**1.3 Radiation environment**

There are two distinct families of radiation particles: primary and secondary. Primary particles originate from the Sun, our galaxy and distant galaxies [14]. They are high-energy charged particles, mostly protons that move at speeds close to that of light. The diverse mix of ionizing radiation in space, ranging from X-rays to heavy ions with energies up to TeV makes it an extremely challenging environment for radiation protection of humans [14]. When primaries interact with matter, such as the lunar surface, a habitat, or Earth's atmosphere, secondary emissions are produced. The nature and properties of secondary particles depend on the type of interaction that occurred. On the Moon, we can distinguish two branches of secondary emissions: the ones that will occur in the habitat and the lunar neutron albedo.
