Interaction of Radiation with Matter

After discussing the terms atoms, atomic nuclei, and radiation, and learning what is meant by radiation energy, radiation intensity, and energy flux density, we will next take a closer look at what happens when radiation strikes atoms or atomic nuclei, i.e., the interaction of radiation with matter.

As described in the previous sections, energy is transported by α-, β-, and γ-radiation as well as by neutrons. When radiation strikes matter, such as a gas, a liquid, or a solid, the energy of that radiation is transferred entirely or partially to the matter. This transfer can occur through collisions (e.g., ionization, excitation) or transformation processes (e.g., Compton scattering, photoelectric effect, pair production) and is generally referred to as interaction of radiation with matter.

The points just mentioned will be discussed in more detail below.

Let us begin with a description of what is meant by matter. Matter consists of atoms and molecules that can interact with each other. Depending on the strength of these interactions and the mobility of the atoms and molecules, three different states are distinguished:

• solid
• liquid or
• gaseous.

These three states of matter form the classical states of aggregation of matter. Plasma is described as a fourth state of aggregation, but this is not discussed further here.

Even though it is already clear from our daily experience, we will initially consider the distinction between the three states of aggregation:

In the solid state, a material usually retains both its shape and its volume. The liquid state differs from the solid state in that its shape adapts to the respective conditions, but its volume remains unchanged. In the gaseous state, the property of volume consistency is also lost, i.e., a gas completely fills the available space.

The reasons for this behavior of the three states of aggregation lie in the structure of the respective matter. In a solid, the atoms and molecules are arranged rigidly. Their positions are fixed through the lattice structure of the solid. The atoms and molecules move slightly around their lattice positions due to the always present thermal energy (heat). In contrast, their loose arrangement in a liquid allows the atoms and molecules to constantly rearrange. In a gas, the atoms and molecules move very rapidly and possess a high kinetic energy that ensures they do not stick together and spread throughout the available space.

We now know that matter can exist in different states of aggregation – solid, liquid, and gaseous – and that a transition between the various states is possible through changes in various parameters such as temperature, pressure, volume, etc.

Furthermore, we know that matter is composed of atoms or molecules.

A question we might still ask concerns the number of atoms that are approximately contained in a specific volume for the three states of aggregation. A generally valid answer can only provide rough values since the precise values also depend on the respective type of atom or molecule as well as the prevailing environmental conditions (temperature, pressure, etc.). However, as rough guidelines, the following values can be cited for a cube with an edge length of 1 mm, i.e., a volume of 1 mm³:

Now that we understand the term matter, we will examine the different ways that energy can be transferred to matter for the various types of radiation. Let’s start with charged particles, i.e., α- and β-radiation, which are directly ionizing.

Heavy charged particles, such as α-particles, interact with matter primarily via the Coulomb force. This describes the force acting between two point charges, in our case between the positive charge of the α-particle and the negative charges of the electrons in the atoms of the matter. Once an α-particle penetrates matter, it immediately interacts with a number of electrons. If the momentum transferred from an α-particle to an electron is sufficiently large, the electron can be ejected from the atom, resulting in ionization of the atom. At the same time, the (kinetic) energy of the α-particle is reduced as it has transferred some of its energy to the electron.

Light charged particles, such as β-particles, can also transfer their (kinetic) energy through the Coulomb force, but they can additionally emit electromagnetic radiation: If a moving β-particle is deflected from its flight direction, for example, as it approaches an atomic nucleus, part of its energy is converted into radiation known as bremsstrahlung. This radiation can further transfer its energy to matter through subsequent indirect ionization.

Gamma and X-ray radiation are electrically neutral electromagnetic waves (photons), which is why they do not interact directly with matter via the Coulomb force. Therefore, they are referred to as indirectly ionizing. Their interaction with matter occurs mainly through three processes:

• Photoelectric absorption (often referred to as the photoelectric effect),
• Compton scattering, and
• Pair production.

Each of these processes results in a partial or complete transfer of radiation energy to the electrons in the matter.

In the photoelectric effect, electrons are ejected from the atomic shell by electromagnetic radiation. The total energy of the radiation is transferred to the electron in the form of kinetic energy, minus the energy needed to liberate the electron from its binding in the atomic shell. This implies that the energy of the electromagnetic radiation must be greater than the binding energy of the electron!

The complete absorption of the photon by a free electron is not possible. Instead, a Compton effect takes place, resulting in a photon of lower energy.

In Compton scattering, the electromagnetic radiation is scattered by an electron. Here, part of the energy of the radiation is transferred to the electron, which is ejected from the atomic shell. The energy of the scattered electromagnetic radiation is then correspondingly lower.

Compton scattering is the dominant interaction process for electromagnetic radiation with matter between roughly 100 keV and 10 MeV.

If the energy of the electromagnetic radiation exceeds 1022 keV, then near atomic nuclei, all of the energy can be converted into mass and kinetic energy in the form of an electron-positron pair (pair production). The rest mass of an electron or positron is 511 keV, meaning a minimum energy of 1022 keV is required to create the two particles from the electromagnetic radiation. The remaining energy is distributed to the two particles in the form of kinetic energy, causing them to move away in opposite directions.

Neutron radiation represents a special case of indirect ionization. The uncharged neutrons can penetrate deeply into matter and transfer kinetic energy through collisions with atomic nuclei or initiate nuclear reactions.

In the case of a collision, all or part of the neutron's energy may be transferred to an atomic nucleus that then continues on as a charged (heavy) particle.

If a nuclear reaction occurs, the neutron is absorbed by the atomic nucleus, resulting in the production of a charged heavy particle. The charged particles can then perform direct ionization.

Now we know how radiation interacts with matter. However, one important point has only been briefly touched upon: the question of how strong these interactions are, or phrased differently, how deeply the radiation penetrates into matter. The answer to this question leads to the measures to be taken in radiation protection: The further radiation can penetrate into matter, the greater efforts must be made regarding the required shielding.

The quantities that play an important role in this context are the so-called cross sections. The cross section is a measure of the probability of a particular interaction of radiation with a specific material (matter), i.e., how likely it is that an α-particle will be scattered by a particular type of atom, or that gamma radiation will perform a photoelectric effect in a certain material, or that a neutron will be absorbed by an atom in a nuclear reaction…

Values of cross sections are tabulated for different types of radiation for various materials and radiation energies.

As previously mentioned, the ranges of α-radiation in the air are a few centimeters, while in tissue it is a few micrometers. A sheet of paper or the surface of the skin is sufficient for its shielding.

The range of β-radiation is larger. It is a maximum of a few meters in air and a few millimeters in tissue. Here, a thin sheet of metal (e.g., aluminum) is sufficient for its shielding.

However, for X-ray and gamma radiation or neutrons, we must actually know the energy of the radiation and the material with which it interacts, and thereby the respective cross section, in order to make a statement about its range.

Then we can adequately approximate the radiation intensity I behind a material of thickness x with the cross section (more precisely: the linear attenuation coefficient) μ, given the radiation intensity upon entering the material is I₀ by the equation

\(I = I_0 \cdot exp(-µ \cdot x)\)

For practical radiation protection, this has the following implications:

• We can thus make a statement about how high the radiation intensity is behind a material, for example behind a wall, a door, a shield, etc., if we know the parameters I₀, μ, and x, and whether we may need to take further protective measures.
• The radiation intensity decreases exponentially . This means that it will never be completely gone. However, we can ensure, through suitable shielding, that it becomes so low that it does not pose any health hazards or other impacts. This can be achieved through appropriate choice of material (which sets the cross section) and its thickness.