2. • Radiation therapy means using radiation for treatment of malignant and some benign
conditions.
✓An understanding of the particles and processes involved in imparting radiation energy to
matter is fundamental to the clinical application of ionizing radiation to patients.
3. • In the irradiation of a biological system, physical and biological events occur in the following
order:
✓Physical events: Physical interactions (e.g., photoelectric, Compton, collisional) result in
ionizations and radiation dose.
✓Chemical events: Ionizations result in broken atomic and molecular bonds or chemical
changes.
✓Biological events: Changes in the chemistry of molecules result in changes in biological
function (i.e., cells have improper or changed function).
✓Clinical events: Biological alteration may result in clinical changes, such as tumor regression,
cancer induction, or tissue fibrosis.
4. Radiation Physics
• Radiation oncology as a field uses energy in the form of ionizing radiation delivered to a
target for cure or palliation.
• A basic understanding of the physical properties of radiation is critical to understand:
✓ What this radiation is.
✓ How it is produced
✓ How it reacts with tissue.
5. • Medical physics is largely, but not exclusively, based on the study and use of ionizing
radiation in medicine;
• Health physics deals with health hazards posed by ionizing radiation and with safety issues
related to use of ionizing radiation
7. The absolute basics Elements and compounds
• Everything is made up of matter. There are two types of matter — elements and compounds.
• An element is a kind of matter that cannot be decomposed into two or more simpler types of
matter. An example of an element is hydrogen.
• A compound is a kind of matter that can be decomposed into two or more simpler types of
matter.
✓A compound is formed when two or more elements combine to produce a more complex
kind of matter. An example of a compound is water, which can be broken down into the two
elements, hydrogen and oxygen.
8.
9. Atoms and molecules
• Atoms are the very smallest particles of an element that can exist without losing the
chemical properties of the element.
• There are 114 types of atom, all defined in the periodic table by their atomic
numbers. The periodic table arranges the atoms in groups and in periods.
✓The rows are called periods and the columns are called groups.
✓Elements/atoms in the same groups are like each other.
10.
11. • Molecules are the smallest particles of a compound that can exist without losing
the chemical properties of that compound — for example, the water molecule
consisting of two hydrogen atoms and one oxygen atom. If the molecule is broken
down further the resulting matter loses the properties of water.
12. Atomic substructure
• Atoms can be broken down into smaller particles. These particles are neutrons,
protons and electrons.
• Neutrons and protons are in the nucleus of the atom and are surrounded by the
electrons.
✓Protons are relatively large particles and have a positive charge.
✓Neutrons are also ‘ large ’ but have no charge.
• Electrons are relatively much smaller and lighter particles. They are attracted to the
nucleus because they have a negative charge, but do not collide with it because the
electrons orbit the nucleus.
13.
14. Atomic and mass numbers
• Each atom has a particular number of protons and neutrons.
• The atomic number is the number of protons in the nucleus, Z.
• The mass number of an atom is the number of protons and neutrons added
together, A.
• The atomic and mass numbers for an atom X are depicted as:
• The atomic number, i.e., the number of protons in an atom, defines the
atom/element.
15. • The atomic mass M is smaller than the sum of individual masses of constituent
particles because of the intrinsic energy associated with binding the particles
(nucleons) within the nucleus.
• The atomic mass is larger than the nuclear mass because the atomic mass includes
the mass contribution by Z orbital electrons while the nuclear mass does not.
16. • If the number of protons is somehow changed, the atom changes into that of another
element.
• In contrast, if the number of neutrons is changed, the atom remains the same, but may
have some different characteristics. Atoms with the same atomic number but different
mass numbers are called isotopes.
• The total number of protons and neutrons determine the nuclide.
• The number of neutrons relative to the protons determines the stability of the nucleus,
with certain isotopes undergoing radioactive decay.
17. • The term nuclide refers to all atomic forms of all elements.
• An element may be composed of atoms that all have the same number of protons, i.e.,
have the same atomic number Z, but have a different number of neutrons, i.e., have
different atomic mass numbers A.
18. Electron shells and energy levels
• Electrons reside around the nucleus in number of ‘ shells ’ . They cannot exist
between these shells.
• The shells are labelled with letters of the alphabet, starting with K at the inner shell.
• Each shell can hold a maximum number of electrons.
• Most shells are made up of sub-shells.
✓The shell closest to the nucleus (K) has one shell, which can hold a maximum of 2
electrons. The next shell out (L) has two sub-shells — one holding a maximum of 2
and the second capable of holding a maximum of 6 electrons. The next shell (M) has
3 sub-shells, holding 2, 6 and 10 electrons.
19.
20. Electron binding energy
• Electrons are bound to the nucleus by the attraction between negative and positive
charge.
• This attraction means that it takes energy from outside to separate the nucleus from
the electron.
• Electron binding energy is the energy required to knock an electron loose:
✓It increases with proximity to the nucleus by radius squared (r2).
✓Electron binding energy increases with increasing charge of the nucleus (Z).
✓The binding energy is greatest for the inner shell and is progressively lower for each
shell moving away from the nucleus.
21. Inner shell electrons
• have a large binding energy because they are very close to the nucleus.
• Even though they have a higher “binding energy” these electrons are said to be at
a “lower energy level”.
Valence (outer) electrons
• Have little binding energy because they are further away and are easily removed.
• Any change in orbit is associated with a change in energy.
• Pushing energy into an atom can knock an electron loose from its valence shell (or
raise the shell to a higher shell).
22. • When an electron moves from a higher shell to a lower shell, it gives off energy,
either in the form of a photon or by kinetic energy and knocking another electron to
a higher shell.
• Binding energies are greater for atoms with a greater number of protons in the
nucleus (i.e., a higher atomic number) because they have a higher positive nuclear
charge, and therefore a greater hold on the orbiting electrons.
• If an electron gains more energy than the binding energy, it can escape from the
attraction of the nucleus and leave the atom. This is called ionization .
• The resulting atom has a net positive charge because it has one less electron than it
has protons — i.e., it is a positive ion.
23.
24. Energy levels
• An electron can also move between shells of different binding energies.
• This happens when an electron gains enough energy to move from one (sub-) shell
to another, but not quite enough to escape the atom completely.
• Each (sub-) shell can therefore be seen as a fixed energy level and electrons can only
exist in these shells if they possess that amount of energy.
• The energy levels are fixed for any atom.
25. • As well as moving from a lower energy level to a higher energy level by gaining
energy from somewhere, electrons can move the other way and release their excess
energy. 1 electron volt (eV) is equal to 1.602 x10 -19 Joules.
26. Electromagnetism, electromagnetic radiation and the electromagnetic spectrum
• There are four fundamental forces of nature:
✓Gravity,
✓Electromagnetism,
✓Weak interaction,
✓Strong interaction.
• They are termed ‘fundamental’ because, they cannot be explained or picked apart
by other forces.
• In order of descending strength these are:
27. 1. Strong Nuclear Force:
✓ The strongest force in nature; “glues” the nucleus together.
✓ Holds the nucleus together, counters the repulsive effect of protons’ positive
charge.
28. 2. Electromagnetic (Coulombic) Force:
✓ ~1/100 as strong as the strong force.
✓ Opposites attract. Electrons are attracted by the positively charged nucleus and
are more attracted as they get closer; Valence electrons are not strongly attracted,
and their movements are responsible for all chemical reactions.
✓ Protons repel each other within the nucleus but are held in place by the strong
force.
29. 3. Weak Nuclear Force:
✓ ~1/1,000,000 as strong as the strong force.
✓ Works inside particles (between quarks) and is responsible for radioactive decay.
4. Gravity:
✓ ~1ᵡ10−39
as strong as the strong force.
✓Not important on the atomic scale
31. • Electromagnetic radiation is a form of energy transfer though space as a
combination of electrical and magnetic fields.
• A moving electrical field generates a varying magnetic field and vice versa. These
combined moving fields form the electromagnetic wave.
• The inexplicable feature of electromagnetic radiation is that it sometimes behaves as
waves and sometimes behaves as particles — summed up in the term ‘wave-particle
duality’.
32.
33. The wave model of electromagnetic radiation
• Electromagnetic radiation causes effects that suggest it behaves as waves.
• For example, it exhibits reflection, refraction and interference.
• All electromagnetic waves travel at a velocity of 3 × 10 8 metres per second in a
vacuum.
34. Waves
• Waves are a series of peaks and troughs and have definable features: Wavelength,
Frequency, Energy.
• Wavelength is the distance between two successive crests or troughs. The symbol is λ and
it is measured in meters.
• Frequency is the number of waves passing a particular point in unit time. The symbol is ν
and the unit is number per second or hertz (Hz).
• The amplitude can be thought of as the energy of the wave
35.
36. The particle behavior of electromagnetic radiation
• Electromagnetic radiation also behaves as particles.
• These particles are discrete packets of energy and are called photons.
✓The energy of these photons is proportional to the frequency of the electromagnetic
wave to which they are linked. So, a short wavelength relates to high energy photons
and a long wavelength to low energy photons.
• There is an equation that relates the energy and frequency — the Planck-Einstein
equation,
✓E=h. v
37. ✓where E is energy, h is Planck's constant (6.626 × 10 −34 Joules per second (J s -1 ))
and v is frequency.
• So, if frequency is the velocity divided by the wavelength,
✓E=h. c / λ
✓where c is the speed of light and λ is the wavelength of the wave.
• In the realm of electromagnetic radiation, the velocity is constant, so frequency and
wavelength vary together.
• At high frequencies and short wavelengths, and therefore higher energies,
electromagnetic radiation has more particle-like behavior.
38. • The range of frequency and wavelengths is called the electromagnetic spectrum.
• Humans have evolved to detect part of this spectrum — visible light.
• The rest of the electromagnetic spectrum on either side of either side of visible light
cannot be sensed.
39. The electromagnetic spectrum
• comprises all types of electromagnetic radiation, ranging from radio waves (low
energy, long wavelength, low frequency) to ionizing radiations (high energy, short
wavelength, high frequency).
• In order of increasing energy: Radio waves! Microwaves !infrared! rainbow colors,
light ! UV rays ! x-rays, gamma rays and Cosmic rays.
✓As a side-note, UV radiation can still cause chemical reactions by exciting valence
electrons, altering chemical bonds without actually ionizing.
✓Therefore, sun-tanning is bad and still cancer-causing even though there is no
“ionizing” radiation involved.
40.
41.
42. • Electromagnetic radiation can also be subdivided into ionizing and nonionizing
radiations.
Types of nonionizing electromagnetic radiation
•Radio waves
•Microwaves
•Infrared light
•Visible light
•Ultraviolet light
43. Types of Ionizing electromagnetic waves :
• Gamma rays: Photons resulting from nuclear transitions.
• Annihilation quanta: Photons resulting from positron–electron annihilation.
• Characteristic(fluorescence)x rays : Photons resulting from electron transitions
between atomic shells.
• Bremsstrahlung x rays: Photons resulting from electron–nucleus Coulomb
interactions.
44. Common features of electromagnetic radiation :
• It propagates in a straight line.
• It travels at the speed of light (nearly 300,000 km/s).
• It transfers energy to the medium through which it passes, and the amount of
energy transferred correlates positively with the frequency and negatively with the
wavelength of the radiation.
• The energy of the radiation decreases as it passes through a material, due to
absorption and scattering, and this decrease in energy is negatively correlated with
the square of the distance traveled through the material.
45. The Essence of Radioactivity
• The sub-atomic particles exist in a particular arrangement.
• The amount of energy in the particles can vary with the arrangement.
• They will always try to settle in an arrangement that has the lowest energy
configuration.
• Some nuclides have unstable nuclear arrangements and shift to a more stable
arrangement over time.
46. • While undergoing this rearrangement they emit one of the following:
✓An alpha particle: consisting of two protons and two neutrons.
✓A beta particle: an electron.
✓A gamma ray: a packet of electromagnetic energy i.e. a photon.
47. • Any element that undergoes this process is called radioactive, and the phenomenon
is called radioactivity.
✓Another way of looking at radioactive materials is that they continuously emit
energy in the form of the alpha particles, beta particles or electromagnetic waves.
• Radioactivity is the spontaneous decay of the nucleus of an atom from which either
alpha, beta or gamma rays are emitted, though all processes may be occurring
simultaneously in a sample of radioactive material.
48. Radioactive decay
•The property of unstable nuclides during which they undergo a spontaneous
transformation within the nucleus. This change results in the emission of energetic
particles or electromagnetic energy from the atoms and the production of an
altered nucleus.
49. • Radionuclides may decay by any one or a combination of six processes:
✓ Spontaneous fission
✓α-decay
✓ β–-decay
✓ β+-decay
✓ electron capture
✓ isomeric transition (IT)
•In all decay processes, the energy, mass, and charge of radionuclides must be
conserved.
50. Spontaneous Fission
• Fission is a process in which a heavy nucleus breaks down into two fragments typically in
the ratio of 60:40.
• This process is accompanied by the emission of two or three neutrons with a mean
energy of 1.5 MeV and a release of nearly 200-MeV energy, which appears mostly as
heat.
• Fission in heavy nuclei can occur spontaneously or by bombardment with energetic
particles.
52. Alpha Decay (α-Decay)
• Usually heavy nuclei such as Radon, Uranium, Neptunium, and so forth decay by α-
particle emission.
• The α-particle is a helium ion with two electrons stripped off the atom and contains two
protons and two neutrons bound together in the nucleus.
• In α-decay, the atomic number of the parent nuclide is therefore reduced by 2 and the
mass number by 4.
53. • An example of α-decay is :
• An α-transition may be followed by β–-emission or γ-ray emission or both.
• The α-particles are monoenergetic, and their range in matter is very short (on the order of
10−6 cm) and is approximately 0.03 mm in body tissue.
54. Beta Decay (β–-Decay)
• When a nucleus is “neutron rich” (i.e., has a higher N /Z ratio compared to the stable
nucleus), it decays by β−-particle emission along with an antineutrino.
• An antineutrino ( v ) is an entity almost without mass and charge and is primarily needed
to conserve energy in the decay.
• In β−-decay, a neutron (n) essentially decays into a proton (p) and a β−-particle; for
example,
55. Positron or β+-Decay
• Nuclei that are “neutron deficient” or “proton rich” (i.e., have an N /Z ratio less than that
of the stable nuclei) can decay by β+-particle emission accompanied by the emission of a
neutrino (v), which is an opposite entity of the antineutrino.
56. Electron Capture
• When a nucleus has a smaller N /Z ratio compared to the stable nucleus, as an alternative
to β+-decay, it may also decay by the so-called electron capture process, in which an
electron is captured from the extranuclear electron shells, thus transforming a proton into
a neutron and emitting a neutrino.
57. Isomeric Transition
• A nucleus can remain in several excited energy states above the ground state that are
defined by quantum mechanics.
• All these excited states are referred to as isomeric states and decay to the ground state,
with a lifetime of fractions of picoseconds to many years.
• The decay of an upper excited state to a lower excited state is called the isomeric transition.
58. • In isomeric transition, the energy difference between the energy states may appear as γ-
rays.
• When isomeric states are long lived, they are referred to as metastable states and can be
detected by appropriate instruments.
• The metastable state is denoted by “m” as in 99mTc.
•In radioactive decay, particle emission or electron capture may be followed by isomeric
transition.
59.
60. Units of radioactivity
• The activity of a quantity of radioactive material is expressed in terms of the number of
spontaneous nuclear transformations taking place in unit time.
• The SI unit of activity is the becquerel (Bq), a special name for the reciprocal second (s-1).
•The expression of activity in terms of the becquerel therefore indicates the number of
transformations per second.
•The historical unit of activity is the curie.
•The curie (Ci) is equivalent to 3.7 x 1010 Bq.
61. Activity and half-life
• The activity of a radioactive material is measured as the number of nuclei that disintegrate
per second.
• The SI unit of activity is the becquerel, the symbol is Bq.
• The activity of any radioactive material reduces with time.
• The activity at any time is dependent on the number of nuclei present at that time. The
proportion of nuclei undergoing disintegration remains constant. This leads to a pattern of
decay called ‘ exponential decay ’.
•Half-life is defined as the time for a radioactive material to lose half of its activity, which is
the same as saying it is the time for half the nuclei in a material to decay.
62.
63. The four “isos”:
• Isotope: same number of protons, different neutrons. Same chemical behavior, different
mass, and different nuclear decay properties.
✓Ex: 125 I and 131 I, both behave like iodine but have different half-lives.
• Isotone: same number of neutrons, different protons.
✓Rarely used.
• Isomer: same nuclide, different energy state (excited vs. non-excited)
✓Isomers release their energy through gamma decay.
✓Ex: 99mTc decays to 99Tc, releasing its excess energy without changing the
✓number of protons or neutrons.
64. • Isobar: same number of nucleons, different nuclide (more protons and less neutrons, or vice
versa).
✓“bar” = same mass—think barbell.
✓Beta decay and electron capture always result in an isobar.
✓Ex: 131I decays to 131Xe, which has the same mass number but is a different nuclide and has
different chemical properties.
66. Radiation
• It is the propagation of energy from a radiative source to another medium.
• This transmission of energy can take the form of particulate radiation or non
particulate radiation (i.e., electromagnetic waves).
• The photon
✓ is the smallest unit of electromagnetic radiation .
✓ Photons have no mass.
67. Classification of radiation
• Depending on its ability to ionize matter radiation is classified into two main
categories:
✓Ionizing radiation
✓Nonionizing radiation
68. • Ionizing radiation
✓Can ionize matter either directly or indirectly because its quantum energy exceeds
the ionization potential of atoms and molecules of the absorber.
✓The ionization energy (IE), also known as ionization potential (IP), of atoms is
defined as the minimum energy required for ionizing an atom and is typically
specified in electron volts (eV).
✓In nature IE ranges from a few electron volts (∼4 eV) for alkali elements to 24.6 eV
for helium (noble gas) with IE for all other atoms lying between the two extremes.
69. ✓ Ionizing radiation can be further divided into
➢ Directly ionizing radiation: Comprises charged particles (electrons, protons, α-
particles, heavy ions) that deposit energy in the absorber through a direct one-step
process involving Coulomb interactions between the directly ionizing charged
particle and orbital electrons of the atoms in the absorber.
70. ➢ Indirectly ionizing radiation:
• Comprises non particulate radiation (photons such as x-rays and γ-rays) that deposit
energy in the absorber through a two-step process as follows:
✓In the first step a charged particle is released in the absorber (photons release either
electrons or electron/positron pairs, neutrons release protons or heavier ions).
✓ In the second step, the released charged particles deposit energy to the absorber
through direct Coulomb interactions with orbital electrons of the atoms in the
absorber.
71.
72. Non-ionizing radiation:
• cannot ionize matter because its energy is lower than the ionization energy of atoms
or molecules of the absorber.
• The term non-ionizing radiation thus refers to all types of electromagnetic radiation
that do not carry enough energy per quantum to ionize atoms or molecules of the
absorber.
• Near ultraviolet radiation, visible light, infrared photons, microwaves, and radio
waves are examples of non-ionizing radiation.
