The NASA Space Radiation Laboratory makes use of a particle accelerator that produces beams of protons or heavy ions. These ions are typical of those accelerated in cosmic sources and by the Sun. The beams of ions move through a 100 m (328-foot) transport tunnel to the 37 m (400-square-foot) shielded target hall. There, they hit the target, which may be a biological sample or shielding material. In a 2002 NASA study, it was determined that materials that have high hydrogen contents, such as polyethylene, can reduce primary and secondary radiation to a greater extent than metals, such as aluminum. The problem with this "passive shielding" method is that radiation interactions in the material generate secondary radiation.