The Detection of Single Positive Ions, Electrons and Photons by a Secondary Electron Multiplier

A secondary electron multiplier tube has been developed and used successfully for counting single positive ions, electrons and photons. This tube has twelve electrodes covered with a thin layer of beryllium. Tests have shown that with 330 volts per stage this tube will multiply the primary current by a factor of 10^5. The use of guard ring insulation and electrostatic shielding for the final collecting electrode has reduced the background current to an extremely low value. The last electrode was connected to the grid of the first tube in a linear amplifier. In order to count single positive ions or electrons the tube was waxed to the end of a simple magnetic spectrograph. Ions having energies from 50 to 20, 000 electron volts and masses from one to 32 were detected. By increasing the gain of the thermionic amplifier electrons could be counted. With still more gain single photons were counted. The multiplier tube has an extremely low background counting rate. Because a pressure of 10^-6 mm of Hg is maintained in the counter, it can be connected directly to the vacuum system through which the electrons or positive ions are accelerated. The absence of a thin window between the counter and the vacuum system should be a great advantage in experiments dealing with low energy positive ions or electrons.