Compared with X-rays, sunlight cannot penetrate the human body. Why? And why can't x-rays penetrate bones?

1895 german physicist roentgen (W.C.R? Ntgen) When studying the gas discharge phenomenon in cathode-ray tubes, a sealed glass tube with two metal electrodes (one is called anode and the other is called cathode) is used, tens of thousands of volts of high-voltage electricity is applied at both ends of the electrodes, and the air in the glass tube is pumped out by an air extractor. In order to cover the light during high voltage discharge (an arc lamp), the glass tube is covered with a layer of black cardboard. When he was doing this experiment in a dark room, he happened to find a piece of cardboard soaked in barium platinum cyanide solution emitting bright fluorescence two meters away from the glass tube. Further experiments show that cardboard, wooden boards, clothes and books with a thickness of about 2000 pages can't stop this fluorescence. Even more surprising, when I reached for this fluorescent cardboard with my hand, I saw an image of a hand bone on it.

At that time, Roentgen decided that it was a kind of invisible ray, but it could penetrate the object. Because we can't explain its principle and don't know its nature, we borrowed the unknown "X" in mathematics as the code name, which is called "X-ray" (or X-ray or X-ray for short). This is the discovery of X-ray and the origin of its name. This name has continued to this day. To commemorate this great discovery of Roentgen, later generations named it Roentgen Ray.

The discovery of X-ray is of great significance in human history, which has opened up a brand-new road for natural science and medicine. For this reason, Roentgen won the first Nobel Prize in Physics at 190 1.

Science is always developing. After repeated practice and research by Roentgen and scientists all over the world, the essence of X-ray is gradually revealed, and it is proved that it is an electromagnetic wave with extremely short wavelength and great energy. Its wavelength is shorter than that of visible light (about 0.00 1 ~ 100 nm, and the wavelength of X-rays used in medicine is about 0.00 1. 0. 1 nm), its photon energy is tens of thousands to hundreds of thousands times greater than that of visible light. Therefore, in addition to the general properties of visible light, X-rays have their own characteristics.

Second, the nature of X-rays.

physical effect

1. Penetration refers to the ability of X-rays to pass through a substance without being absorbed. X-rays can penetrate substances that ordinary visible light cannot penetrate. Because visible light waves are long, their energy is very small. When it touches an object, part of it is reflected, most of it is absorbed by matter, and it cannot penetrate the object. On the other hand, X-rays have short wave length and high energy. When they shine on matter, only a part of them are absorbed by matter, and most of them pass through atomic gaps, showing strong penetration ability. The ability of X-rays to penetrate matter is related to the energy of X-ray photons. The shorter the wavelength of X-ray, the greater the photon energy and the stronger the penetrating power. X-ray penetration is also related to the density of matter. Substances with high density absorb more X-rays and transmit less. Low density has less absorption and more penetration. Using the characteristics of differential absorption, soft tissues such as bones, muscles and fat with different densities can be distinguished. This is the physical basis of X-ray fluoroscopy and photography.

2. When the ionized substance is irradiated by X-rays, the electrons outside the nucleus leave the atomic orbit, which is called ionization. In the process of photoelectric effect and scattering, the process in which photoelectrons and recoil electrons leave their atoms is called primary ionization, and the process in which these photoelectrons or recoil electrons collide with other atoms in the process of traveling is called secondary ionization. In solids and liquids. Ionized positive and negative ions will recombine quickly and are not easy to collect. The forgotten charge in gas is easy to collect, and the X-ray exposure can be measured by ionizing charge: the X-ray measuring instrument is made according to this principle. Due to ionization, gas can conduct electricity; Some substances can react chemically; Various biological effects can be induced in organisms. Ionization is the basis of X-ray injury and treatment.

Fluorescence is invisible because the wavelength of X-ray is very short. However, when it irradiates some compounds such as phosphorus, platinum barium cyanide, zinc cadmium sulfide, calcium tungstate and so on. The atom is excited by ionization or excitation. When the atom returns to the ground state, it radiates visible light or ultraviolet light due to the energy level transition of valence electrons. This is fluorescence. X-rays make substances fluoresce, which is called fluorescence. The intensity of fluorescence is directly proportional to the number of x-rays. This function is the basis of X-ray application in perspective. This fluorescence can be used for X-ray diagnosis, making fluorescent screen, intensifying screen, input screen of image intensifier, etc. Fluorescent screen is used to observe the image of X-rays passing through human tissues during fluoroscopy, and intensifying screen is used to enhance the sensitivity of film during photography.

4. Most of the X-ray energy absorbed by hot matter is converted into heat energy, which raises the temperature of the object. This is a thermal action.

5. Interference, diffraction, reflection and refraction are the same as visible light. It has been applied to X-ray microscope, wavelength measurement and material structure analysis.

(2) Chemical effects

1. Photosensitization is the same as visible light, and X-rays can make films photosensitive. When X-rays irradiate the silver bromide on the film, silver particles can be precipitated, and the film will have a "photosensitive effect". The photographic intensity of film is in direct proportion to the number of x-rays. When X-rays pass through the human body, the density of each tissue in the human body is different, the absorption of X-rays is different, and the sensitivity obtained on the exposed film is also different, thus obtaining X-ray images. This is the basis of using X-ray for photographic inspection.

2. Some substances play a coloring role, such as platinum, barium cyanide, lead glass, crystal, etc. Dehydration and discoloration after long-term X-ray irradiation is called coloring effect.

(3) Biological effects

When X-rays irradiate organisms, biological cells are inhibited, destroyed or even necrotic, which leads to different degrees of physiological, pathological and biochemical changes in organisms, which is called biological effect of X-rays. Different biological cells have different sensitivities to X-rays. Maple Leaf X-ray can treat some diseases of human body, such as tumor. On the other hand, it is harmful to normal health, so we should protect the human body. Biological effects of X-rays < The base of the mortar is caused by X-ray ionization. Because x-rays have hunger on them! Therefore, it is widely used in industry, agriculture, scientific research and other fields, such as industrial flaw detection, crystal analysis and so on. In medicine, X-ray technology has become a specialized subject for diagnosing and treating diseases, and plays an important role in medical and health undertakings.

Third, the application of X-ray in medicine.

(1) X-ray diagnosis

The application of X-ray in medical diagnosis is mainly based on its penetrability, differential absorption, photosensitivity and fluorescence. Because X-rays will be absorbed in different degrees when passing through the human body, for example, the amount of X-rays absorbed by bones is more than that absorbed by muscles, so the amount of X-rays after passing through the human body is different. It carries the information of the density distribution of various parts of the human body, and the intensity of fluorescence or light sensitivity caused on the fluorescent screen or photographic film is very different, so shadows with different densities will be displayed on the fluorescent screen or photographic film (after development and fixation). According to the contrast between light and dark, combined with clinical manifestations, laboratory examination results and pathological diagnosis, we can judge whether a certain part of the human body is normal or not. Therefore, X-ray diagnosis technology has become the earliest non-evisceration inspection technology in the world.

(2) X-ray therapy

The application of X-ray in treatment mainly depends on its biological effect. When X-rays with different energies are used to irradiate the tissue of human lesions, the irradiated tissue can be destroyed or inhibited, thus achieving the purpose of treating some diseases, especially tumors.

(3) X-ray protection

At the same time of using X-rays, people have found problems that lead to patients' hair loss, skin burns, visual impairment of staff, leukemia and other radiation injuries. In order to prevent X-rays from harming human body, corresponding protective measures must be taken. The above constitute the three links of X-ray application in medicine-diagnosis, treatment and protection.

Four. Brief history of medical x-ray equipment development

Since 1895, X-ray diagnosis and treatment technology has developed rapidly, and the main progress can be divided into the following stages:

(1) ion X-ray tube stage (1895 ~ 19 12)

This is the early stage of X-ray equipment. At that time, the structure of the X-ray machine was very simple, using a gas-containing cold cathode ion X-ray tube with low efficiency, using a bulky induction coil to generate high voltage, and the exposed high-voltage part was not equipped with precise control devices. X-ray machine has small capacity, low efficiency, weak penetration, low image clarity and lack of protection. According to the data, it took 40 ~ 60 min to take an X-ray pelvic image at that time, but the skin of the subject was burned by X-ray after taking the photo.

(2) Electron X-ray tube stage (19 13 ~ 1928)

With the development of electromagnetism, high vacuum technology and other disciplines, in 19 10, American physicist W.D. Coolidge published a report on the successful manufacture of tungsten X-ray tubes. 19 13 has been put into practical use. Its biggest feature is that the tungsten filament is heated to incandescent state to provide electrons for the tube current, so the tube current can be controlled by adjusting the heating temperature of the filament, so that the tube voltage and tube current can be adjusted independently, which is exactly what is needed to improve the image quality.

The invention of grid in 19 13 partially eliminated the scattered lines and improved the image quality. 19 14 years, the fluorescent screen of cadmium tungstate was made, and the application of X-ray fluoroscopy began. 1923 invented the dual-focus X-ray tube, which met the needs of X-ray photography. The power of X-ray tube can reach several kilowatts, and the side length of rectangular focus is only a few millimeters, so the quality of X-ray image is greatly improved. At the same time, with the gradual application of contrast agent, the diagnostic range of X-ray is also expanding. It is no longer a simple tool for simply shooting bone images, but an important medical diagnostic facility for examining the gastrointestinal tract, bronchus, blood vessels, ventricles, kidneys, bladder and other human tissues and organs with poor natural contrast (poor absorption of X-rays). Meanwhile, X-rays have been used for treatment.