X-RAY THEORY

In a high resolution x-ray source, electrons are emitted from a cathode and accelerated to speeds close to the speed of light. These electrons are then focused by a magnetic lens on to a very small spot on a metallic target.

On impact with the atoms in the target, the electrons lose energy through a series of glancing collisions. A small percentage of these interactions produce X-rays, the vast majority produce heat that must be removed from the target material.

There are two x-ray production mechanisms:

• A high energy electron is decelerated by the electrons orbiting an atom in the target. This deceleration produces radiation in the form of X-rays. This process is is called Bremsstrahlung - translated from German as braking radiation. This is the dominant X-ray production process and produces a wide spectrum of X-ray energies up to the accelerating voltage.
• A high energy electron can eject electrons from a target atom. The resulting atom is unstable as it has a "hole" in one of its inner electron orbits. In order to return to a stable state the atom shuffles its electrons to fill the hole and releases an X-ray. This process is called characteristic X-ray emission because the energy of the resulting X-rays depends upon the target material.

In our x-ray sources, electrons emitted from a fine wire are accelerated by up to 255 thousand volts. These high-energy electrons are then focused by a magnetic lens on to a spot on a metallic target, as shown:

1. filament
2. electron beam
3. magnetic lens
4. target

Obtaining a sharp, high-resolution real time x-ray image requires a very small spot size. If the x-ray spot size is too large, unsharp regions are created in the x-ray image, as shown:

Moving the sample closer to the x-ray source increases the magnification of the resulting image: