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Ever wondered how X-rays work and operate?
X-rays are electromagnetic waves of very high frequency and very short wavelength (0.001nm-10nm). Because of their high frequency and high energy they will penetrate flesh and cause ionisation of atoms they encounter. In 1895, Wilhelm Roentgen was studying cathode rays using a Crookes tube when he noticed a fluorescent screen glowing on a nearby table. Roentgen investigated this further and showed that the fluorescence was due to invisible rays coming from the Crookes tube. These rays were capable of passing through opaque black paper that was wrapped around the tube. Unknowingly, he named them X-rays. Basically X-rays are emitted from a cathode ray tube when the cathode ray tubes strike the glass of the tube. Modern X-ray tubes are mainly used to produce X-rays. * A cathode which is a filament of wire through which a current is passed. Electrons are emitted from the hot filament and a metal focusing cup to help direct the electrons in the right direction towards the anode. An accelerating potential of 25000-250000 volts (high potential difference between anode and cathode) is generated. The anode is usually made of tungsten that can withstand the high temperatures generated. When the electrons strike the tungsten, they are absorbed and some of their energy is converted to X-rays. The face of the anode is set at 45 degrees to the electron beam, ensuring that the X-rays which are emitted perpendicularly to the electron beam direction emerge through the sidewall of the tube, sending it in a predetermined direction. In many cases a thin ‘window’ is put in the tube wall to facilitate the exit of X-rays. When the electrons emitted from the heated filament are accelerated by the high potential difference and strike the tungsten target, the electrons rapidly decelerate and produce X-rays and heat. X-ray production is very inefficient as approximately 99% of the electron’s energy is converted to heat in the anode with only 1% being converted into X-rays. The heat generated in the target per second is enough to heat a cup of water to boiling point in 1 sec so overheating must be avoided. Copper(good conductor of heat is used for anode mountings and oil circulating in the outer region near the anode helps the cooling by convection. Cooling can also occur by rotating the target at a rapid rate of 3600 revolutions per minute, allowing the heat produced to be distributed over a large area. A graph of intensity versus frequency for X-rays show that :The X-rays cover a continuous range of frequencies (referred to as bremsstrahlung, German for braking radiation), For a given accelerating voltage there is a definite maximum frequency which increases as the accelerating voltage increases, ‘Spikes’ appear on top of the continuous spectrum. These are characteristic of the target material and the X-rays are referred to as characteristic X-rays, that is different target materials have different ‘spikes’. They occur when an incident electron displaces an inner shell electron and an outer shell electron falls back to the vacancy and emits a photon of X-radiation.
A 3mm thick aluminium filter absorbs the soft X-rays ensuring that the harder X-rays are used as they produce a clearer image. Absorption of the soft X-rays also prevents them burning the skin of the patient. The high frequency of X-rays makes them able to penetrate deeply into materials including those opaque to light. The degree of penetration depends on the material. High-density materials absorbs X-rays more readily than loew-density materials. Ex, bone absorbs more X-rays than soft tissue and cancerous tissue absorbs differently from healthy tissue.