Space radiation has been identified as the main health hazard to crews involved in long-term interplanetary space missions. In deep space environment, without the protection of the terrestrial magnetic field, Solar Particle Events (SPEs) emitted by the Sun and the much more energetic Galactic Cosmic Rays (GCRs) that originated and accelerated in the galaxy are the two types to be considered. According to the measurements of the Radiation Assessment Detector (RAD) instrument on the Mars Science Laboratory (MSL) during the cruise to Mars, GCRs contributed 95% to the total dose during the journey. Thus, during the foreseen 500-day mission to and from Mars, the dose due to GCRs will far exceed the limitation recommended by the International Commission on Radiological Protection (ICRP). Therefore, radiation shielding has become a major concern for future manned missions, especially outside the low Earth orbit. In a research paper recently published in Space: Science & Technology, Minghui Cai and Jianwei Han from National Space Science Center, Chinese Academy of Sciences developed two kinds of hydrogenous-rich composites for shielding space radiation, and conducted the experiments and simulations to evaluate the shielding performances of these new composites.
The basic process of composite fabrication proposed by the authors is as follows. In the first step, the special resin with high hydrogen containing was synthesized through the design of molecular structure. In the second step, the high-density polyethylene fiber was used as reinforcement to prepare high hydrogen containing composites. In the third step, boron powder was used as an additive to optimize the shielding performance of the composites.
- In the preparation of high hydrogen-containing resin, the molecular structure of the resin matrix is first designed. Afterwards, the resin solution with a reactive end-capping agent is prepared, which has the characteristics of low viscosity and high solid content. Then, due to the high curing temperature of the resin system, a special curing agent and catalyst were added to the resin system to reduce the curing temperature to adapt to composite with high-density polyethylene (HDPE) fiber.
- In the preparation of hydrogenous-rich composite (name the material with PE), the cured sheet with the designed size was first prepared by hot pressing technology. Afterwards, the resin, curing agent, and catalyst were mixed in the mass ratio of 1:0.69:0.02 and evenly coated on the polyethylene fiber. Then, the coated cloth was cured in an oven, folded repeatedly, wrapped with glass cloth and polyimide film, and finally pressed into a plate.
- In the preparation of hydrogenous-rich composite with modified boron fillers (name the material with PE/B), boron was chosen and added into the high hydrogen-containing resin to improve the shielding ability of secondary neutrons produced by cosmic ray bombardment materials. Moreover, different contents of boron were added to the resin solution to study its influence on the viscosity of the resin. The results show that the higher the filler content, the higher the viscosity of the resin. When the boron filler is 20%, the resin viscosity is suitable for fiber coating.
Since the samples for two kinds of hydrogenous-rich composites were prepared as 50 mm×50 mm size with different shielding thicknesses. beams of 80~400 MeV/n 12C of Heavy Ion Research Facility in Lanzhou were used to represent the complex GCR radiation field and measure the shielding properties of composite samples. Experimental results show that the composite with more hydrogen content has higher shielding ability for 80 and 400 MeV/n 12C particles. On the other hand, the addition of boron has no obvious effect on improving the shielding performance of the composite.
However, one of the main disadvantages of the ground accelerator test is that it can only use single energy fixed Z heavy-ion beam, which rarely represented the kinds of residual particles that make up the GCR spectrum. To assess the shielding effectiveness of a given space radiation shielding material, the radiation could be simulated dose in target as a function of shielding. Thickness using Monte Carlo radiation transport codes, such as Geant4 and FLUKA. For the simulation of space radiation interactions with shielding materials, the following conditions are considered by the authors: (1) the spectra of the primary radiation components in free space; (2) accurate information on the elemental composition, density, and geometry of shielding material; (3) the physical processes describe the interactions of the particles with the material nuclei and atoms. All these simulation aspects have been realized by the implementation of a full 3D model of the shielding material using the Geant4 simulation toolkit with proper distribution of the primary particles and considering suitable interaction processes in Geant4. The simulation results show that hydrogenous-rich composite has an obvious advantage in space radiation shielding compared with traditional aluminum. When the shielding thickness is larger than 10 g/cm2, the mass needed for shielding can be reduced by more than 77% and 33% for GCRs and SPEs, respectively.
