Exploring Crookes Dark Space in Low-Pressure Gases

[mahblah]

Homework Statement

From the book: "When electrodes are placed in a gas at normal atmospheric pressure no current passes and the gas act as an insulator until the electric field is increased to above 3 or 4 MV/m when sparking takes place. In crontast, at lower pressures, a steady current can be mantained in a gas. At pressures of about 1mm of mercury, the discharge is accompanied by the enmision of light, but at still lower pressures a dark region forms near the cathode. The dark region, called the Crookes dark space, increases in size as the pressure falls, filling the discharge tube at pressures of 0.001 mm and below. If under these low-pressure conditions, a small hole is made in the anode, a green glow is observed on the glass wall of the discharge tube."

The Attempt at a Solution

I'm not sure about mine explanation of the phenomena:

if you have a great ddp, you can see light from cathode to anode becouse the electrons hits gas particle and ionize them. When the ddp falls the electron has not enough energy.. so you can only see a ray outside of the tube because they want to loose their momentum by releasing photons?sorry form my bad english and thanks all
Mahblah.

 

[EWH]

If ddp= pressure, then yes, at moderate pressures the gas is ionized by the electrons. At low pressures there is less likelihood of hitting a gas molecule, and there is no glow in the evacuated region. The electrons still have enough energy, they just aren't hitting much gas.

(For a given electron energy (voltage) and gas density, there is an approximate distance that electrons will travel (statistically speaking) and most of the energy will be deposited at around that distance. The effect is much more pronounced with protons. See http://en.wikipedia.org/wiki/Particle_therapy . In gasses once the ionization is started there is a positive feedback loop - ionization increases plasma conductivity, which in turn makes the plasma more likely to interact with the electrons, so you get a certain distance where the ionization seems to suddenly start. Also plasma has very complex self-organizing properties which lead to the formation of layers and other structures in the plasma.)

With a hole in the anode, some electrons fly through the hole and continue until they hit the glass, where they excite impurities in the glass (or phosphors in a regular CRT), causing a glow. (And some x-rays, too from bremsstrahlung.)