principles of physics of diagnostic radiology

1. UG LECTURES
RADIOLOGY 1
PRINCIPLES OF PHYSICS OF
DIAGNOSTIC RADIOLOGY

2. PRINCIPLES OF PHYSICS OF DIAGNOSTIC RADIOLOGY
Learning objectives
• At the end of this session the student should be able to:
• Describe simple atomic theory
• Describe the structure of an atom
• Describe the process of ionization of atoms

3. SIMPLE ATOMIC THEORY
OUTLINE
08-09-2012
• Structure of the Atom
• The Nucleus
• Atomic shells
• Electrons in shells
• Electron binding energies
• Ionization and excitation of atoms
• Emission of EM radiation from atoms

4. ATOMIC STRUCTURE
• Made up of a central
nucleus surrounded by
orbiting electrons
• Electrons orbit the
nucleus in confined spaces
called shells
• No electrons in spaces
between the shells

5. THE NUCLEUS
• A tiny positively charged mass at the centre of the atom
• Made up of protons(+ve charge) and neutrons(no charge)
• The number of protons is called the atomic number(Z)
• Atomic number determines nuclear charge and atomic
chemistry.
• The higher the atomic number the greater the nuclear charge

6. NUCLEUS cont
• Sum of proton numbers and neutron numbers is
called the mass number(A)
• Nearly all of the atoms’ mass is contained in the
nucleus
• Electron mass=1/1840 mass of a proton or
neutron. It is the smallest unit of electric charge
• Charge of proton= charge of electron but in
opposite sign
• In a neutral atom number of electrons=number
of protons in the nucleus

7. Atomic shells cont
• The shell nearest the nucleus is called K shell,
others in order of increasing distance from the
nucleus are L,M,N etc.
• Shell radii, i.e. distance from the nucleus,…
increase rapidly with increasing shell number
• n=1 ,K shell
• n=2 , L shell
• N=3 , M shell etc

8. Electrons in shells
• Different numbers of electrons are
accomodated in different shells
• Max number of electrons in shell number n is
given by the formula 2n squared. Therefore
max e’ in K,L,M shells is 2,8,18 etc., as such
most e’ in atoms occupy the outer shells

9. ELECTRON BINDING ENERGIES
• There’s an electrostatic force of attraction between the nucleus(+) and
electrons(-)
• The force depends on:
- nuclear charge
- electron charge
- distance between electrons and nucleus(shell radii)
• Therefore:
- Electrostatic force increases with Z
- The force decreases rapidly as the shell number increases
• An electron may be ejected from its shell by application of an external
source of energy. However, the energy supplied must overcome the
electrostatic force of attraction

10. Electron binding energies cont
• The minimum energy required to eject an electron from its shell is called
the binding energy
• The B.E. of a free e’ is zero
• In a given atom, B.E. is highest for K shell and decreases rapidly with
increasing shell number
• In corresponding shells, B.E. increases with Z

11. ELECTRON BINDING ENERGIES
example
MATERIAL Z B.E(K.ev)
shell K L M
TUNGSTEN 74 69.5 11.0 2.8
COPPER 29 9.0 1.1 0.12

12. IONIZATION, EXCITATION OF ATOMS
• An atom may be ionized by application of sufficient energy to eject an e’
from a shell
• Following ionization an e’ vacancy is created in the affected shell
• The atom is left in an excited state
• An excited atom has a tendency to undergo rearrangement so as to revert
to its normal configuration and energy state(de-excitation)
• De-excitation takes place through a series of e’ transitions from outer to
inner shells to fill e’ vacancies
• At the end of the series a free e’ will have come in to complete the de-
excitation process

13. EMMISION OF ELECTROMAGNETIC RADIATION FROM ATOMS
• EM radiation is produced whenever a charged
particle, e.g an e’ is attracted rapidly to fill a
vacancy in an inner shell
• Rarely a free e’ may be attracted fom outside
the atom to fill the shell vacancy.