Xray Production - Xray Tube

Xray Production

How Xray Produce?

In this section will include a close look at a number of pieces of xray equipment. To better appreciate their purposes, it will be helpful to understand how xray are produced. There are 4 basic requirements for the production of xrays:

• A vacuum
• A source of electrons
• A target for the electrons
• A high potential difference (voltage) between the electron source and the target.

The container of the vacuum is the xray tube itself, sometimes referred to as a glass envelope. It is made of borosilicate glass (PYREX) to withstand heat and is fitted on both ends with connection for the electrical supply. All of the air is removed from the tube so that gas molecules will not interfere with the process of xray production.

Coolidge Tube: simplified understanding of xray production

Source of Xray

The source of electron is a wire filament at the electrically negative cathode end of the tube. It is made of the element tungsten, a large atom with 74 electrons orbiting around its nucleus. An electric current flows through the filament to heat it. This speeds up the movement of the electron and increases their distance from the nucleus. Electrons in the outermost orbital shells get so far from the nucleus that they are no longer in orbit but are instead flung out of the atom, forming an electron cloud around the filament. These free electron, called a space charge, provide the needed electrons for xray production.

Target

The target is at the electrically positive anode end of the tube, the end opposite the filament. The smooth, hard surface of the target is the site to which the electrons travels, and is the place where the xrays are generated. The target is also made of tungsten, which has a high melting point to withstand the heat produced at the anode during xray exposure.

High Voltage Transformer

The voltage required for xray production is provided by a high voltage transformer. The two ends of the xray tube are connected in the transformer circuit so that during an exposure, the filament or cathode end is negative and the target or anode end is positive. The high positive electrical potential at the target attract the negatively charge electrons of the electron cloud, which move rapidly across the tube, forming an electron stream. When these fast moving electrons collide with the target, the kinetic energy of their motion must be converted into a different form of energy. The great majority of this kinetic energy is converted into heat (>99%), but a small amount is converted into the energy form that we know as xrays.