Researchers have designed a high-performance lead-free surface modified gadolinium oxide / boron carbide / high-density polyethylene composite shielding material, and its protective performance is even better than the original boron doped polyethylene collimating shield in China's large scientific device - all superconducting tokamak scientific experimental device.

Usually in people's impression, nuclear radiation protection materials are often inseparable from heavy lead. For example, the protective door used in the hospital X-ray examination room is made of lead material. However, the biological toxicity of lead is not friendly to the environment, which limits its application.

Recently, the research team of Institute of plasma physics, Hefei Research Institute, Chinese Academy of Sciences has made a new progress, which is expected to change people's traditional understanding of nuclear radiation protection materials. The team developed a kind of high-performance, lead-free neutron and gamma ray composite shielding materials, and carried out experimental research and simulation calculation verification around the shielding performance and mechanism of the materials. The relevant results were published in the nuclear science and technology journal nuclear materials and energy, and applied for an invention patent.

Traditional shielding materials are difficult to meet the needs of radiation protection in modern society

Neutron is an electrically neutral particle, which is not affected by Coulomb force, has strong penetration, and will produce secondary gamma rays in the process of collision. It is the research focus of modern nuclear radiation protection. The scientific and efficient neutron shielding scheme will select high atomic number (atomic number refers to the serial number of elements in the periodic table) materials and low atomic number materials, as well as neutron absorbing materials for composite shielding. For example, the commonly used lead boron polyethylene plate composed of lead, boron and polyethylene is this composite shielding material.

Lead boron polyethylene is a traditional shielding material, in which polyethylene has high hydrogen content and hydrogen atom has good moderation effect on fast neutrons; Boron atom can absorb thermal neutrons; In addition to shielding fast neutrons with certain energy, lead atoms are also particularly effective in shielding gamma rays. Compared with other nuclear shielding materials, pbpe not only has high-efficiency nuclear shielding performance, but also has the characteristics of light weight and small volume. It has been widely used in nuclear protection in nuclear power, nuclear power, military industry, aviation, medical treatment and other fields.

However, with the development of atomic energy industry, people must take strict protective measures to ensure the physical health and environmental safety of nuclear personnel. Traditional materials such as lead, boron and polyethylene have single shielding function and limited shielding performance, and some have poor thermodynamic performance, which is difficult to meet the requirements of modern society for nuclear radiation protection. Moreover, these lead containing protective materials often lose their protective effect after a few years of use, and flow into the environment after elimination, which will pollute the surrounding environment.

The new protective material has excellent comprehensive shielding performance

The rare earth element gadolinium usually exists in the form of non-toxic gadolinium oxide in nature, and its average thermal neutron absorption cross section is very high. It not only has high temperature resistance, but also has good gamma ray shielding performance. According to its material characteristics, researchers designed a high-performance lead-free surface modified gadolinium oxide / boron carbide / high-density polyethylene composite shielding scheme.

Firstly, the researchers used coupling agent to modify the surface of gadolinium oxide, which improved its interfacial compatibility and dispersion in the matrix, and made the radiation particles interact more fully with the functional components in the material, so as to decay rapidly. Secondly, the composite designed by the researchers uses the gadolinium hydrogen boron system to slow and absorb neutrons. Using the interaction characteristics between light and heavy nuclei and neutrons and the characteristics of high thermal neutron absorption cross section of gadolinium and boron, the high-energy incident neutrons produce inelastic collision with gadolinium, elastic collision with hydrogen, carbon and oxygen until they become thermal neutrons, and finally absorbed by gadolinium and boron. Gadolinium, as a heavy nuclear element, also has the function of absorbing gamma rays.

Through further research, researchers found that the performance improvement of modified nano gadolinium oxide on the composite was significantly better than that of modified micron gadolinium oxide and unmodified nano and micron gadolinium oxide, and the improvement of radiation shielding performance of the composite was particularly obvious when the material thickness was thinner than 6cm.

Then, the researchers sent the new lead-free nuclear radiation protection materials developed by them to Beijing Radiation Application Research Center for sample shielding actual test. The test results are satisfactory: under the irradiation environment of Cf-252 neutron source, the neutron shielding rate of the composite reaches 98% when the thickness is 15 cm; Under the irradiation environment of cesium-137 and cobalt-60 gamma sources, the gamma shielding rates of the composites reached 72% and 60% when the thickness was 15 cm, respectively.

It is worth mentioning that the comprehensive shielding performance of this new lead-free nuclear radiation protection material is even better than the original boron doped polyethylene collimating shield in the all superconducting tokamak scientific experimental device, a large scientific device in China. It shows that this new lead-free nuclear radiation protection material can be used as an improved alternative material or as a protective material for other neutron gamma hybrid fields, and provide a better means of nuclear radiation protection in the scientific research of controlled nuclear fusion.