1 interaction of radiation with matter

Lecture 5

Shahid Younas

INTERACTION OF RADIATION WITH MATTER

Introduction

Lecture 5

Manner of interactions

Nature of Materials

Nature of Incident particle

Interactions are probabilities

Noble Prize

Types and sources of ionizing radiation

Lecture 5

The important types of ionizing radiation

-rays X-rays Fast Electrons Heavy Charged Particles Neutrons

-rays

Lecture 04

Electromagnetic radiation emitted from a

nucleus or in annihilation reactions

between matter and antimatter.

Practical range of photon energies emitted

by radioactive atoms extends from 2.6 keV

to 7.1 MeV

X-rays

Lecture 04

Electromagnetic radiation emitted by

charged particles (usually electrons) in

changing atomic energy levels (called

characteristic or fluorescence x-rays)

or in slowing down in a Coulomb field

(continuous or bremsstrahlung x-rays).

X-rays

Lecture 04

20 – 120 kV Diagnostic-range x-rays

120 – 300 kV Orthovoltage x-rays

300 kV – 1 MV Intermediate-energy x-rays

1 MV upward Megavoltage x-rays

X-rays

Lecture 04

Do you know the name of x-rays of range 0.1 – 20 kV.

Soft x-rays or Grenz rays

Fast Electrons

Lecture 04

Do you know the name of electrons if they result from a charged-particle collision.

Delta Rays “-rays”

Fast Electrons

Lecture 04

Positrons if positive in charge

If emitted from a nucleus they are

usually referred to as -rays

(positive or negative)

Intense continuous beams of

electrons up to 12 MeV available

from Van de Graff generators.

Heavy Charged Particles

Lecture 04

Acceleration by a Coulomb force field.

Alpha particles also emitted by some

radioactive nuclei.

Proton – the hydrogen nucleus

Deuteron – the deuterium nucleus

Triton – a proton and two neutrons bound by

nuclear force

Alpha particle – the helium nucleus

Heavy Charged Particles

Lecture 04

Do you know the name of some other heavy charged particles

Pions – negative -mesons

Neutrons

Lecture 04

Neutral particles obtained from nuclear reactions [e.g., (p, n) or fission].

They cannot themselves be accelerated electrostatically

ICRU Terminology

Lecture 04

Directly ionizing radiation

Fast charged particles

Deliver their energy to matter directly, through many small Coulomb-

force interactions along the particle’s track

ICRU Terminology

Lecture 04

Indirectly ionizing radiation

X- or -ray photons or neutrons.

First transfer their energy to charged particles.

Resulting fast charged particles then in turn deliver the energy to

the matter as above

Specific Ionization

Lecture 04

Number of primary and secondary ion pairs

produced per unit length of charged particle’s path

is called specific ionization.

Expressed in ion pairs (IP)/mm.

Specific Ionization

Lecture 04

Increases with electrical charge of particle.

Decreases with incident particle velocity.

~ 7000 IP/mm in air by Alpha particle.

Specific Ionization

Lecture 04

Do you know the medical usage of specific ionization?

Radiotherapy

Specific Ionization

Lecture 04

7.69 MeV Alpha Particle from Polonium 214.

Charged Particle Tracks

Lecture 03

Do you know the difference between “Range” and “Path Length” for ionizing particles?

Path length is actual distance particle travels whereas range is actual depth of penetration in matter


Lecture 03

Electrons follow tortuous

paths in matter as the

result of multiple

scattering events.

Ionization track is thin

and non-uniform


Lecture 03

Larger mass of heavy

charged particle results in

dense and usually linear

ionization track.

Linear Energy Transfer

Lecture 04

Amount of energy deposited per unit path length is called the

linear energy transfer (LET).

Expressed in units of eV/cm.

LET is the product of specific ionization (IP/cm) and the

average energy deposited per ion pair (ev/IP).


Lecture 04

Radiation LET (keV/ um)

1 MeV Gamma Rays 0.5

100 kVp X-rays 6

20 keV β-particles 10

5 MeV neutrons 20

5 MeV α-particles 50

1 MeV Electron 0.2

100 keV Electron 0.3


Lecture 04

Can you establish a relation between LET and biological damage?

High LET Radiation (alpha particles, protons etc)are more dangerous to tissue

than low LET radiation (gamma, X-rays, electrons)


Lecture 04

What is the probability of charged particle passing through a

medium without interaction.

ZERO


Lecture 04

What is the probability of charged particle passing through a

medium without interaction.

ZERO

Scattering

Lecture 04

Interaction resulting in the deflection of a particle or photon from its original

trajectory.

Elastic : Billiard Ball

In-elastic

Scattering

Lecture 04

Is there any effect of scattering on image quality.

bone

air

soft tissue bone

primary diaphragm

film, fluorescent screen or image intensifier

primary radiological

image

intensity at detector

scattered radiation

grid

Image taken from: Johns & Cunningham, The Physics of Radiology, 4th Edition

Scattering

Lecture 04

Veil over image.

Add patient dose.

Particle interactions

Lecture 04

Energetic charged particles interact with matter by electrical forces and lose

kinetic energy via:

Excitation

Radiative losses

Ionization

~ 70% of charged particle energy deposition leads to nonionizing excitation

Particle interactions

Lecture 04

A: Excitation (left) and

de-excitation on (right)

B: Ionization and the

production of delta rays

INTERACTIONS

Lecture 04

All Interactions are Probabilities.

Different passengers in a car; faces different level of injuries

or enjoy different grades of safety in the situation of an

accident.

PHOTON INTERACTIONS

Lecture 04

Chance of event happening

Relative predictions can be made,

energy of photons

type of matter with which photons are gong to interact

Radiative Interactions-Bremsstrahlung

Lecture 04

Path of the electron is deflected / de-

accelerated by the positively charged

nucleus.


Lecture 04

Angle of emission changes with the incident electron energy.

Probability of production is ~ Z2 of the absorber.

Energy emission varies inversely with the square of the mass.

Protons and alpha particles produce less than one-millionth the amount

of bremsstrahlung radiation as electrons of the same energy.


Lecture 04

Disadvantages:

two edged sword

It is not especially useful for therapeutic

Bremsstrahlung produced by a beta electron is more harmful to the

technologist than the beta particle that produces it, because of the

penetrability of an electromagnetic ray.


Lecture 04

Ratio of electron energy loss by bremsstrahlung production to that lost by

excitation and ionization,

kinetic energy of incident electron * atomic number

820

Radiative Interactions-Characteristics

Lecture 04

If a high speed beta particle approaches

an electron in an inner orbital.

Ejected from the atom.

Neutron interactions

Lecture 04

Don’t interact with electrons.

Don’t create direct ionization.

They do interact with atomic nuclei,

sometimes liberating charged particle.

Neutrons may also be captured by atomic

nuclei-Retention of the neutron converts the atom to a different nuclide (stable or radioactive)

Rayleigh Scattering

Lecture 04

Incident photon interacts with

and excites the total atom as

opposed to individual electrons.

Rayleigh Scattering

Lecture 04

Electric field of the incident photon’s electromagnetic wave expands energy.

Causing all of the electrons in the scattering atom to oscillate in phase.

Atom’s electron cloud immediately radiates this energy.

Emitting photon of same energy but slightly different direction.

Rayleigh Scattering

Lecture 04

Occurs mainly with very low energy diagnostic x-rays, as used

in mammography (15 to 30 keV)

Less than 5% of interactions in soft tissue above 70 keV; at

most only 12% at ~30 keV.

Rayleigh Scattering

Lecture 04

Compton Scattering

Lecture 04

Predominant interaction in the diagnostic energy range

with soft tissue.

Predominate Energy Region: 26 keV to 30 MeV.

Most likely to occur between photons and outer

(“valence”) shell electrons.

Compton Scattering

Lecture 04

Electron ejected from the atom.

Binding energy comparatively small

and can be ignored.

Photon scattered with reduction in

energy.

Compton Scattering

Lecture 04

Energy of scattered photon can be calculated by,

)cos1(1 20

0

0

0

cmE

EE

EEE

sc

esc

Compton Scattering

Lecture 04

Ionization of the atom.

Ejected electron lose K.E. by excitation and ionization of atoms

in the surrounding material.

Compton Scattering

Lecture 04

As incident photon

energy increases,

scattered photons and

electrons are scattered

more toward the

forward direction.

Compton Scattering

Lecture 04

In diagnostic imaging (18 to 150 keV), the majority of the incident

photon energy is transferred to the scattered photon.

In x-ray transmission imaging, these photons are much more likely to

be detected by the image receptor.

Thus reducing image contrast.

Compton Scattering

Lecture 04

Probability of interaction increases as incident photon energy

increases.

probability also per atom of the absorber depends on the number

of electrons available as scattering targets and therefore increases

linearly with Z.

Compton Scattering

Lecture 04

Laws of conservation of energy and momentum place limits on

both scattering angle and energy transfer.

Energy of the scattered electron is usually absorbed near the

scattering site

Compton Scattering

Lecture 04

Do you know the angles of maximum energy transfer and scattering

of Compton Electron.

Maximal energy transfer 180-degree- photon backscatter

Maximal Scattering angle is 90 degrees

PHOTOELECTRIC EFFECT / ABSORPTION

Lecture 04

All of the incident photon energy is transferred to an electron, which

is ejected from the atom.

Kinetic energy of ejected photoelectron (Ec) is equal to incident

photon energy (E0) minus the binding energy of the orbital electron

(Eb).

Ec = Eo - Eb

Photoelectric absorption

Lecture 04

Incident photon

energy must be

greater than or

equal to the

binding energy of

the ejected

photon.

The Photoelectric Effect / Absorption

Lecture 04

Atom is ionized, with an inner shell

vacancy.

Electron cascade from outer to inner

shells- Characteristic x-rays or Auger

electrons


Lecture 04

Probability of characteristic x-ray emission decreases as Z decreases

Does not occur frequently for diagnostic energy photon interactions in soft

tissue


Lecture 04

Most likely to occur • With inner-shell electrons

• With tightly bound electrons.

• When the x-ray energy is greater

than the electron-binding energy.


Lecture 04

As the x-ray energy increases• Increased penetration through

tissue without interaction.

• Less photoelectric effect relative

to Compton effect.

• Reduced absolute absorption.


Lecture 04

As the atomic number of the absorber increases

As mass density of the absorber increases

• Increases ~ Z3.

• Proportional increase in photoelectric effect.


Lecture 04

Low atomic number target atoms such as soft tissue have low binding energies.

Therefore the photoelectric electron is released with kinetic energy nearly equal to the incident x-ray.

Higher atomic number target atoms will have higher binding energies.


Lecture 04

Probability of photoelectric absorption per unit mass is approximately

proportional to,

No additional non-primary photons to degrade the image.

33 / EZ


Lecture 04

1 / E3 explains why image contrast decreases when higher x-ray energies are used in

imaging process.

For 1 / E3 there is an exceptions.

Absorption Edges (Discontinuities).


Lecture 04

Photon energy

corresponding to an

absorption edge is the

binding energy of electrons

in a particular shell or

subshell


Lecture 04

• The photoelectric effect predominates when lower energy photons interact

with high Z materials like Lead and Iodine.

• Compton scattering will predominates at most diagnostic photon energies

in materials of lower Z such as tissue and air.

Principle of radiological image formation

Lecture 04

Attenuation of an X Ray beam

Air: negligible

Bone: significant due to relatively high density (atom mass number of Ca)

Soft tissue (e.g. muscle ): similar to water

fat tissue: less important than water

Principle of radiological image formation

Lecture 04

Attenuation of an X Ray beam

lungs: weak due to density.

bones can allow to visualize lung structures with higher kVp

(reducing photoelectric effect)

body cavities are made visible by means of contrast products (Iodine,

Barium).

Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle) and Bone

Lecture 04

Water (Muscle) Bone

Lecture 04

X Ray penetration in human tissues

60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs

Lecture 04

X Ray penetration in human tissues

Higher kVp reduces photoelectric effect

The image contrast is lowered

Bones and lungs structures can simultaneously be

visualized

Note: body cavities can be made visible by means

of contrast media: iodine, barium

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1 interaction of radiation with matter