Neutrons are nuclear particles without any charge. They can be used for plant mutation breeding, in nuclear reactors for producing nuclear energy, for boron neutron capture therapy for cancer treatment, neutron imaging, neutron activation analysis and neutron microscopy. It easily passes through most materials and reacts with the nuclei of the target atom.
Neutron shielding materials are critical components for radiation protection in many nuclear facilities. In some nuclear fusion experimental devices, diagnostic systems like Neutron cameras require the installation of a collimated shielding shell to achieve location measurements of neutron emissivity. Any such shielding materials must withstand neutron and gamma radiation for a long time in a very harsh environment. Because of the limitations of space and mobility, the weight and volume of radiation shielding materials are quite restricted. In addition to excellent shielding performance, radiation shielding materials also need mechanical durability, low specific gravity, small volume, long service life and other properties.
Dr. Huo Zhipeng and his student Zhao Sheng from the Hefei Institute of Physical Science of the Chinese Academy of Sciences has been working on developing a lead-free neutron and gamma ray composite shielding material that has high shielding properties and is environmentally friendly. Dr. Huo has been involved in radiation and environmental protection for years. The results of his research have been published in Nuclear Materials and Energy.
The composite is a modified-gadolinium oxide/boron carbide/high density polyethylene (Gd2O3/B4C/HDPE). It was tested safe and effective to shield neutron and gamma rays through a series of complex and comprehensive experiments.
Neutrons always emit secondary gamma rays during particle collision processes. The scientific and efficient scheme of shielding neutrons is to select high atomic number (Z) and low atomic number (Z) materials and neutron absorbing materials simultaneously for combined shielding. Lead has often been used but its uses are restricted because of its high biological toxicity.
The rare earth element gadolinium usually exists in the form of non-toxic Gd2O3 in nature. It has always shown high average thermal neutron absorption, high temperature resistance and good gamma shielding performance.
The Chinese research team studied the shielding mechanism first. Then they adopted the coupling agents to modify the surface of the Gd2O3 to improve the interfacial compatibility and dispersion of the Gd2O3 in the matrix.
Dr. Huo explained in his report how this new radiation shielding worked. Fast neutrons collide with gadolinium inelastically and collide with hydrogen elastically. They become thermal neutrons which are absorbed by the high Z element Gd and boron.
The experimental results show that the neutron shielding rate of the composite can be as high as ninety eight percent under the conditions of fifteen centimeters of thickness in californium-252 environments. In cesium-137 and cobalt-60 environments, the gamma shielding rates of the composite are seventy two percent and sixty percent, respectively, at the same thickness.
The comprehensive shielding performance of the new composite is better than conventional boron-polyethylene collimating shielding. It is suitable for gamma spectrum diagnosis systems of Experimental Advanced Superconducting Tokamak (EAST). It is expected to be a promising radiation shielding material for neutron-gamma mixed fields, according to Dr. Huo.