by Vladimir BUSHUYEV, Dr. Sc. (Phys. & Math.), Physics Department, Lomonosov Moscow State University
In 1991 Russian scientists developed and patented a new method of producing X-ray images - roentgenograms - of weakly absorbing objects, including medico-biological ones. Since the discovery of X-rays by the German physicist, Prof. Wilhelm Roentgen, in 1895, they have found a whole range of applications in various fields of science and technology, including many uses in clinical practice. In this latter case we use the ability of X-ray to pass through materials and substances while being partially absorbed by them. As a result, doctors can, not only examine the inner organs of a patient, but take photographs of them.
X-rays, we know are hard electromagnetic radiation with a wavelength measuring about 1 A (10 -8 cm). Their most common source is the X-ray tube-a device consisting of an elongated glass vessel of 30-40 cm in length from which air is evacuated. At one end of this tube there is filament (cathode), and on the opposite end - a massive anode, made of some high- melting metal (copper, molybdenum, silver, tungsten). The voltage applied depends on the material and varies from 30 to 100,000 V and more. The electrons emitted by the cathode are accelerated by the strong magnetic field between the electrodes and bombard the anode at near-light velocity. The atoms of the latter are excitated, producing X-rays (consisting of what is known as bremsstrahlung with a broad spectrum and characteristic radiation - with a narrow one) which escapes from the tube through a special aperture, or window.
During radioscopy of an object under investigation the intensity of X-rays is decreased, being transformed into the inner energy of the irradiated material through the excitation and ionization of its atoms and molecules. The degree of damping of radiation depends on the absorption factor and thickness of
Typical diagram of absorption r ... Read more