The "Past and Present Lives" of X-ray Testing

Discovery of X-rays
                                                                        

On the night of November 8, 1895, German physicist William Conrad Roentgen discovered a strange phenomenon in his laboratory. The cathode ray tube was wrapped in black paper with only a narrow gap. When connected to a high-voltage current for the experiment, he discovered that a fluorescent screen coated with barium cyanide platinate emitted a faint light green flash 2 meters away. Once the power was cut off, the flash immediately disappeared. For more than a month in the future, the persistent physicist convinced his wife to act as the experimental subject in order to capture the source of this mysterious phenomenon. He placed his hand in front of a fluorescent screen, and the surprising scene was recorded in history. The fluorescent screen clearly displayed the bone in his hand and the ring on his ring finger, which was the first X-ray film in history. In 1901, Roentgen was also named the first Nobel laureate in physics for his discovery of X-rays.

02-X-Ray Generation Principle

X-rays are generated in an X-ray tube (a vacuum tube with cathode and cathode, tungsten wire as the cathode, and a metal target as the anode). A high direct current voltage (tube voltage) is applied between the cathode and anode. When the cathode is heated to an incandescent state, a large number of electrons are released. These electrons are accelerated in a high-voltage electric field, flying from the cathode to the anode (tube current), and eventually collide with the metal target at a high speed, losing their kinetic energy. The vast majority of these kinetic energy is converted into heat energy, with only a small portion being converted into X-rays for radiation around.

The X-rays emitted by X-ray tubes can be divided into two types: one is X-rays with continuous wavelengths, which form a continuous X-ray spectrum similar to visible white light, also known as polychromatic X-rays or white X-rays, commonly used for flaw detection; Another method is to superimpose several spectral lines with a certain wavelength on the basis of a continuous spectrum to form a characteristic X-ray spectrum. It is similar to monochromatic light of visible light, so it is also known as monochromatic X-ray and is commonly used in crystal diffraction.

03 Film Radiographic Testing

The discovery of X-rays was quickly applied in the medical field, pioneering medical imaging technology. X-rays have strong penetrating ability, and when passing through a substance, their intensity is attenuated by the scattering of absorbed nuclei, and the degree of attenuation depends on the attenuation coefficient of the substance and the thickness of the ray penetration. Usually, materials with higher density have a greater degree of attenuation of radiation. Therefore, when radiation passes through the human body, bones with higher density produce greater attenuation compared to organic soft tissues, thus presenting the internal bone structure on the negative film.

In the field of industrial production, X-ray photography technology is widely used for defect detection of metal and non-metallic material parts, especially for internal quality inspection of castings, welds, etc. In the following figure, after the X-ray passes through the stepped test block, the thicker part has a greater attenuation, and the film exposure is low, resulting in a lower blackness on the negative film. The presence of hole defects in the test block results in a local decrease in thickness, increased attenuation, and higher blackness, resulting in defect projection in the negative film (but the thickness location cannot be provided)