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Interaction of Electromagnetic Radiation with Matter

There are many ways that electromagnetic (EM) radiation can interact with matter, causing several different phenomena. EM radiation is often emitted by matter undergoing a change in energy state. Additionally, it can interact with matter causing a change in the medium or releasing more radiation.

The way that EM radiation interacts with a medium provides valuable insight into both the radiation itself and the properties of the medium. By observing how the medium responds to EM radiation, and what secondary products the medium emits, we can measure properties of the incoming radiation and properties of the medium itself.

The study of interactions between EM radiation and matter has many useful applications including in:

  • Particle detection
  • Medical diagnostics
  • Gamma spectroscopy
  • Radiobiology
  • Photodetector and photoelectron spectroscopy
  • Industrial materials characterization

How Can Electromagnetic Radiation Interact With Matter?


According to quantum mechanics, EM radiation can behave as a wave and as a particle. Therefore, EM radiation can be characterized by its energy, E, momentum, p, wavelength, λ, and frequency, f:

Equation to characterize EM radiation

 

At high energies and short wavelengths, EM radiation typically behaves like particles, called photons. An example of these energy quanta are gamma rays. At low energies and long wavelengths, EM radiation behaves more like a wave. Examples include radio- and micro-waves.

Electromagnetic radiation: wavelegth and frequency determine placement on EMR spectrum
Spectrum of electromagnetic radiation

There are several ways that EM radiation and matter interact:

  • Absorption: EM radiation can be completely absorbed by an electron, boosting it into an excited state or removing it from an atom completely.
  • Scattering: Like particle-particle scattering, photons can transfer some of its energy to a particle causing them to scatter.

EM radiation can also be emitted by a particle, if it changes energy states, if a reaction between particles releases energy, or if that particle accelerates.

When Will EM Radiation Interact With Matter?


The likelihood of EM radiation interacting with a medium depends on several factors.

In a dense medium like a solid, the electron density is high, meaning there are many electrons within a given volume. Conversely, in a less dense medium like a gas, the electrons are more sparsely distributed. A higher electron density in the medium increases the probability of photon-electron interactions. Since elements with a high atomic number tend to be more dense and have more electrons in their outer shells, media containing heavy elements are more likely to interact with EM radiation.

The energy or wavelength of EM radiation influences how likely it is to interact with a medium. Also, the electromagnetic properties of the medium influence the interaction probability. Maxwell's equations for electromagnetism can be used to model these interactions.

EM waves, such as microwaves and radio waves, typically interact with media by increasing the rotational or vibrational energy of the molecules. At higher energies, photons can interact with electrons by exciting the energy state of electrons. As a photon travels through a material, it undergoes one or more interactions with electrons, depositing its energy when it interacts.

Examples of EM Radiation Interacting with Matter


There are many ways in which EM radiation will interact with a medium. All have the net effect of transferring some or all of the initial energy to the surrounding medium. These interactions can result in the production of free or excited electrons, possibly causing further release of radiation. The presence of these free electrons can serve as a detectable signature of a photon-matter interaction. Also, the energy transferred to a conducting medium by an electromagnetic wave can be detected by electronic circuits.

Photoabsorption

The most significant interaction mechanism of EM radiation with matter, especially at low energy and long wavelengths, is the absorption of energy by electrons. When a bound electron absorbs energy, it enters an excited state. If the transferred energy is greater than a specific threshold, the photoelectric effect is observed.

The photoelectric effect describes the effect of a material absorbing a photon, then subsequently emitting an electron, often termed a photoelectron.

This effect occurs when the photon's energy, hf, exceeds the binding energy of the electron within its atomic orbital, Eb. The kinetic energy of the ejected photoelectron, Ee-, can be determined by the following equation:

Equation for energy of photoelectron

The ejection of an electron leaves the atom in an ionized state. This vacancy is quickly filled by another electron from a higher energy level within the atom or from a neighboring atom. This atomic relaxation process often results in the emission of characteristic X-rays. Measuring these X-rays can tell you a lot about a material.

If the energy absorbed by the electron is below the threshold for the photoelectric effect, the excited electron will typically relax back into a lower energy state. The excess energy released during this relaxation is typically lower, emitting EM radiation within the UV, visible, or infrared range. This process is called photoluminescence, which includes fluorescence and phosphorescence.

Compton Scattering

Compton scattering is where a high-energy photon scatters inelastically off an electron within a material. During Compton scattering, an incident photon transfers some of its energy to the electron, causing both the particle and the photon to scatter in different directions. This interaction is very important as it demonstrates the particle-like nature of light, as it cannot be fully explained by classical wave theory.

The change in the photon's wavelength (and consequently its energy) due to Compton scattering can be described by the following equation:

Equation for the change a the photon's wavelenght due to Compton scattering

where λ’ is the wavelength of the scattered photon, λ is the wavelength of the incident photon, h is Planck’s constant, me is the electron mass, c is the speed of light, and θ is the scattering angle of the photon.

Just like the photoelectric effect, if the amount of energy transferred to the electron during the scattering process exceeds the electron's binding energy, the electron could be ejected from the atom entirely while the photon continues on to interact with another electron.

Pair Production

Pair production is a significant interaction mechanism for very high-energy photons. When a photon with an energy exceeding 1.022 MeV interacts with the strong electric field surrounding an atomic nucleus, it can undergo a remarkable transformation: the photon disappears, and in its place, an electron and a positron are created.

This process is a striking example of energy-mass equivalence. The energy of the photon is converted into the mass of the electron-positron pair. The threshold energy of 1.022 MeV is the combined rest mass energy of the electron and positron (0.511 MeV/c² each). Once created, the positron will quickly encounter an electron and annihilate, producing a pair of gamma-ray photons in the process. The probability of pair production increases with increased photon energy.

Energy Transfer to Medium

The distance a photon travels before being fully absorbed depends on several factors, including the photon energy, material density, and the atomic number and composition of the absorbing material. The average distance traveled by a single photon before it interacts is random but can be modeled by considering a beam of identical photons. The energy loss of the beam as the photons traverse a material can be mathematically modeled by an exponential decay function:

Exponential decay function

where I(x) is the intensity (number of remaining photons) at a depth x within the material, I(0) is the initial intensity of the photon beam, and μ is the linear attenuation coefficient, which accounts for the probability of photon interactions within the material. This equation will encompass all possible energy transfer mechanisms such as photoabsorption, Compton scattering, and pair production.

Sources of EM Radiation


Just as EM radiation can have effects on matter, reactions or processes within matter can lead to the emission of EM radiation. Here are some examples of situations where EM radiation is emitted from a particle or process.

Relaxation From Excited States

EM radiation is often emitted when a system transitions from a higher energy state (excited state) to a lower energy state. One prominent example is the emission of characteristic X-rays.

When a high-energy particle, such as an electron or a proton interacts with an atom, it can eject an inner-shell electron, leaving an electron vacancy. This vacancy is quickly filled by an electron from a higher energy level, releasing energy in the form of a characteristic X-ray. The energy of this released X-ray will be specific for each different element. This differs from photoluminescence, which is when an outer shell electron relaxes to a lower state.

Alternatively, gamma rays are emitted when a nucleus (rather than an electron) transitions from an excited state to a lower energy state. This can occur after various nuclear processes, including radioactive decay (e.g., following alpha or beta decay), nuclear reactions (e.g., radiative neutron capture), or internal conversion, where nuclear excitation energy is directly transferred to an atomic electron.

Particle Annihilation

Photons can also be produced through annihilation. Here, a particle and its corresponding antiparticle meet and mutually annihilate each other.

A common example is positron-electron annihilation. Positrons, the antiparticles of electrons, are emitted in certain types of radioactive decay, such as beta-plus decay.

Since our universe is predominantly composed of matter rather than antimatter, a positron will quickly encounter an electron and annihilate. In this process, the mass of both particles is entirely converted into energy, resulting in the emission of two gamma-ray photons traveling in opposite directions to conserve momentum.

Each of these photons carries an energy of 511 keV (kilo-electronvolts, about 1.6×10−16 Joules). The detection of these 511 keV gamma rays serves as a characteristic signature for the occurrence of positron-electron annihilation, providing valuable information in fields like positron emission tomography (PET) imaging.

Particle Acceleration

EM radiation can be produced when charged particles undergo acceleration. This phenomenon occurs in two primary ways:

  • Bremsstrahlung: When a charged particle, such as an electron, passes through a dense material, it interacts with the atomic nuclei. These interactions cause the particle to decelerate, emitting EM radiation. This radiation is known as Bremsstrahlung, which is German for "braking radiation".
  • Synchrotron Radiation: This type of radiation arises when the path of charged particles, typically electrons, is bent in a magnetic field, such as within a circular particle accelerator. As the particles are forced to move in a curved path by the magnetic field, they undergo continuous centripetal acceleration, emitting EM radiation in a narrow, intense beam.

The emitted EM radiation exhibits a spectrum of energies, ranging from zero up to the initial kinetic energy of the accelerated particle.

More Resources


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Beta decay, a term coined by Ernest Rutherford in 1899, describes a class of radioactive decay processes that involve the emission of either energetic electrons or energetic positrons. These reactions are known as beta minus decay (β-) and beta plus decay (β+) respectively, and the emitted electron or positron is often referred to as a beta particle.

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Contributors


Written by

Dr. Matthew Thiesse

Product Developer

Diagrams by

Sam Force

Graphic Designer

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