Radiation shielding

Water-shield Greenhouse Concept

Shielding against radiation is considered a very difficult task. For example, a proton or alpha particle cosmic ray of "medium" energy can pass through more than a metre of aluminium, not counting the effects of secondary radiation. With this in mind, it is clear that any Martian colonists would have to take a holistic approach, reducing their radiation exposure at every possible opportunity through shielding and risk-mitigating behaviour.

Passive shielding

The Mars Foundation concept for a side-lit greenhouse.

In most cases, matter placed between a person (or radiation-sensitive equipment) and radiation source reduces the amount of radiation they absorb. The effectiveness of the shielding increases with the mass of the shielding and decreases with the atomic mass of the elements used for the shielding. For example, 1kg of hydrogen offers more protection then 1kg of aluminium, 2kg of aluminium offers more protection than 1kg of aluminium and 1kg of hydrogen offers more protection than 2kg of aluminium.

Mars One's solution is a thick layer of regolith on top of the settlement modules. An effective shield will require at least several hundred grams of regolith per square centimeter, according to one study. Using a regolith density estimate of 1.4 g/cm³, this means the regolith layer would need to be over 2 meters deep. For concrete with an average density of 2.4 g/cm3 the required thickness might be less.

The attenuation of radiation follows the Beer Lamberth law.

I_x=I_o*e^-ux

Where:	I	=	the intensity of photons transmitted across some distance x
	I0	=	the initial intensity of photons (or radiation in general)
	s	=	a proportionality constant that reflects the total probability of a photon being scattered or absorbed
	µ	=	the linear attenuation coefficient
	x	=	distance traveled (thickness of material)

Linear Attenuation Coefficients (in cm^-1) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.

Absorber	100 keV	200 keV	500 keV
Air	0.000195	0.000159	0.000112
Water	0.167	0.136	0.097
Carbon	0.335	0.274	0.196
Aluminium	0.435	0.324	0.227
Iron	2.72	1.09	0.655
Copper	3.8	1.309	0.73
Lead	59.7	10.15	1.64

the linear attenuation coefficient µ is not commonly found in the litterature, the mass attenuation coefficient µ_m is usually used instead. The coefficient is also dependent on the type of radiation, so a complete solution for radiation protection requires multiple analysis of the type of radiation to be protected against.

Conversion is quite simple as:

µ=µ_m*density of the material

List of mass attenuation coefficients can be found at the NIST website. https://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html

Another common way of evaluating radiation shielding is to use the half value, that expresses the thickness of absorbing material which is needed to reduce the incident radiation intensity by a factor of two, or I_x=I_o / 2.

The Half Value Layer for a range of absorbers is listed in the following table for three gamma-ray energies:

Half Value Layers (in cm) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.

Absorber	100 keV	200 keV	500 keV
Air	3555	4359	6189
Water	4.15	5.1	7.15
Carbon	2.07	2.53	3.54
Aluminium	1.59	2.14	3.05
Iron	0.26	0.64	1.06
Copper	0.18	0.53	0.95
Lead	0.012	0.068	0.42

The first point to note is that the Half Value Layer decreases as the atomic number increases. For example, the value for air at 100 keV is about 35 meters and it decreases to just 0.12 mm for lead at this energy. In other words 35 m of air is needed to reduce the intensity of a 100 keV gamma-ray beam by a factor of two whereas just 0.12 mm of lead can do the same thing. The Half Value Layer increases with increasing gamma-ray energy. For example, from 0.18 cm for copper at 100 keV to about 1 cm at 500 keV.

Active shielding

Active shielding against radiation involves a manmade magnetic field which deflects ionized particles in the same manner as the Earth's. Such fields would require infeasible amounts of energy to generate and would also pose a major risk to anyone approaching the craft or base, as it would create bands of trapped particles similar to the Van Allen belts.

Nevertheless, it might be possible to situate a base in such a location that one of the residual Martian magnetic fields offers a nett benefit. Care should certainly be taken not to situate it where the fields concentrate radiation.

Also, it might be possible (assuming one could generate the required magnetic field in some way) to have the radiation belts of the habitat pass through some sort of physical barrier, which scrubs them of particles.

Risk-mitigating behaviour

The possible sources of radiation on Mars are manmade sources, such as nuclear reactors or medical equipment, solar radiation, galactic cosmic radiation and naturally occuring radioactive elements on Mars.

Possible behavioural choices which minimize the risk from these include:

• Avoiding daytime EVA when there is a significan risk from solar radiation.
• Working preferentially close to natural or manmade objects, such as habitats, rovers or cliffs which provide additional (if not omni-directional) shielding.
• Entering a storm shelter when there is a high-radiation risk from solar particle events.