Introductory lecture for BIOPHYSICS I.pdf

BIOPHYSICS I
DEPARTMENT OF BIOMEDICAL ENGINEERING
DR. RAMY ABDLATY

Radiation, types of
ionizing radiation and
their interaction with
matter
DEPARTMENT OF BIOMEDICAL ENGINEERING
DR. RAMY ABDLATY

Contents
 Material Structure
 Ionizing Radiation
 Types of Ionizing radiation
 Types of non-Ionizing radiation
 Interaction of Ionizing Radiation with matter

Material Structure
Electron volt is a special unit defined as the energy required to move one electron against a potential
difference of one volt. Conversely, it is also the amount of kinetic (motion) energy an electron acquires if it
“falls” through a potential difference of one volt. It is a very small unit on the everyday scale, at only 1.6 × 10-
19 joules (J), but a very convenient unit on the atomic scale.

Ionizing Radiation
Radiation:
It is the process of emitting and propagating radiant energy through space or a
material medium in the form of waves or particles. It can also be shortly defined as
energy in transit.
Ionization:
The process of propagating an external radiation through a medium (gas, crystal,
or a semiconductor) that results in removing an electron from an atom or molecule.
Total Ionization:
This phenomenon takes place when the propagating radiation loses its total kinetic
energy while passing through the medium. The ionized atom eventually loses a
slight part of its mass and acquires a net positive charge.

Ionization process
 The difference between
the ionization and total
ionization processes is
the lost quantity of
energy with respect to
the original incident
energy.
 If the transmitted
ionizing radiation
through a medium is
equal to zero, the
process is called total
ionization.

Ionizing Radiation
 Ions:
Atoms or molecules that that have lost/ gained one or more electron/s.
Ionizing Radiation:
The types of radiation that are potentially capable of transforming atoms/
molecules from the neutral status to ions.
Ionizing radiation can be a naturally occurring from radioactive isotopes likewise
potassium-40 (k-40), or radium-226 (Ra-226).
K-40: is a radioactive isotope of potassium which has a long half-lifetime of
1.251x109 years. Its atomic number is 19.
Ra-226: is a highly radioactive, however the most stable among radium isotopes,
isotope of radium which has a long half-lifetime of 1600 years. Its atomic number is
88.

Ionizing Radiation
Cobalt 60 and K-40

Types of Ionizing Radiation
 1- Particulate Radiation:
✓ It is directly ionizing radiation with its mass and often with a charge.
✓ This type of radiation propagates with a high speed, however, never equal to
the the light speed in vacuum.
✓ Particulate radiation has a small penetration ability.
✓ Examples of particulate radiation are alpha particles (α-particle), beta-particles
(β-particle), and heavy nuclei.
✓ Some uncharged particles, such as neutrons, are called indirectly ionizing
particulate radiation since they liberate directly ionizing particles when they
interact with matter.

Types of Ionizing Radiation
 2- Electromagnetic Radiations:
✓ This type of radiation includes X-rays and gamma-rays (γ-rays).
✓ X-rays and γ-rays have no mass nor charge but they are highly penetrating ionizing radiations.
✓ X-rays and γ-rays are indirectly ionizing radiations: since most of the produced ionization occur
due to high energetic electrons resulting from the interaction of the rays with matter.
✓ Table 1-A in the course book: [energy, ionizing particle]
1. X-rays
2. α-particle
3. β-particle
4. γ-rays
5. Electron beams
6. Neutron beams
7. Proton beams

Types of non-Ionizing Radiation
 Electromagnetic Radiation:
✓ The non-ionizing radiations belong to the electromagnetic radiation.
✓ The energies of the non-ionizing radiations are far from being strong enough to
completely remove an electron and thus cause an ionization process.
✓ Examples of the non-ionizing radiations are low-energy ultraviolet (UV) light,
visible light, infrared (IR), TV waves, broadcasting waves.
✓ Refer to table (1B) for more details regarding the non-ionizing radiation.

Interactions of Ionizing Radiation with matter
 There are 4 main types of ionizing radiations:
1. A charged particle which its mass is comparable to the nuclear mass of the
targeted atom.
2. Beta particles (electrons)
3. Beta particles (positrons)
4. Electromagnetic radiation
The charged particles interact mainly with atomic electrons to produce either:
1. Ionization
2. Excitation
Excitation process takes place when the incident particle gives its energy to an
orbital electron provided that the given energy is insufficient to eject that electron
out of the atom but moves it to a higher orbit for a certain time.

 The interaction of ionizing radiation with the atomic nuclei that results in a
radiative loss of energy is called “Bremsstrahlung” phenomenon.
 Bremsstrahlung occurs for much more likely electrons than for heavier charged
particles.

Bremsstrahlung Interaction
 Bremsstrahlung (from bremsen “to brake”
and Strahlung “radiation”; i.e., “braking
radiation” or “deceleration radiation”)
is electromagnetic radiation produced by
the deceleration of a charged particle
when deflected by another charged
particle, typically an electron by
an atomic nucleus.
 Bremsstrahlung is used to refer to the
process of producing the radiation. It has
a continuous spectrum, which becomes
more intense and whose peak intensity
shifts toward higher frequencies as the
change of the energy of the decelerated
particles increases. The maximum radiation
frequency is related to the kinetic energy
of the electrons by the relationship.

 1- Interactions of heavy charged particles (α-particles) with matter:
1. Heavy charged particles (ex: α-particle) traversing a medium occasionally will collide
elastically with the target nuclei.
2. The electrostatic repulsion of the heavy charged particle with the target nuclei will result in a
deflection force to change the particle propagation direction in a phenomenon called
Rutherford scattering.
3. Rutherford scattering represents the electrostatic effect (repulsion/ attraction) from the
nucleus that deflect the heavy charged particles.
4. Elastic type of nuclear collisions give rise to large changes in the direction of the impinging
particle but not, on the average, to significant energy losses.
5. A heavy charged particle may collide also with atomic electrons, where the former collision is
responsible for a greater loss of the particle energy.
6. The lost energy yields a slow down for the particle movement and 1 out of 2 possibilities for the
targeted atom:
➢ Excitation
➢ Ionization

 Interactions of heavy charged particles with matter: Rutherford scattering:

 1- Interactions of heavy charged particles with matter: Important definitions:
1- The Range:
it is the minimum amount of absorber thickness that stops a heavy charged particle
likewise α-particle.
2- Specific Energy Loss
It is the energy lost by the charged particle along the track per unit path length.
3- The stopping power of the medium
It is the average value of the energy lost per unit length of the particle in the absorbing
medium.
4- The relative stopping power of the medium
It is the range of the α-particles in air divided by the range of α-particles in the medium.
5- Specific Ionization
It is the number of ion pairs produced per unit path length. Specific ionization is
fluctuating and thus we usually calculate the mean specific ionization.

 2- Interactions of Beta particles (β-particles) with matter:
I. β-particle may have a negative charge (β-) or a positive charge (β+) of an electron
or positron, respectively.
II. A great practical difference between the behavior of heavy particles and
electrons arises from the fact that trajectories of electrons in matter are not straight
lines.
III. Electrons suffer from multiple scattering which in turn changes its direction (zigzag
path) along the path of propagation especially for low-energy electrons.
IV. As a result, the actual path of an electron passing through two points may be
longer than the Cartesian distance between the two points.
V. Therefore, electrons of the same energy can travel different distances in the same
medium before coming to rest.
VI. The electron at rest loses its high energy until it becomes equal to thermal energy
and captured by the surrounding atoms.

 2.1- Interactions of Negative Beta particles (β--particles)
with matter:
I. At low energies of β- (E < m0 c2 ), the loss by ionization
process is dominant but not the only one.
II. At higher energies, an electron may interact with the
electromagnetic field of a nucleus and rapidly
decelerated. the deceleration of the electron means
a loss of the kinetic energy of the electron. This loss is
transferred into Bremsstrahlung phenomenon.
III. The rate of energy loss increases with the increase in
the electron energy and the atomic number of the
medium (Z).
IV. The Bragg peak is the peak point on the Bragg curve
which plots the energy loss of ionizing radiation during
its travel through matter. The peak occurs immediately
before the particles come to rest.
“Variation of Bragg Curve Characteristic Induced by Changing
the Position of material, Journal of the Korean Physical Society,
Vol. 58, No. 2, February 2011, pp. 187-197”

 2.1- Interactions of Negative Beta particles (β--
particles) with matter:
I. Bragg peak is the highest value on the Bragg
curve which plots the energy loss of ionizing
radiation during its travel through matter. The
peak occurs immediately before the particles
come to rest.
II. When a fast charged particle moves through
matter, it ionizes atoms of the material and
deposits a dose along its path.
III. A peak occurs because the interaction cross
section increases as the charged particle's
energy decreases. Energy lost by charged
particles is inversely proportional to the square of
their velocity, which explains the peak occurring
just before the particle comes to a complete
stop.
“Variation of Bragg Curve Characteristic Induced by Changing
the Position of material, Journal of the Korean Physical Society,
Vol. 58, No. 2, February 2011, pp. 187-197”

 2.2- Interactions of Negative Beta particles
(β+-particles) with matter:
I. The positron has the same charge of
electron but in the positive form.
II. The positron tends to combine with an
electron (for example: the electrons at the
outer orbit of the atoms) in a very short time
after being liberated.
III. The result of the positron-electron
combination is an “Annihilation Radiation =
[Energy + Positronium]”
IV. Positronium is a short-lived hydrogen-like
atom composed of an electron and a
positron (rather than an electron and a
proton).

Comparison between
Alpha & Beta Particles
Alpha Particle Beta Particle
Alpha particles are released when a nucleus is too heavy - this
means there are so many protons and neutrons in the nucleus that
the nucleus becomes unstable.
The alpha particle consists of two protons and two neutrons, so it is
essentially a helium nucleus, and this decreases
the isotopes atomic mass by four and its atomic number by two,
meaning it becomes a different type of atom.
Alpha particles are the largest form of ionizing radiation (radiation
that knocks electrons off atoms when it collides with them) and
are therefore the most ionizing - the bigger a particle is, the more
likely it is to collide with atoms.
However, it is the least penetrating (again, because they are large
particles they hit atoms quickly and get absorbed) and can be
absorbed by just a sheet of paper. In air, it has a range in air of
about 5cm.
They are produced when an atom has too many neutrons, one of the
neutrons changes into a proton to make the atom more stable.
Because a proton is positive whilst a neutron is neutral, this means that
somehow we need to find a way to account for the lost negative
charge - so as well as a proton, the neutron also produces a high
energy electron. This causes the atomic mass to remain unchanged
but the atomic number increases by one, because the atom has
essentially gained a proton and lost a neutron.
Beta particles are essentially fast moving electrons.
An example is Carbon-14 (atomic number 6) releasing a beta particle
to become Nitrogen-14 (atomic number 7).
Note that the atomic number (number of protons) has increased by
one, and the mass has stayed the same.
Beta particles are between alpha particles and gamma rays - in
terms of how strongly ionizing they are. They are in the middle for how
penetrating they are and will pass through paper but be stopped by
aluminum foil (a few millimeters thick). It has a range in air of around
15cm.

 3- Interactions of Electromagnetic radiation (γ-rays) with matter:
I. γ-rays interact with matter in a different way than charged particles (α , β).
II. γ-rays are distinguished by it high penetrating power that has no definite range
compared to the charged particles.
III. There are more than 12 processes of interaction between photons and matter (as
documented in the text book page 6)
IV. The most probable interactions:
I. Photoelectric effect
II. Compton scattering
III. Pair production
V. Each of the former interactions is accompanied by secondary effects.
I. Photoelectric effect is accompanied by Auger electrons and fluorescent radiation.
II. Compton scattering is accompanied by recoil electrons.
III. Pair production is accompanied by annihilation of positrons.

 3.1- Photoelectric Effect:
I. The photoelectric absorption occurs in
the vicinity of an atom nucleus when
an incoming photon’s energy is totally
used for releasing an electron at the
inner atomic shells.
II. The released electron is called
photoelectron. This photoelectron
emerges with a kinetic energy (T) is
given by:
𝑇 = 𝐸γ − 𝐵𝑒
𝐵𝑒/BE is the binding energy of the electron to
the atom
𝐸γ (hν) is the energy of the incident photon

III. The release of the photoelectron creates a hole in
the inner orbits of the atom that needs to be filled.
For this purpose, an outer orbit electron leaves its
location and falls down to occupy the former hole.
IV. The difference in energy between the two orbits for
the electron transition releases a characteristic
radiation of an X-ray photon.
V. The X-ray photon usually causes the ejection of an
outer electron from the same atom which is called
Auger electron.
VI. In this case the original incident photon (γ-rays)
energy is totally lost.

VII. Auger electrons are usually monoenergetic
escaped photons.
VIII. Photoelectric effect is a dominant interaction
of γ-rays with matter in case of
i. low photon energies and
ii. High atomic numbers of the target atom
IX. The cross-section for absorption of a photon
(𝜎𝑃ℎ𝑜𝑡𝑜
𝐾
) is proportional to the atomic number
(Z5) and inversely proportional to the photon
energy (𝐸γ
7/2).
X. The photoelectric effect is very important in
the absorption of low energy photons by
heavy elements.

 3.2- Compton Scattering:
I. When X-rays or γ-rays of well-
defined wavelength (λ0)
propagate through an atom, it
might be scattered by an angle
(φ).
II. The scattered photon contains a
component of well-defined
wavelength (λ’) which is longer
than (λ0), this phenomenon is
called Compton Effect.
III. ℎ𝜈′
is the energy of the scattered
photon, ℎ𝜈0 is the energy of the
incident photon, 𝐸𝑒 is the kinetic
energy of the ejected electron.
ℎ𝜈′
= ℎ𝜈0 − 𝐸𝑒

 3.2- Compton Scattering:
IV. The cross-section of Compton scattering
(𝜎𝑃ℎ𝑜𝑡𝑜) is proportional to the atomic
number (Z).
V. X-rays or γ-rays of low energy creates
Compton scattering with the electrons of
relatively small binding energy in the outer
shells of the atom or “free electrons”.
VI. The free means that the binding energy in
the outer shells of the atom is less than the
bombarding photon.
VII. As the energy of X-rays or γ-rays increases
more, a plenty of electrons in the atom
becomes available for Compton
scattering.
ℎ𝜈′ = ℎ𝜈0 − 𝐸𝑒

 3.3- Pair production:
I. This phenomenon is restricted to the
presence of γ-ray photon energy > 1.02
MeV.
II. This high energetic photon may interact
strongly with the electromagnetic field of
an atomic nucleus to form an electron-
positron pair as shown in figure.
III. 1.02 MeV is the minimum amount of
energy that can provide the rest masses
of the electron and positron (0.511 MeV
each).
IV. Excess energy (> 1.02 MeV) are carried
equally by the electron and the positron
which produce ionization as they travel in
the material.

 3.3- Pair production:
V. The produced positron is eventually
captured by an electron and
annihilation of the two particles occurs.
VI. Annihilation radiation produces two
photons equal in energy (0.511 MeV) but
opposite in direction.
VII. The produced photons then lose their
energy by either photoelectric effect or
Compton scattering.
VIII. The cross section per atom for pair
production is equal to zero for photon
energies < 1.02 MeV. For greater
energies of photons, it increases first
slowly, then more rapidly. It is also
proportional to (Z2).

 3.4- Summary of Interactions:
I. The shown figure displays the
Predominant types of interaction for a
range of incident photon energies on
the x-axis and absorber atomic numbers
on the y-axis.
II. For low energy incident photons, the
probability of photoelectric effect (PE)
and Compton scattering (CS) to take
place dominate over pair production.
III. The governing factor for which PE or CS
to take place is the atomic number of
the absorber material.
IV. The range of photon energies used in
the medical field is approximately
confined between 50-800 KeV.
V. Therefore the most dominant
interactions are either PE or CS.

Introductory lecture for BIOPHYSICS I.pdf

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