CHARACTERISTICS OF
INTERACTIONS

In a radiation interaction, the radiation
and the material with which it interacts
may be considered as a single system.
• When the system is compared before and
•
•
after the interaction, certain quantities will be
found to be invariant.
Invariant quantities are exactly the same
before and after the interaction.
Invariant quantities are said to be conserved
in the interaction.


One quantity that is always conserved in
an interaction is the total energy of the
system, with the understanding that
mass is a form of energy.
Other quantities that are conserved
include momentum and electric charge.

Some quantities are not always
conserved during an interaction.
• For example. the number of particles may not
be conserved because particles may be
• fragmented,
• fused,
• created (energy converted to mass), or
• destroyed (mass converted to energy).

Interactions may be classified as either
• elastic or
• inelastic.

An interaction is elastic if the sum of the
kinetic energies of the interacting entities
is conserved during the interaction.
• If some energy is used to free an electron or
•
nucleon from a bound state, kinetic energy is
not conserved and the interaction is inelastic.
Total energy is conserved in all interactions,
but kinetic energy is conserved only in
interactions designated as elastic.
ENTC 4390
DIRECTLY IONIZING
RADIATION

When an electron is ejected from an
atom, the atom is left in an ionized state.
• Hydrogen is the element with the smallest
atomic number and requires the least energy
(binding energy of 13.6 eV) to eject its K-shell
electron.


Radiation of energy less than 13.6 eV is
termed nonionizing radiation because it
cannot eject this most easily removed
electron.
Radiation with energy above 13.6 eV is
referred to as ionizing radiation.


If electrons are not ejected from atoms but
merely raised to higher energy levels (outer
shells), the process is termed excitation, and
the atom is said to be “excited.”
Charged particles such as electrons, protons,
and atomic nuclei are directly ionizing
radiations because they can eject electrons
from atoms through charged-particle
interactions.

Neutrons and photons (x and g rays) can
set charged particles into motion, but
they do not produce significant ionization
directly because they are uncharged.
• These radiations are said to he indirectly
ionizing.

Energy transferred to an electron in excess of
its binding energy appears as kinetic energy of
the ejected electron.
•

An ejected electron and the residual positive ion
constitute an ion pair.
An average energy of 33.85 eV termed the Wquantity or W, is expended by charged
particles per ion pair produced in air.
•
The average energy required to remove an electron
from nitrogen or oxygen (the most common atoms in
air) is much less than 33.85 eV.

The W-quantity includes not only the
electrons binding energy but also the
average kinetic energy of the ejected
electron and the average energy lost as
incident particles excite atoms, interact
with nuclei, and increase the rate of
vibration of nearby molecules.
• On the average. 2.2 atoms are excited per ion
pair produced in air.
ENTC 4390
MEDICAL IMAGING
Interactions of Radiation
Particle Interactions

Particles of ionizing radiation include
charged particles, such as:
• alpha particles
• protons
• electrons,
• beta particles
• positrons, and
• neutrons.

The behavior of heavy charged particles
is different from that of lighter charged
particles such as electrons and
positrons.
Excitation, Ionization, and
Radiative Losses

Energetic charged particles all interact
with matter by electrical forces and lose
kinetic energy via:
• excitation,
• ionization, and
• radioactive losses.

Excitation and ionization occur when
charged particles lose energy by
interacting with orbital electrons.
• Excitation is the transfer of some of the
incident particle’s energy to electrons in the
absorbing material, promoting them to
electron orbits farther from the nucleus.
• Higher energy levels.

In excitation, the energy transferred to
an electron does not exceed its binding
energy.
• The electron doesn’t leave the atom.

Following excitation, the electron will
return to a lower energy level,
• With the emission of the excitation energy in
the form of
• Electromagnetic radiation or
• Auger electrons.
Atomic Emissions
L
L
K
K
Second
electron
ejected
from
atom
Photon or
bundle of
em-radiation
Characteristic Radiation
Auger Electron
Note: 20181Tl → 20180Hg by absorbing an orbital electron.
We primarily image the K X-rays of 20180Hg.
Characteristic X-Rays

Electron transitions between atomic
shells results in the emission of radiation
in the visible, ultraviolet, and x-ray
portions of the electromagnetic (EM)
spectrum.
• Characteristic x-rays are named according to
the orbital in which the vacancy occurred.
Electromagnetic radiation
Electromagnetic Radiation


Electromagnetic radiation consists of
oscillating electric and magnetic fields.
An electromagnetic wave requires no
medium for propagation,
• That is, it can travel in a vacuum as well as
through matter.

The wavelength of an electromagnetic
wave is depicted as the distance
between adjacent crests of the
oscillating fields.
• The constant speed of electromagnetic
radiation in a vacuum is the product of the
frequency n and the wavelength l of the
electromagnetic wave.
c = ln

Often it is convenient to assign wavelike
properties to electromagnetic rays.
• At other times it is useful to regard these
radiations as discrete bundles of energy termed
photons or quanta.
• The two interpretations of electromagnetic
radiation are united by the equation
E = hn
• where E represents the energy of a photon and n
represents the frequency of the electromagnetic
wave. The symbol h represents Planck’s constant,
6.62 x 10 -34 J-sec.

The frequency n is
n = c/l
and the photon energy may be written as
E = (hc)/l

The energy in keV possessed by a
photon of wavelength, l, in nanometers
is
E = 1.24/l
• Electromagnetic waves ranging in energy
from a few nano-electron volts up to
gigaelectron volts make up the
electromagnetic spectrum.
Ultraviolet Light

Ultraviolet (UV) light is usually
characterized as nonionizing.
• UV light is used to
• sterilize medical instruments,
• destroy cells,
• produce cosmetic tanning, and
• treat certain dermatologic conditions
Visible Light

Visible light is the part of the EM
spectrum to which the retina is most
sensitive.
Infrared

Infrared is the energy released as heat
by materials near room temperature.
• Infrared sensitive devices can record heat
•
signatures.
To date, they have not found a good medical
imaging application.

The radiation resulting from a vacancy in
the K shell is called a K-characteristic xray.
• The radiation resulting from a vacancy in the
L shell is called a L-characteristic x-ray.

If the vacancy in one shell is filled by the
adjacent shell it is identified by a
subscript alpha.
• L  K transition = Ka and
• M  L transition = La

If the electron vacancy is filled from a
nonadjacent shell it is identified by a
subscript beta.
• M  K transition = Kb

The energy of the characteristic x-ray is
the difference between the electron
binding energies (Eb) of the respective
shells.
-2.5 keV
-11 keV
–
–
-69.5 keV
-67 keV Kb
Characteristic
X-Ray
–
–
Vacant
–
L
K
M
Auger Electrons

An electron cascade does not always
result in a characteristic x-ray.
• A competing process that predominates in low
Z elements is Auger electron emission.
• The energy released is transferred to an orbital
electron, typically in the same shell as the
cascading electron.
-2.5 keV
-11 keV
–
–
–
-69.5 keV
–
Vacant
–
L
K
M
64.5 keV Auger
electron

If the transferred energy exceeds the
binding energy of the electron, ionization
occurs, whereby the electron is ejected
from the atom.
• The result of ionization is an ion pair
consisting of the ejected electron and the
positively charged atom.
• Sometimes the ejected electrons possess enough
energy to produce further ionizations called
secondary ionization.

The term interaction may be used to
describe
• the crash of two automobiles
•
• (an example in the macroscopic world) or
the collision of an x ray with an atom
• (an example in the submicroscopic world).

Interactions in both macroscopic and
microscopic scales follow fundamental
principles of physics such as
(a) the conservation of energy and
(b) the conservation of momentum.

Neutrons are uncharged particles.
• Neutrons do not interact with electrons and do
not directly cause excitation and ionization.
• They do, however, interact with atomic nuclei,
sometimes liberating charged particles or nuclear
fragments.
• Neutrons may also be captured by atomic nuclei.


In some cases the neutron is reemitted.
In other cases the neutron is retained,
converting the atom to a difference nuclide.
•
In this case, the binding energy my be emitted via
spontaneous gamma-ray emission:
1
H  n H  g
1
2
Eg  2.22 MeV
–
–
–
–
+
–
–
++++
+
+
+
++++
–
–
–
–
–
–
–
–
–
–
–
–
Neutron
–
–
–
–
–
–
–

Pair production only occurs when the
energies of x-ray or gamma ray exceed
1.02 MeV.
• An x-ray or gamma ray interacts with
the electric field of the nucleus of an
atom.
0.51 MeV
annihilation

The photon’s energy
–
is transformed into
an electron-positron
b
– – –
pair.
–
–
–
–
–
–
–
– –
–
++++
+
– –
+
+
++++
–
–
– –
–
–
–
–
b
–
– –
0.51 MeV

When the positron (a form of antimatter)
comes to rest, it interacts with a negatively
charged electron, resulting in the formation of
two oppositely directed 0.511 MeV annihilation
photons.
•
Positrons are important in imaging of positron emitting
radiopharmaceuticals in which the resultant
annihilation photon pairs emitted from the patient are
detected by positron emission tomography (PET)
scanners.
Ionizing Radiation