Electrical and Luminescent Properties of Willemite Under Electron Bombardment

An insulated surface uniformly bombarded with electrons of a given energy attains a potential relative to other elements in a high vacuum tube such that exactly as many electrons leave the surface by secondary emission as there are primary electrons arriving. A tube and circuit have been constructed for determining accurately this potential for willemite surfaces while being bombarded by electrons of energy range up to 10,000 volts. It was found that the surface potential was only slightly negative with respect to the most positive anode of the tube over the lower range of energy but finally reached an upper limit of potential relative to the cathode usually between 6000 and 7000 volts. The resistance between wires covered by the willemite was found to be independent of the bombarding current and voltage except in so far as this bombardment changed the temperature of the material. A special potassium phototube is described which was used to determine the light output from the bombarded surface as a function of the current density and the electron energy. The light was found to be accurately proportional to the current for densities below 2.5×10−6 amp. per sq. cm but above this a saturation effect set in quite gradually so that with a current of 20×10−6 amp. per sq. cm the light output was only 65 percent of the value which it would have been had the light intensity per unit current density remained constant. The light output for a constant current was found to increase with (V – V0)2 where V0 is the ``dead voltage'' when the bombarding energy was less than that required to penetrate through the individual grains of the willemite. An experimental nine-inch cathode-ray tube was also studied for screen potential and light output as a function of current density and anode voltage. The difference in potential between the screen and the anode was again small for the lower range in voltage but with anode voltages above 7000 volts, the apparent screen potential depended on the current density so that a change of potential of the screen of as much as 1500 volts was observed as the current density was increased from one to ten milliamperes per sq. cm. The light output increased with the square of the bombarding potential up to 1500 volts but above this it increased with the 1.2 power of the voltage. The light output per unit current density observed at 10−2 amp. per sq. cm was only two percent of that observed at 10−6 amp. per sq. cm on account of the severe ``saturation'' effect found at high current densities.