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Ionization Potentials

Author: J.B. Hoag

Fig. 18 E. A gas-filled diode. (From E. & N. P.)

Fig. 18 F. The effect of gases in a diode. (From E. & N. P.)

Figure 18 E shows a hot-cathode diode filled with a gas such as helium at a pressure of 1 or 2 mms. The current-voltage curve of this tube is shown at 0AP1 in Fig. 18 F. The rise of this curve above that for a highly evacuated diode, OAP0, is due to the creation of charged particles when the electrons from the filament collide with neutral gas atoms. If the pressure in the tube is greater, the amount of this additional current will be greater, as indicated by the curves P2, P5. It will be observed, however, that these curves all rise above AP0 at the same plate voltage. This means that when the electrons are sped up sufficiently, to the voltage V1 they are just able to knock an electron out of an atom. The potential V1 is called the minimum ionizing potential. It gives a measure of the energy needed to remove the most loosely bound electron in the atom. It is therefore numerically equal to the energy which this electron possessed while in a normal undisturbed atom.

Ionizing potentials range between 3.88 and 24.5 volts. The following values are of interest in commercial tubes: mercury vapor, 10.39; argon, 15.7; nitrogen, 16.3; neon, 21.5; helium, 24.5.

The number of atoms ionized per electron collision increases rapidly from a zero value, just below the ionizing potential, to a maximum value at 135 electron volts for mercury vapor, 140 for argon, 175 for nitrogen, 340 for neon, and 210 for helium. For electrons of still greater energy this ionization probability decreases slowly.

Last Update: 2010-11-21