2 Materials

Materials in Radiant are defined by a Radiant.Material object. A material is characterized by an optional human-readable tag, a mass density, a state of matter and a list of elements with their corresponding mass weight fractions.

2.1 Instantiating a Material

A new material is instantiated with the Material constructor, which accepts an optional string tag used to identify the material. This tag must be unique within a calculation if more than one material is defined.

water = Material("water")          # Tagged constructor (recommended)
unnamed = Material()               # Tag can also be set later with set_tag(...)
unnamed.set_tag("aluminum")

2.2 Predefined Materials

For convenience, Radiant ships with a large library of predefined materials based on the NIST database of elements, compounds and mixtures (water, air, lead, compact bone, A-150 tissue-equivalent plastic, and many more). Each one is a zero-argument constructor that returns a fully-configured Material, with its density, mean excitation energy, state of matter and elemental composition already set to the NIST-recommended values:

w   = Water()                       # liquid water
air = Air_Dry_Near_Sea_Level()      # dry air near sea level
pb  = Lead()                        # elemental lead

These are simply shortcuts for a material assembled by hand as in the next sections; any property can still be overridden afterwards (e.g. w.set_density(0.998)). The complete list of available constructors, together with the exact density, mean excitation energy and composition preset for each one, is given in the Predefined Materials page of the API reference.

2.3 Defining the Composition

Once a material is instantiated, elements are added with add_element. For each element, the chemical symbol and its mass weight fraction (between 0 and 1) are provided. The weight fractions across all the elements of a material must sum to at most 1.

water = Material("water")
water.set_density(1.0)             # Density in g/cm³
water.add_element("H",0.1111)      # Hydrogen with mass weight of 11.11 %
water.add_element("O",0.8889)      # Oxygen with mass weight of 88.89 %

For a monoelemental material, the weight fraction can be omitted; it defaults to 1.0 and the density is automatically set to the tabulated density of that element at 20°C:

al = Material("aluminum")
al.add_element("Al")               # 100 % Al, density set to 2.70 g/cm³

If set_density(...) is called later, it overrides this default value.

2.4 State of Matter

By default a material is "solid". The state of matter influences density-effect corrections in the inelastic-collision cross-sections. The accepted values are "solid", "liquid" and "gaz":

water.set_state_of_matter("liquid")

2.5 Mean Excitation Energy

The mean excitation energy $I$ is the key material parameter of the collisional (inelastic) stopping power. By default Radiant determines it automatically:

1. for a few recognized compositions, the ICRU-recommended value is used (e.g. $78$ eV for liquid water, $85.7$ eV for dry air);
2. for any other material, $I$ is computed from the elemental values through the Bragg additivity rule.

The additivity rule systematically overestimates $I$ for many compounds because it ignores chemical-binding and phase effects (for example it yields $≈ 87$ eV for an ICRP soft-tissue composition and $≈ 92$ eV for striated muscle, versus the ICRU/ESTAR values of about $73$ eV and $74.7$ eV). Since $I$ directly shifts the stopping power, this can move the Bragg peak noticeably. For tissues and other compounds it is therefore recommended to impose the ICRU/ESTAR value explicitly with set_mean_excitation_energy (in eV):

muscle.set_mean_excitation_energy(74.7)   # ICRU/ESTAR value for striated muscle

A value set this way takes precedence over both the tabulated value and the additivity rule. The getter get_mean_excitation_energy() returns the user-defined value, or missing if none was set (in which case the automatic value is used internally).

2.6 Inspecting a Material

Calling println(material) prints a summary of the material:

Material:
   Density (g/cm³):              1.0
   Elements in the compound:     ["h", "o"]
   Weight fractions:             [0.1111, 0.8889]
   State of matter:              liquid

A series of getter methods is also provided to retrieve individual fields:

water.get_tag()                    # "water"
water.get_density()                # 1.0
water.get_state_of_matter()        # "liquid"
water.get_number_of_elements()     # 2
water.get_atomic_numbers()         # [1, 8]
water.get_weight_fractions()       # [0.1111, 0.8889]
water.get_mean_excitation_energy() # missing (78 eV used internally for water)

2.7 Summary of the Material API