X-ray Tube - Crookes Tube

Crookes Tube

Historically, X-rays were discovered radiating from experimental discharge tubes called Crookes tubes invented by British physicist William Crookes and others. As the medical and other uses of X-rays became apparent, workshops began to manufacture specialized Crookes tubes to produce X-rays. These were the first X-ray tubes. These first generation cold cathode or Crookes X-ray tubes were used until the 1920s.

Crookes tubes generated the electrons needed to create X-rays by ionization of the residual air in the tube, instead of a heated filament, so they were partially but not completely evacuated. They consisted of a glass bulb with around 10−6 to 5×10−8 atmospheric pressure of air (0.1 to 0.005 Pa). An aluminum cathode plate at one end of the tube created a beam of electrons, which struck a platinum anode target at the center generating X-rays. The anode surface was angled so that the X-rays would radiate through the side of the tube. The cathode was concave so that the electrons were focused on a small (~1 mm) spot on the anode, approximating a point source of X-rays, which resulted in sharper images. The tube had a third electrode, an anticathode connected to the anode. It improved the X-ray output, but the method by which it achieved this is not understood. A more common arrangement used a cupped plate anticathode (similar in construction to the cathode) in line with the anode such that the anode was between the cathode and the anticathode.

To operate, a DC voltage of a few kilovolts to as much as 100 kV was applied between the anodes and the cathode, usually generated by an induction coil, or for larger tubes, an electrostatic machine. This created and then accelerated a small number of ions from the low pressure gas in the tube. These struck further gas atoms, knocking electrons off them, creating more positive ions in a chain reaction. All the positive ions were attracted to the cathode. When they struck it, they knocked electrons out of the metal, which were accelerated along with the electrons knocked from the gas atoms toward the anode target. When these high speed electrons struck the atoms of the anode, they created X-rays by one of two processes, either Bremsstrahlung or X-ray fluorescence.

Crookes tubes were unreliable. As time passed, the residual air would be absorbed by the walls of the tube, reducing the pressure. This increased the voltage across the tube, generating 'harder' X-rays, until eventually the tube stopped working. To prevent this, 'softener' devices were used (see picture). A small tube attached to the side of the main tube contained a mica sleeve or chemical that released a small amount of gas when heated, restoring the correct pressure.

The glass envelope of the tube would blacken in use due to the X-rays affecting its structure.