*1.3.2 Solar particle events (SPEs)*

The Sun continuously emits particles which make up the solar wind. These are mostly low-energy protons and electrons which are stopped by thin shielding and are thus normally not considered a threat to human space exploration [14]. However during the periods of high activity, the Sun's ejected protons can be accelerated by the chock of a coronal mass ejection or during a solar flare to very high energies. When the energies and flux of the accelerated particles reach high values and extend over a certain period of time, they are registered as Solar Particle Events (SPEs).

SPEs contain mostly protons; include helium ions as well as some highly charged and energetic (HZE) ions. The flux of protons above 30 MeV can exceed 1010 cm−2 in several hours or days and particles above 50 MeV can penetrate spacesuits and spacecraft [14].

Although SPEs are related to the solar activity and cycle, their appearance remains rather unpredictable [23–25], especially far into the future as explorationtype missions are typically planned. SPEs differ in the prevalent proton energies and particle flux. Some SPEs have been observed and recorded, and data from those events are typically used for radiation protection in space. In 2018, NASA published a report [23] recommending to use the October 1989 series of events as a reference design case for missions beyond LEO, based on SPE storm shelter requirements provided in [26].

#### *1.3.3 Secondary emissions*

When primary radiation enters a habitat wall, it reacts with the target molecules and produces secondary emissions. Depending on the nature and energy of the primary-target pair of agents, the produced results will differ from knocked-off electrons to nuclear spallation and formation of ions, neutrons, pions, muons, etc. The most commonly present secondary particles in metallic space vehicles and habitats will be protons of slightly reduced yet still very high energies (when compared to primary protons), neutrons, helium nuclei [27], X and γrays, and

metallic ions of low energies [28]. Some of the secondary emissions (e.g. neutrons) can travel longer in the human body than the primary incoming particle, thus potentially being more harmful. Therefore, secondary emissions must be specifically considered in habitat construction and counter-acted upon, namely in the choice of supplementary materials.

#### *1.3.4 Lunar neutron albedo*

Interactions between the primary particles and the lunar soil cause the formation of lunar radiation albedo. It consists mainly of neutrons that are formed from the constant GCR bombardment of regolith and which shoot upwards from the surface. It has been estimated that the neutron albedo can contribute up to 20% of the effective dose on the Moon [17]. Therefore, any human activity on the surface has to take the lunar neutron albedo into account.
