Radioactivity and X-ray Interaction with Matter

Radioactivity and X-ray Interaction with Matter

RADIATION SCIENCE

CHAPTER 2

• Radioactivity
• Thermionic Emission
• Photoelectric Effect

✓Compton Scattering

• Beam Attenuation
• Half Value Layer
• X-ray interaction with matter✓Radioactivity
• Radioactivity is the spontaneous disintegration of
• unstable atomic nuclei, releasing energy in the form of
• radiation.
• It was discovered in 1896 by Henri Becquerel and later
• studied by Marie and Pierre Curie.
• Atoms that undergo radioactive decay are called
• radioisotopes.
• Causes of Radioactivity
• Nuclei are radioactive if they are unstable due to:
• Unfavorable proton-to-neutron ratio.
• Excess energy within the nucleus.• To achieve stability, these nuclei disintegrate and emit
• radiation.
• Types of Radioactive Radiation
• (a) Alpha (α) Particles
• Composition: 2 protons + 2 neutrons (Helium nucleus).

• Charge: +2.

• Mass: Heavy.
• Penetration: Very low (stopped by paper or skin).
• Effect: Highly ionizing, dangerous if ingested or inhaled.(b) Beta (β) Particles
• Two types:
• o β- (electron emission): Neutron → Proton + Electron +
• Antineutrino.
• o β+ (positron emission): Proton → Neutron + Positron +
• Neutrino.
• Charge: -1 (β-) or +1 (β+).
• Penetration: Moderate (stopped by aluminum sheet).
• Effect: Medium ionizing power.
• (c) Gamma (γ) Rays
• Composition: Electromagnetic radiation (photons).

• Charge: None.

• Mass: None.
• Penetration: Very high (needs lead or thick concrete to
• stop).
• Effect: Low ionizing but very penetrating.Units of Radioactivity
• Becquerel (Bq): 1 disintegration per second.
• Curie (Ci): 3.7 × 10¹⁰ disintegrations per second.
• Gray (Gy): Unit of absorbed dose (1 Gy = 1 J/kg).
• Sievert (Sv): Biological effect of absorbed dose (dose
• equivalent).
• Detection of Radioactivity
• Geiger–Müller counter
• Scintillation counter
• Cloud chamber
• Film badges (for monitoring exposure in
• radiology/medicine)
• Applications of Radioactivity
• 1.Medicine
• o Cancer treatment (radiotherapy, e.g., Cobalt-60).
• o Diagnostic imaging (PET scan using positron
• emitters).
• 2.Industry
• o Tracers for leaks.
• o Thickness control in manufacturing.
• 3.Agricultureo Food preservation (irradiation).
• o Mutation breeding.
• 4.Archaeology & Geology
• o Carbon-14 dating.
• o Uranium-lead dating of rocks.
• 5.Energy
• o Nuclear power generation.
• Dangers of Radioactivity
• Causes cell damage, cancer, and mutations.
• Acute exposure → Radiation sickness (nausea, hair loss,
• death at high doses).
• Requires strict safety measures: shielding, monitoring,
• and controlled exposure✓Thermionic Emission
• Thermionic emission is the release (emission) of
• electrons from the surface of a metal when it is heated to
• a high temperature.
• At high temperatures, electrons gain sufficient
• kinetic energy to overcome the work function (the
• minimum energy required for an electron to escape the

metal surface).

Basic Principle

• Metals have free electrons in the conduction band that
• move randomly.• At ordinary temperatures, these electrons lack enough
• energy to escape.
• When heated strongly:
• o Electrons gain energy from thermal vibrations.
• o If energy > work function → electrons are emitted

from the surface.

Factors Affecting Thermionic Emission

• 1.Temperature – higher temperature increases emission.
• 2.Work function of material – metals with lower work
• function emit electrons more easily.
• 3.Surface condition – impurities or oxides can hinder
• electron escape.
• 4.Vacuum condition – prevents electrons from colliding
• with air molecules.Applications of Thermionic Emission
• 1.Cathode Ray Tubes (CRT)
• o Used in old television and oscilloscope screens.
• o Electrons emitted by heated filament are accelerated
• and focused to form images.
• 2.X-ray Tubes
• o Thermionically emitted electrons from a heated
• cathode are accelerated to strike a metal target →
• production of X-rays.3.Vacuum Tubes / Valves
• o Early amplifiers, rectifiers, and oscillators.
• 4.Electron Microscopes
• o Hot filament acts as electron source for imaging.
• 5.Radio Transmitters
• o Thermionic valves were used before semiconductor✓X-ray Interaction with Matter
• X-rays interact with matter in the following five ways:
• 1.Coherent scattering
• 2.Compton scattering
• 3.Photoelectric effect
• 4.Pair production
• 5.Photodisintegration.
• Only Compton scattering and photoelectric effect are
• important in making an x-ray image (diagnostic
• radiology).
• The probability and type of interaction to occur
• depends on;
• a)X-ray photon energy
• b)The atomic number (Z)
• c)Density of the material.
• d)Thickness of the material
• Mechanisms of X-ray Interaction with Matter
• 1.Coherent (Classical/Rayleigh) Scattering
• Process:
• Occurs when a low-energy X-ray photon (<10
• keV) interacts with atoms.
• The photon changes direction but does not lose
• energy.❖There is no energy transfer and therefore no
• ionization.
• Result:
• Only contributes slightly to image noise, the
• general graying of an image that reduces image
• contrast.
• Significance:
• Coherent scattering is of little importance

(negligible effect) to diagnostic radiology.❖This is because coherent scattering primarily involves low-energy x-rays, which contribute little to the medical image.

• 2.Compton Scattering
• Process:
• In Compton scattering, the incident x-ray photon
• interacts with an outer-shell electron and ejects it
• from the atom, thereby ionizing the atom.
• X-rays at the diagnostic range can undergo
• Compton scattering but mainly the moderate
• energy x-rays.
• Dependence:
• Probability depends mainly on electron density,
• not atomic number.
• Dominates at moderate photon energies (20–150
• keV).
• Significance:
• Major source of scatter radiation in diagnostic
• imaging.
• Reduces image contrast.
• Primary contributor to radiation dose to staff.
• 3.Photoelectric Effect
• Process:
• An X-ray photon transfers all its energy to a
• tightly bound inner-shell electron.
• The electron is ejected (photoelectron).
• The incident photon is totally absorbed.
• The vacancy is filled by another electron,
• releasing characteristic radiation or Auger
• electron.Significance:
• Main contributor to image contrast in diagnostic
• radiology (especially bone vs. soft tissue).
• Also increases patient dose because energy is
• absorbed.
• Most important process in diagnostic radiology.
• 4.Pair Production
• Process:
• Occurs when photon energy > 1.022 MeV.
• Photon interacts with the nucleus’ electric field and
• converts into an electron-positron pair.
• Significance:
• Not relevant in diagnostic radiology (too high
• energy).
• Important in radiation therapy and PET imaging.5.Photodisintegration
• Process:
• Occurs at photon energies > 10 MeV.
• Photon is absorbed by nucleus, causing it to emit
• a nucleon (proton or neutron).
• Significance:
• Only relevant in high-energy radiation therapy.Factors Affecting X-ray Interaction
• 1.Photon Energy (kVp)
• Low kVp → More photoelectric effect (high contrast,
• high dose).
• High kVp → More Compton scattering (lower
• contrast, lower dose).
• Very high energies (>1 MeV) → Pair production,
• photodisintegration.
• 2.Atomic Number (Z) of absorber
• Higher Z → Stronger photoelectric effect.
• Example: Bone (calcium, high Z) absorbs more than
• soft tissue (low Z).
• 3.Density of absorber
• Denser material → More interactions.
• 4.Thickness of absorber
• Thicker material → More interactions.Practical Applications in Radiology

• Image Contrast:

• Determined mainly by balance between photoelectric
• effect (contrast) and Compton scattering (degrades
• contrast).
• Radiation Protection:
• Lead (high Z) is effective shielding → photoelectric

absorption dominates.

• Clinical Imaging:

• Low kVp for extremities → maximize photoelectric
• effect.
• High kVp for chest → minimize absorption, allow
• penetration.
• Radiotherapy:
• Pair production and photodisintegration become
• important at therapeutic energies.✓Beam Attenuation
• Definition
• Beam attenuation is the reduction in the intensity
• (quantity) of an X-ray beam as it passes through matter.
• Occurs due to absorption (mainly photoelectric effect)
• and scattering (mainly Compton).
• Process
• As X-rays pass through tissue, some photons are
• removed from the primary beam:
• Absorbed → contribute to patient dose.
• Scattered → reduce image quality and increase staff
• exposure.✓ Half Value Layer (HVL)
• Definition
• The thickness of a specified material required to reduce
• the intensity of an X-ray beam to one-half of its original
• value.
• Units
• Measured in mm of Aluminum (mm Al) for diagnostic Xrays.
• For higher energies (therapy), expressed in mm of lead
• (Pb) or cm of concrete.Significance
• 1.Beam Quality Indicator
• HVL measures the penetrability (hardness) of the
• beam.
• Higher HVL = more penetrating (harder beam).
• 2.Filtration Effect
• Adding filters (e.g., Al sheets) increases HVL by
• removing low-energy photons.
• 3.Radiation Protection
• Determines shielding requirements.
• 4.Quality Control
• Routine HVL testing ensures X-ray equipment is

producing beams of appropriate quality.