Production of X-Rays
By- Sidhartha Dev Pattanaik
Department of Radiotherapy
VIMSAR, Burla
X-Ray Discovery
• X-rays were discovered on November 8, 1895 by Wilhelm
Roentgen (1845–1923) from the University of Wurzburg. He
made this discovery by accident. While using a vacuum tube
covered with cardboard to block visible and UV light, he
noticed that a screen placed nearby started to glow
(fluoresce).
• When Roentgen accidentally placed his hand between the
tube and the screen, he saw the outline of his bones — his
skeleton — on the screen. This mysterious type of radiation
was later called X-rays (sometimes also called Roentgen rays).
• X-rays also affected photographic film, so the images they
created could be saved. The first X-ray image ever taken was
of the hand of his wife, Anna Bertha Roentgen. Roentgen
officially presented his discovery in a research paper at the end
of December 1895.
Properties of X-rays
• X-rays are invisible. • X-rays darken photographic film just like visible
• They have no mass. light.
• They are absorbed by metal and bone, which is
• X-rays travel at the speed of light in a vacuum.
why bones show up on X-ray images.
• They move in straight lines.
• X-rays can cause photoelectric emission.
• X-rays have a very short wavelength.
• They are produced when high-energy electrons
• They are not affected by electric or magnetic hit a metal target.
fields.
• X-rays cannot be refracted.
These properties make X-rays very useful for
• X-rays cause ionisation (they can add or medical diagnosis and treatment.
remove electrons from atoms or molecules).
• They pass through healthy body tissue.
Electron Interaction with the target
When the electron arrives at the target ,
it interacts in four ways:
1. Ionization of target atoms
2. Production of characteristic X-rays
3. Interaction with nuclear field
4. Interaction with nucleus
BREMSSTRAHLUNG X-RAYS
• When an incident electron reaches nearer to nucleus of
an atom in the target, since the electron is a negative
particle, it is attracted by the positive nucleus. It is made
to orbit partially around the nucleus, decelerates and
goes out with reduced energy. The loss of energy
appears in the form of X-ray photons, these X-Rays are
known as Bremsstrahlung X rays.
• The photon energy can take any continuous value from
a certain minimum to maximum value .
• The energy of the X ray photon depends on the degree
to which the electron is decelerated by Nuclear
attraction.
• Bremsstrahlung is a German word, meaning breaking
radiation.
• Some electrons stop in one impact and release all their energy at once, while others lose
fractions of their total energy in successive deflections until all energy is spent. The electrons
which are stopped in one impact produce photons of maximum energy, i.e., x-rays of minimum
wavelength. Such electrons transfer all their energy eV into photon energy so that
SWL : short-wavelength limit
Characteristic X-rays:
Comparison between Continuous X-Ray and Bremsstrahlung X-Ray
Feature Characteristic X-ray Bremsstrahlung X-ray
Produced when an inner-shell electron is ejected and an Produced when a high-speed electron is decelerated or deflected
Origin outer-shell electron fills the vacancy, releasing energy as X-ray by the electric field of the nucleus, releasing energy as X-ray
photons. photons.
Discrete energies (specific to the target atom’s electron Continuous spectrum of energies from zero up to the maximum
Energy Spectrum
binding energies). electron energy.
Strongly dependent on the atomic number (Z) of the target Depends on Z for intensity (higher Z → higher output) but not for
Dependence on Target Material
(binding energies differ for each element). the continuous shape of the spectrum.
Occurs only if the incident electron energy exceeds the No specific threshold energy (occurs at any electron energy,
Threshold Energy
binding energy of the target atom’s inner shell. though intensity increases with higher energy).
Production Mechanism Electron–electron interaction (collision with bound electron). Electron–nucleus interaction (deceleration in nuclear field).
Sharp peaks (e.g., Kα, Kβ lines) superimposed on the
Appearance in X-ray Spectrum Smooth continuous background spectrum.
continuous spectrum.
Dominant at higher tube voltages above threshold for specific Dominant at lower tube voltages and for most of the spectrum in
Relative Contribution
shells (and in high-Z targets). diagnostic radiology.
Equal to the difference in binding energies of the two shells Can take any value up to the maximum kinetic energy of the
Energy Values
involved in the transition. incoming electron.
X-Ray Attenuation Factors Affecting Absorption
• X-rays are high-energy photons that can pass through a) Photon Energy: Lower energy → higher
matter. As they travel through material, their intensity absorption (photoelectric effect).
decreases due to absorption and scattering. Absorption b) Atomic Number (Z): Higher Z → higher
is a key principle behind medical imaging and radiation
protection. probability of photoelectric absorption.
c) Density: Denser material → more attenuation.
• When an X-ray beam interacts with matter, photons d) Electrons per gram: More electrons per gram
lose energy mainly by Photoelectric Effect, Compton
Scattering and Pair Production (only above 1.022 MeV). → more attenuation
Linear Attenuation Coefficient ( μ ) Mass Attenuation Coefficient ( μ/ρ )
• The linear attenuation coefficient is a measure of • The mass attenuation coefficient is the linear attenuation
how easily a material can attenuate (reduce the coefficient divided by the density of the material.
intensity of) an X-ray or gamma ray beam. It expresses how easily a material attenuates photons per
unit mass rather than per unit thickness.
• It is the fractional decrease in intensity per unit
thickness of the absorbing material.
• Units
cm⁻¹ or m⁻¹
Half-Value Layer (HVL)
Half-Value Layer (HVL) is defined as the thickness of an absorber required to reduce the
intensity of a radiation beam to one-half of its original value.
Tenth-Value Layer (TVL)
Tenth-Value Layer (TVL) is the thickness of absorber needed to reduce the intensity of
radiation to one-tenth of its original value.
