hv=e(V+\ensuremath{\Phi})$ where $\ensuremath{\Phi}$ is the work function of iron whose value is calculated to be 5 volts. Correcting for this, about 23 radiation potentials were located at 7.3, 14.1, 16.5, 24.3, 28.8, 34.3, 48, etc. volts, and 9 ionization potentials at 11.1, 19.4, 41.2, etc., volts. The ionization potentials at 818.5, 704.3 and 54.6 volts are identified with the lines, ${\mathrm{L}}_{\mathrm{I}}$, ${\mathrm{L}}_{\mathrm{III}}$, and ${\mathrm{M}}_{\mathrm{III}}$.