Tech Help

WHAT IS GOING ON INSIDE THAT X-RAY TUBE?

HEAT, that is the key word.  Only 1% of all the applied energy is converted into x-rays, and 99% of applied energy is converted into heat.  Filament temperature can reach 2,600 degrees Celsius (4,682 F), just to give you a feel for how much heat is produced.  The temperature on the Anode reaches 2,000 degrees Celsius (3,632 F).  To give you a  better appreciation of these temperatures, lava is only around 700-900 degrees Celsius (1,292 - 1,652 F) - considerably cooler than what the insert’s components are subjected to.

The cathode, anode, bearings and the envelope have to quickly change from relatively cool temperatures to extremely high temperatures many thousands of times during their lifetime.  Meanwhile, the surface of the tube housing cannot exceed 80 degrees Celsius, or 175 degrees Fahrenheit.  The materials that can withstand such extremes are rare indeed, and a minor miracle that x-ray tubes work at all.

The PROBLEM is getting as much heat as possible away from all the components of the insert.  How do the manufacturers solve this problem? First, the accumulated heat energy in the anode is dissipated by way of radiation. Second, the anode radiation warms the envelope and the oil surrounding it which conducts  heat away from the components inside the envelope. Then, the heat in the oil is conducted into the housing. Third, when the housing cannot radiate enough of the heat away, a fan blowing air over the housing may be used. A heat exchanger option also may be used to carry the heat away from the envelope. The heat exchanger is kept cool by a fan system, chilled water or oil.

X-RAYS, after all, is what it’s all about. How do they occur?  When a stream of very fast, high-energy electrons strike the Tungsten electrode (anode), the electrons are slowed down, and some even penetrate into the metal.  The sudden braking of the electrons produces an electromagnetic radiation of very short wavelength called X-rays or Roentgen rays.  This radiation is generated by electrons penetrating the tungsten and interacting and colliding with the tungsten atoms. This produces well-defined wavelengths which are characteristic of the structure of the tungsten atom.

As a general rule, X-ray tubes are constructed such that a stream of electrons are directed against the anode or target.  The collision of the electrons against the anode causes the latter to become very hot - so hot that some sort of cooling is necessary in modern-day applications. The most effective (and common) method of directly cooling the anode is to spin or rotate it as it is bombarded, thus spreading the heat over a larger area.