In a process called neutron activation, neutrons are absorbed by nuclei of atoms in a nuclear reaction. This most often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next-heavier isotope, many of which are unstable. Cosmic radiation is not a common concern on Earth, as the Earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a significant problem for satellites and astronauts, especially while passing through the Van Allen Belt or while completely outside the protective regions of the Earth's magnetosphere. Frequent fliers may be at a slightly higher risk because of the decreased absorption from thinner atmosphere. Cosmic radiation is extremely high energy, and is very penetrating. Electromagnetic radiation Electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength. X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium or thorium are used, but lead is much more common; several cm are often required. Barium sulfate is used in some applications too. However, when the cost is important, almost any material can be used, but it must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water-cooled layer of lead on the inside to protect the porous concrete from the coolant inside. The concrete is also made with heavy aggregates, such as Baryte or MagnaDense (Magnetite), to aid in the shielding properties of the concrete. Gamma rays are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray. Ultraviolet (UV) radiation is ionizing in its shortest wavelengths but is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing, and protective eyewear. Protection from UV is simpler than for the other forms of radiation above, so it is often considered separately. In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily. For example, although high atomic number materials are very effective in shielding photons, using them to shield beta particles may cause higher radiation exposure due to the production of Bremsstrahlung x-rays, and hence low