Secondary Emission from Metals Due to Bombardment of High Speed Positive Ions

A beam of ${\mathrm{K}}^{+}$ ions from the iron catalyst source discovered by Kunsman was collected in a Faraday cylinder. A target could be interposed at the mouth of the cylinder by a magnetic control. The difference in current measured in these two cases gave the amount of secondary emission. A transverse magnetic field could be applied at the target to prevent emission of electrons and thus separate electron emission from positive ion reflection. The percentage secondary emission due to bombardment of positive ions having speeds up to 1000 volts has been found for three metals; viz., aluminum, nickel, and molybdenum under a variety of surface conditions. Heat treatment in general reduced the secondary electron emission. The secondary electron emission could not be detected (was less than 0.5%) at positive ion velocities less than 200 volts for Al, 300 volts for Ni, and 600 volts for Mo after heat treatment. The secondary emission increased from these values to 7.0% for Al, 4.2% for Ni, and 3.8% for Mo at 1000 volts. Without heat treatment the emission was detected at lower voltages and reached about double the above values at 1000 volts. The secondary electrons emitted were of low speed, a retarding field of a fraction of a volt was enough to stop nearly all of them. In the cases of molybdenum and nickel positive ion reflection did not exceed 2%, and was undetectable in the case of aluminum.