Radiation and Matter Interaction

• Faculty of Medicine of Mostaganem presents information about interaction between radiation and matter, presented by Dr. Nouairi Hafida.

Objectives

• To describe Compton and photoelectric effects.
• To describe the interactions of particles with matter.
• To use the law of radiation attenuation.
• To cite the consequences of radiation/matter interaction.

Plan

• Introduction
• Definition of radiation interaction with matter
• Interaction between photons and matter
• Interaction between particulate radiation and matter
• Laws of attenuation of a photon beam
• Conclusion

Introduction

• The interaction between radiation and matter results in energy transfer.
• Interaction is essential for radiation detection, crucial in diagnostic imaging
• Energy transfer is the first step in how radiation affects biological systems.
• Radiation interacts with matter either directly which is obligatory or indirectly.

Types of Radiation

• Ionizing radiation includes directly ionizing charged particles and indirectly ionizing non-charged particles.
• Non-ionizing radiation includes visible light and infrared radiation.

Definition of Interaction

• Interaction refers to any phenomenon occurring when radiation passes through any medium.
• Interactions manifest through energy exchanges.
• The nature of radiation whether REM or particle, radiation properties like charge and characteristics of the traversed matter determines the presence of a phenomenon.

Photon Interaction with Matter

• A photon can deposit all its energy through total absorption.
• A photon can deposit part of its energy, resulting in diffused radiation
• A photon can be transmitted without interaction, constituting primary radiation

Photon-Matter Interaction Mechanisms

• Excitation
• Photoelectric effect
• Compton effect
• Pair production

Excitation

• REM provides energy to an atom. If E(hv) < E(L), electrons move to farther orbits than their fundamental level
• The atom enters an excited state and returns to normal through fluorescence emission.

Photoelectric Effect

• A photon interacts with an electron bound to an atom where E(hv) ≥ E(L).
• The incident photon disappears after transferring all its energy to the electron.
• The ejected electron carries kinetic energy where E(c) = E(hv) - E(L).
• This mechanism dominates for low-energy photons interacting with deep shell electrons.

Compton Effect

• An incident photon hits an electron with energy E(hv) >> E(L) where the electron is removed from its orbit, moving directionally with kinetic energy
• The incident photon deflects at an angle, keeping energy not transferred to the electron.
• Prevails for high-energy photons interacting with peripheral electrons.

Consequences of Photon Interaction

• After expelling an electron from the electronic cloud, an electron from the outer shell fills its place
• This leaves a vacancy to be filled by an even more external electron.
• The process results in characteristic or fluorescence rearrangement electronic emission.

Auger Effect

• The interaction of a fluorescence photon from electronic rearrangement with an electron happens in the same atom.
• Results in the ejection of an Auger electron.

Pair Production

• If an incident photon with energy exceeding 1.022 MeV passes near an atom's nucleus, it creates a pair of ions: a positron and a negatron.

Creation of Pairs

• A positron is quickly drawn to an electron, resulting in annihilation which is marked by emitting two photons, each with 0.511 MeV, emitted at 180° to each other.
• This is the base principle for PET scans.

Interaction Domain

• The photoelectric effect dominates with photons with a low energy which is less than 100 KeV and high Z.
• Compton effect dominates with average energy between 100 to 1 MeV and average Z.
• Pair production dominates with high energy exceeding 1.02 MeV and high Z.

Particulate Radiation-Matter Interactions

• The interaction between an ionizing material particle and matter is characterized by energy transfer due to direct collisions or distance collision.
• The process slows the particle and causes physical effects such as ionization until the particle stops.
• Electrons in the periphery or the nucleus are involved.

Interaction with Atom Electrons

• If ΔE is the energy transferred by an incident particle to a target electron with binding energy E(l) and the three possible outcomes are:
• If ΔE ≥ E(l), ionization happens. The electron is ejected with kinetic energy E(c) = ΔE - E(l), further causing secondary ionizations.
• If ΔE < E(l), excitation occurs.
• If ΔE is very small, thermal dissipation results.

Interaction with Nucleus

• A charged particle close to a nucleus is deflected, slowing down.
• Slowing results in an energy release as "Bremsstrahlung" radiation, the principle behind X-ray production.

Linear Energy Transfer (LET)

• The quantity of energy transferred by a particle to the medium along its path or trajectory.
• Formula: TEL = dE/dx (keV/µm).
• During each interaction, the particle transfers part of its energy to the medium until its speed reaches zero.

Measuring LET

• LET is measured by linear ionization density (DLI).
• Formula: TEL = DLI × ω, where ω is the average ionization energy, 34 eV in air.

Linear Ionization Density (DLI)

• The number of ion pairs created per unit length, measured in pairs of ions per µm.
• Formula: DLI = dN/dx.
• DLI increases towards the end of the particle path and the maximum ionization occurs at the end of the range as shown by the Bragg curve.

Beta Radiation

• Consist of light electrons with either a negative charge also known as electrons (β-) or a positive charge which is also known as positrons (β+).
• Beta radiations are weakly ionizing, so their path through matter is a broken line with segment length decreasing.
• Range for beta radiation is up to 1.5 cm.

Types of Beta Interaction

• Beta particles undergo two types of interactions with atoms whether colliding with cloud electrons or experiencing proximity braking to the nucleus.
• Both lead to X-ray formation.

Beta + Radiation

• Beta + radiation annihilates matter when it encounters electrons creating two gamma rays.

Laws of Photon Beam Attenuation

• When a narrow, unidirectional beam of monoenergetic photons passes through material, a part is stopped, another part is deflected or diffused, and another part remains transmitted in original direction without deviating.
• The beam's total energy is reduced or attenuated.

Attenuation Law

• The random nature of photon interactions leads to an exponential attenuation law.
• N = N(o)e^(-µx) is the number of photons that will pass, given a screen of width x with a linear attenuation coefficient of µ, when receiving N(o) photons.
• The width x is measured in cm, and the coefficient µ in cm^-1, where N or N(o) can be replaced by I or E.

Attenuation Coefficient

• The linear attenuation coefficient µ determines the opportunity of photon's interaction with material. This coefficient relies on both the nature and the energy of the photons that are arriving.
• µ/p indicates mass attenuation where the material measured in cm²/gr. The given formula is E = E(o)e^-(µ/p)x where x represents density in matter.

Half-Value Layer

• Layer thickness where only half of the amount original incident photons get to pass.
• N(cda)=N(o)e^-(µp)CDA is the amount of the original photons that are passing from an incidents medium that is being measured and N(o)/2=N(o)e^-(µ/p)CDA
• µ CDA = ln 2 and CDA = Ln 2/µ

Factors Affecting CDA

• CDA value relies on the absorbing environments state such as a gas and whether it is liquid or is a solid of an atomic amount of Z alongside the amount that is being emitted from photon itself.
• It is impossible to completely stop photon beam

Conclusions

• The study of interaction phenomena among different radiation and differing material made significant advancements among medical practices and the field of medicine.
• The formation of rays of X alongside of the principle of the positron emission tomography called TEP are two of the diagnostic and radiological effects base as it is on the photoelectric effects
• Also, the ionising capability alongside damaging power are a foundation in both radio therapy alongside radio bioligy.
• Phenomena such as the creation of diffused radiation through Compton and the mitigation of emitted beam from the the photons have allowed to promote radiation safety.