Technology
What is Real-Time Micro-Focal X-ray?
Traditional film or film replacement technology X-ray systems rely on a "contact print". This simply means the sample is placed in direct contact with the detector, which in most cases is radiographic film. When this combination is illuminated with X-rays the sample shades the film from the X-rays leaving a one to one scale image on the film.
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.
Real-Time Micro-Focal X-ray systems differ from traditional X-rays systems in two fundamental ways:
- Detector
- X-ray Source
Detector
The detector is the key to the unique real-time aspect of these X-ray systems. Samples may be viewed live under X-ray luminescence, whilst being re-positioned for optimum viewing. Most real-time systems employ an X-ray image intensifier as the detector.
Above is a representation of an image intensifier, as used in real-time X-ray systems. The camera may be either analogue outputting composite video or digital using a digitiser or firewire port.
X-ray Source
A Micro-Focal X-ray source differs from conventional X-ray sources because of its ability to tightly control the emissive area - this is known as the 'focal spot size'. This is represented in the drawings below:
It is this tightly controlled emissive area that gives the Micro-Focal X-ray source its ability to produce high-resolution magnified images of the sample. This is referred to as 'Geometric Magnification' and is demonstrated by the illustration below:
The more the 'focal spot' tends toward a point source rather than a finite area the more resolute is the shadow cast by an object illuminated from this source.
The position of the sample relative to the X-ray source and the detector determine the geometric magnification.
