KAHIBARO
7.3. Common Materials
Table of Contents
Air
In most Geant4 simulations the default world material is air, because it represents ordinary laboratory conditions while being almost transparent to many particles. For beginners, the key point is that you rarely need to define air by hand. Instead you typically obtain it from the NIST material database through G4NistManager. A common way is to ask explicitly for "G4_AIR". This name corresponds to dry air at standard conditions with a predefined mixture of nitrogen, oxygen and small fractions of other gases. The density is about $1.29\times10^{-3}\,\text{g/cm}^3$.
Air is usually used in three main roles. First, as the material of the world volume that surrounds the whole detector, because it is realistic and keeps particle paths close to those in a real laboratory. Second, as the filling material between detector components such as between shielding layers or around beam lines. Third, as a reference for vacuum. When you want a vacuum region you can either use a special vacuum material from NIST, for example "G4_Galactic", or define your own extremely low density gas.
For low energy photons and electrons air can already have a visible effect on attenuation and scattering if paths are long enough. For most small detector setups in the MeV range its influence is modest, but for shielding studies or large experimental halls air can contribute non negligible scattering, especially of electrons and low energy photons.
Always use a physically meaningful material for the world volume, typically NIST "G4_AIR" or a vacuum material, and avoid leaving the world in an undefined or placeholder material.
Water
Water is one of the most important materials in Geant4 simulations. It appears as a stand in for many types of soft biological tissue, as a calibration material, and as an absorber or moderator for neutrons. The NIST database provides "G4_WATER", which represents liquid water at a standard density of about $1.0\,\text{g/cm}^3$. This definition includes the correct elemental composition of hydrogen and oxygen and is suitable for most basic applications.
In medical physics water is often used to build water phantoms that approximate human tissue for dose calculations. Because the stopping power and scattering properties in water are well studied, water phantoms allow a straightforward comparison between simulation and reference data, for example depth dose curves for photon or proton beams. In radiation protection studies water can model human bodies or organs inside a shielding environment.
Water also has a special role in neutron physics. Hydrogen rich materials slow down fast neutrons efficiently through elastic scattering. This makes water an effective moderator and shield. In such simulations the correct hydrogen content and density are essential. For dosimetry and shielding studies beginners should use the NIST "G4_WATER" without modification unless there is a specific reason to adjust temperature or density.
In optical simulations water can act as a medium for Cherenkov radiation or scintillation, but detailed optical properties such as refractive index and absorption length are not part of the basic NIST definition. These optical properties are added separately to the material in dedicated optical photon examples and will be covered in an optical chapter.
When comparing dose or attenuation in water with published data, always ensure that you use the standard NIST "G4_WATER" material with its default density of about $1.0\,\text{g/cm}^3$.
Aluminum
Aluminum is a light structural metal commonly used in detector housings, support structures, and beam line components. It has atomic number $Z = 13$ and a typical density in Geant4 of about $2.7\,\text{g/cm}^3$. You can obtain it easily from the NIST database with the name "G4_Al". Since aluminum is an element, its material definition in Geant4 is simpler than that of compounds or mixtures, and using the NIST version avoids mistakes in atomic weight or density.
In geometry, aluminum often appears as the material of vacuum chambers, shielding boxes, and frames for scintillators or semiconductor detectors. Although aluminum provides some attenuation for photons, it is much less effective than heavy materials such as lead. It is therefore typically used where mechanical support and low mass are more important than maximum shielding, for example in satellite payloads or detector mounts that must minimize scattering.
For electron and proton transport simulations aluminum produces noticeable energy loss and secondary radiation. For thin windows, such as entrance windows of detectors or beam pipes, the correct thickness and density of aluminum can influence the energy of the beam that reaches the sensitive region. In such cases geometric thickness in millimeters combined with the standard density determine the areal density in $\text{g/cm}^2$, which directly affects stopping power.
In many introductory examples aluminum is chosen as a simple single element detector or as a test absorber, because it demonstrates basic interactions while remaining easy to understand.
When modeling structural components, always distinguish between high Z shielding materials such as lead and lighter structural metals such as aluminum. Their attenuation properties differ strongly even for the same thickness.
Silicon
Silicon plays a central role in modern detector technology. It is the base material of many semiconductor detectors used in tracking, vertex detection, and X ray detection. In Geant4, silicon as a pure element is available from NIST with the name "G4_Si". Its atomic number is $Z = 14$ and the typical density used is about $2.33\,\text{g/cm}^3$. This density corresponds to crystalline silicon.
In particle tracking detectors, thin silicon layers record the passage of charged particles through ionization. The thickness is often on the order of a few hundred micrometers. Even though the geometric thickness is small, the relatively high density means that silicon can still provide precise position measurements with limited multiple scattering. In X ray and gamma ray detection, thicker silicon detectors can be used to convert photon energy into electron hole pairs which are then collected as signal.
Silicon is also a convenient material for teaching examples, because it is a simple element with well known properties and widely available reference data. When you use silicon in Geant4 as a detector material, you typically combine it with sensitive detectors and scoring to track energy deposition and hit positions. The details of charge collection, electronics response, and noise are usually modeled separately or approximated with energy smearing rather than being handled directly by Geant4.
In optical photon simulations silicon can represent the active surface of photodetectors such as silicon photomultipliers or photodiodes, but detailed quantum efficiency and timing behavior are not part of the bare material definition. Those effects are introduced through optical surfaces and detection models later in the simulation chain.
Use NIST "G4_Si" for silicon detector materials to ensure correct density and atomic properties, and handle electronic response and resolution outside the basic Geant4 material definition.
Lead
Lead is a heavy metal that is widely used in radiation shielding and collimation. It has atomic number $Z = 82$ and a density of about $11.34\,\text{g/cm}^3$ in Geant4. This combination of high Z and high density gives lead a very strong attenuation power for gamma rays and X rays. In Geant4 you usually access it via the NIST material name "G4_Pb".
In shielding simulations lead blocks are used to reduce the intensity of photons that reach a detector or a defined region. The choice of lead thickness for a given photon energy determines the transmission that you will measure. In a beginner friendly shielding study, for example, you might place a lead slab between a gamma source and a detector and then vary its thickness to study attenuation. Because lead is so effective, even a few millimeters can have a large effect at moderate photon energies.
Lead is also common in collimators, where it shapes or narrows a beam by absorbing off axis particles. In such applications, the geometry of the lead structure and the presence of any air gaps or openings can strongly influence the observed beam profile. The same nucleus that makes lead a good photon absorber also means that electromagnetic showers and secondary particles are important at higher energies.
Although lead provides powerful shielding, it is heavy and toxic, so in realistic designs it is often combined with other materials and mechanical supports. In Geant4, however, you typically model it simply as a homogeneous material defined by the NIST database unless you have a special requirement such as a specific lead alloy.
For gamma and X ray shielding studies in Geant4, always use a correct high Z material such as NIST "G4_Pb", and remember that small thickness changes in lead can cause large changes in transmission.
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