2.1 Shielding of Galactic Cosmic Rays
The GCRs of interest have charge number, Z from 1 to 28, and energy from less than 1 MeV/u
to more than 10,000 MeV/u with a median energy of about 1,000 MeV/u. The GCRs with
energies less than about 2,000 MeV/u are modulated by the 11-year solar cycle, with more than
two-times higher GCR flux at solar minimum when the solar wind is weakest compared to the
flux at solar maximum. The most recent solar minimum was in 2008-2009, and the next will
occur in 2019-2020. Engineering considerations on material strength, temperature, ultraviolet
degradation, flammability, etc., must be considered alongside of radiation protection, and the
composite picture must be analyzed. Materials with the smallest mean atomic mass are usually
the most efficient shields for both SPE and GCR, as described next. The composition of the
radiation field changes as particles lose energy and suffer nuclear interactions in traversing
structural materials, instruments, and the tissues of astronauts. Both the energy loss and the
changes in particle fluence are related to the number of atoms per unit mass (in units such as
grams) in the traversed material, which, in turn, is proportional to Avogadro's number divided by
the atomic mass number, AT, for each element of the material. The energy loss by ionization of
a single component of shielding material with atomic number ZT is proportional to the number of
electrons per atom and thus proportional to ZT/AT. However, the energy lost per gram of material
and per incident fluence (e.g., in units of particles per cm2
), the “mass stopping power,” is also
inversely proportional to the density, (e.g., in g/cm3
) of the material, so that the energy lost by
one incident particle per cm2
per unit mass is proportional to Z/A.
The number of nuclear interactions per unit mass and per unit incident fluence is proportional to
/A, where is the total nuclear reaction cross section (Wilson et al., 1991; 1995). To a first
approximation, is proportional to A2/3, so that the nuclear transmission is proportional to 1/A1/3
.
The ratio of electronic stopping power to nuclear interaction transmission is therefore
proportional to Z/ A2/3. Materials with small atomic mass have the highest number of electrons
per nucleon (e.g., Z/A is 1 for hydrogen, 0.5 for carbon, 0.48 for aluminum, 0.46 for iron, and
0.40 for lead). Light mass materials have smaller nuclei and therefore more of them can fit into a