Ray shielding

- Dec 09, 2019-

There are three elements of ray protection are distance, time and shielding, or the main methods of ray protection are time protection, distance protection and shielding protection, commonly known as the three methods of ray protection.


X-ray penetration of the body will produce certain biological effects. If the amount of X-ray exposure is too much, exceeding the allowable amount of radiation exposure, there may be a radioactive reaction, and even a certain degree of radiation damage. However, if the X-ray exposure is within the allowable range, the effect is usually minimal. People do not have to refuse necessary x-rays and CT scans because of the radiation, not to mention being afraid to enter the radiology section of a hospital.


In terms of technology, the principle of shielding and distance protection can be adopted. Shielding is the use of substances with a high atomic number, usually lead or leaded, as a barrier to absorb unwanted x-rays. Distance protection means using the principle that the amount of X-ray exposure is inversely proportional to the square of the distance to reduce the amount of exposure by increasing the distance between the X-ray source and the human body.


Shielding material

Common shielding materials for x-rays are lead plates and concrete walls, or barium cement (added with barium sulfate - cement that also weighs quartz powder) walls. The concept of a half-value layer (half-value layer) is usually used in the thickness estimation of shielding materials. In X-ray detection, wide-beam X-ray is used. The following table shows the approximate half-valent layer thickness t1/2 and 1/10 in lead and concrete. Note: due to the purity and purity of the lead plate, the formulation of concrete and the inevitable differences in the structure, the half-price layer thickness given in the table can only be used as a reference value, and it is necessary to consider increasing the insurance amount in practical application.


In shielding calculation, two factors need to be considered, that is, the primary radiation shield through which the ray source passes directly through the shield, and the scattered radiation caused by the ray on the shield. Generally speaking, the key point of shielding protection is to place a shielding material of sufficient thickness between the ray source and the human body to effectively absorb the rays.


Elemental protective material: (1) lead, density I 1.35g1cm3, is the earliest material used in radiation protection. Lead has a high attenuation capacity for low or high energy X and Y photons, and is easy to process and inexpensive. However, the application of lead is limited due to its high backscattering to low energy X-ray, low hardness, high temperature resistance and toxicity. (2) iron, with a density of 7.8 and cm3, has good mechanical properties, low price and easy to obtain, and the reverse scattering percentage of iron is only 65 of that of lead. Therefore, iron is a kind of shielding material with good protective and structural properties. Iron can be made into sheets of different thicknesses and shapes and used as linings for various protective materials. Iron oxide can also be used as filler for other protective materials.


Concrete: concrete is often used for structural shielding of radiation sources and is still in use. Although a material obtained by sintering a compound of s. pb.fe has been reported as an alternative to cement, no practical applications have been reported.


Lead rubber: lead rubber is made of high quality natural or synthetic rubber mixed with metal oxides such as lead oxide and vulcanized rubber products. It is mainly used for making personal protective clothing for various protective purposes and local protective hanging curtain for matching with X-ray machine. Mobile protective screens and various forms of contact shielding for use by patients during medical exposure.


Radiation proof inorganic lead glass:


In the manufacturing process of glass, in addition to the inherent components of optical glass, the addition of heavy metal oxides with the ordinal number of plateau particles, such as PbO. BaO. Bi203, etc. can form transparent radiation-proof glass. For X-ray(y-ray) protection needs transparent shielding ray equipment, radioactive workplace, such as X-ray screen lead glass, fixed protection facilities on the observation window, radioisotope application of local protection shield, commonly known as ZF series. At present, there are dozens of radiation-proof inorganic lead glass manufacturers in China, whose product performance has approached the international level.


Radiation proof organic lead glass:


Organic lead glass is a transparent shielding material in which lead oxide is introduced into ordinary organic glass. The main raw materials are methacrylic acid, methyl methacrylate vinegar and lead oxide. In the late 1970s, Japan's Concorde chemical industry co., ltd. first developed and patented X-ray resistant organic lead glass [Is], and then reported patents one after another.


Fiberglass composite protective material: this kind of material is a kind of protective material developed in the early 1980s. It is a plastic material with lead, barium and other oxides, sulfides, tungsten, steel, drill and other metal elements as anti-radiation components on the basis of ordinary fiberglass reinforced materials. At present the use of the molding material more radiation resistance, anti - aging performance of good unsaturated poly - vinegar resin, reinforcement materials used roving and fiberglass cloth.


Shielding effect


Shielding materials are designed to shield rays. The capacity of a material to absorb photons, that is, the shielding effect, depends on the incidence of photoelectric effect and Compton effect between incident photons and the material.


Photoelectric absorption cross section should be considered in designing shielding materials. When the energy of the incident photon is enough to remove one of the K electrons from the nucleus of the absorbing substance from the atom, resonance absorption is caused. Therefore, in addition to increasing the density of the material, the number of electrons absorbed from outside the nucleus, the size and number of energy levels in the inner orbital, and the distribution of orbital electrons also affect the absorption performance of the material.


Effects of material density on shielding properties


For the photoelectric effect, since the photoelectric effect is the interaction between photons and electrons in the inner shell, the number of electrons in the inner shell is the same for elements with different subordinal Numbers. Therefore, the number of electrons in the inner shell per unit volume depends on the density of the absorbing material. A material with a high mass density has more atoms per unit volume that absorb atoms, and a corresponding number of electrons in its inner shell. For photons of a specific energy, the photoelectric effect is more likely to occur. For the Compton effect, the higher the number of electrons and free electrons in the shell layer (the element with the larger main quantum number), the more likely the Compton effect is to occur, which is consistent with the photoelectric effect in the requirement of material density. Obviously, the selection of ordinal number and high quality density elements as absorption materials will have a great contribution to the absorption performance.


Absorbing the effects of the electron energy levels in the outer and inner orbits of the nucleus


Analysis of all kinds of material mass attenuation coefficient and the relationship between the photon energy, for any kind of specific material, there are one or more energy, in the energy, absorption coefficient will happen suddenly increase, this makes the absorption coefficient spurt of energy is called the absorption edge of this substance or absorption edge, its value is equal to the inner shell electron binding energy. Since the complete disappearance of photon energy is finally completed in the process of photoelectric effect, if there are multiple absorption limits in the absorption material, the contribution of photoelectric effect can improve the absorption efficiency of the continuous energy photon beam in multiple places. The incident photons are absorbed much more at the K absorption limit. By using this feature to combine a variety of elements in a certain proportion, the shielding effect of the composite shielding material to the photons in a certain energy range will be significantly better than that of any single material that constitutes the composite shielding material.


Effects of electron absorption distribution outside the nucleus on shielding performance


An incident photon must collide with an electron outside the nucleus of the atom to attenuate its energy. If the incident photon fails to collide with the electron in any way in its direction of advance, the energy of the photon will not be consumed. The probability of a photon colliding with an electron depends not only on the density of the electron, but also on the distribution of electrons outside the nucleus. The direction of the orbital extension is related. In terms of material design, the interaction probability of photons and electrons in the material will be greatly increased if the extrarenal electrons absorbing atoms have multiple spatial extension directions, and if the multi-extension directions (orbitals) can be "drawn together", "crossed" and "overlapped".