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Basic Radiation Physics
Pradeep Kumar S
Assistant Professor of Radiology Physics
Govt. Royapettah Hospital, Chennai

• The periodic table of elements lists the elements in ascending order of atomic number

1.2 ATOMIC AND NUCLEAR STRUCTURE
1.2.1 Basic definitions for atomic structure
• The constituent particles forming an atom are:
• Proton
• Neutron
• Electron
Protons and neutrons are known as nucleons and form the
nucleus.
• Atomic number Z
Number of protons and number of electrons in an atom.

1.2.1 Basic definitions for atomic structure
• Atomic mass number A
Number of nucleons (Z+N) in an atom,
where
• Z is the number of protons (atomic number) in an atom
• N is the number of neutrons in an atom.

ATOMIC AND NUCLEAR STRUCTURE
Basic definitions for atomic structure
In nuclear physics the convention is to designate a
nucleus X as
where
A is the atomic mass number
Z is the atomic number
For example:
• Cobalt-60 nucleus with Z = 27 protons and A = 33 neutrons is
identified as .
• Radium-226 nucleus with 88 protons and 138 neutrons is
identified as .
Z
A
X
88
226
Ra
27
60
Co

1.2.2 Rutherford’s model of the atom
• Rutherford’s atomic model is based on results of the
Geiger-Marsden experiment of 1909 with 5.5 MeV
alpha particles scattered on thin gold foils with a
thickness of the order of 10-6 m.

Review of Radiation Oncology Physics: A
Handbook for Teachers and Students -
1.2.2 �ݺ�ߣ 2
• At the time of the Geiger-Marsden experiment Thomson
atomic model was the prevailing atomic model.
• The model was based on an
assumption that the positive
and the negative (electron)
charges of the atom were
distributed uniformly over the
atomic volume
(“plum-pudding model”).

1.2.2 �ݺ�ߣ 4
• Ernest Rutherford concluded that the peculiar results of
the Geiger-Marsden experiment did not support the
Thomson’s atomic model and proposed the currently
accepted atomic model in which:
• Mass and positive charge of the
atom are concentrated in the
nucleus the size of which is
of the order of 10-15 m.
• Negatively charged electrons
revolve about the nucleus in
a spherical cloud on the periphery
of the Rutherford atom with a
radius of the order of 10-10 m.

1.2.2 �ݺ�ߣ 5
• Based on his model and four additional assumptions,
Rutherford derived the kinematics for the scattering
of alpha particles on gold nuclei using basic principles
of classical mechanics.
• The four assumptions are related to:
• Mass of the gold nucleus.
• Scattering of alpha particles.
• Penetration of the nucleus.
• Kinetic energy of the alpha particles.

1.2.2 �ݺ�ߣ 6
• The four assumptions are:
• Mass of the gold nucleus >> mass of the alpha particle.
• Scattering of alpha particles on atomic electrons is
negligible.
• Alpha particle does not penetrate the nucleus, i.e., there
are no nuclear reactions occurring.
• Alpha particles with kinetic energies of the order of a few
MeV are non-relativistic and the simple classical
relationship for the kinetic energy EK of the alpha particle is
valid: 

2
K
2
m
E

1.2.3 �ݺ�ߣ 1
1.2.3 Bohr’s model of the hydrogen atom
• Niels Bohr in 1913 combined Rutherford’s concept of
the nuclear atom with Planck’s idea of the quantized
nature of the radiation process and developed an
atomic model that successfully deals with one-
electron structures, such as the hydrogen atom, singly
ionized helium, etc.
• M nucleus with mass M
• me electron with mass me
• rn radius of electron orbit

1.2.3 �ݺ�ߣ 2
• Bohr’s atomic model is based on four postulates:
• Postulate 1: Electrons revolve about the Rutherford nucleus
in well-defined, allowed orbits (planetary-like motion).
• Postulate 2: While in orbit, the electron does not lose any
energy despite being constantly accelerated (no energy
loss while electron is in allowed orbit).
• Postulate 3: The angular momentum of the electron in
an allowed orbit is quantized (quantization of angular
momentum).
• Postulate 4: An atom emits radiation only when an electron
makes a transition from one orbit to another (energy
emission during orbital transitions).

1.2.3 �ݺ�ߣ 9
• Energy levels En of
orbital electron shells
in a one-electron
Bohr atom are:
• ER = Rydberg energy
En
 ER
Z
n






2
 13.6 eV
Z
n






2

1.2.4 �ݺ�ߣ 4
1.2.4 Multi-electron atom
Energy level diagram for
multi-electron atom (lead)
Shell (orbit) designations:
n = 1 K shell (2 electrons)
n = 2 L shell (8 electrons)
n = 3 M shell (18 electrons)
n = 4 N shell (32 electrons)
……

1.2.5 �ݺ�ߣ 1
1.2.5 Nuclear structure
• Most of the atomic mass is concentrated in the
atomic nucleus consisting of Z protons and A-Z
neutrons where Z is the atomic number and A the
atomic mass number (Rutherford-Bohr atomic
model).
• Protons and neutrons are commonly called nucleons
and are bound to the nucleus with the strong force.

1.2.5 �ݺ�ߣ 2
• In contrast to the electrostatic and gravitational forces
that are inversely proportional to the square of the
distance between two particles, the strong force
between two particles is a very short range force,
active only at distances of the order of a few
femtometers.
• Radius r of the nucleus is estimated from: ,
where ro is the nuclear radius constant (1.4 fm).
r  ro
A
3

1.2.5 �ݺ�ߣ 3
• The sum of masses of the individual components of a
nucleus that contains Z protons and (A-Z) neutrons is
larger than the mass of the nucleus M.
• This difference in masses is called the mass defect
(deficit) and its energy equivalent is called
the total binding energy EB of the nucleus:
m mc2
EB
 Zmp
c2
 (A  Z)mn
c2
 Mc2

1.2.5 �ݺ�ߣ 4
The binding energy per nucleon (EB/A) in a nucleus varies
with the number of nucleons A and is of the order of 8
MeV per nucleon.
EB
A

Zmp
c2
 (A  Z)mn
c2
 Mc2
A
Nucleus EB/A (MeV)
1.1
2.8
2.6
7.1
8.8
7.3
2
1H
3
1H
3
1He
4
1He
60
27Co
238
92U

1.2.6 �ݺ�ߣ 1
1.2.6 Nuclear reactions
• Nuclear reaction:
Projectile (a) bombards target (A)
which is transformed into nuclei (B) and (b).
• The most important physical quantities that are
conserved in a nuclear reaction are:
• Charge
• Mass number
• Linear momentum
• Mass-energy
A  a  B  b or A(a,b)B

RADIO NUCLIDES
• Nuclei having different number of protons, neutrons, or both are
called nuclides.
• Unstable nuclides are called Radionuclides ,and atoms with unstable
nuclei are called Radioisotopes.

Nuclear Terminology
• Nuclides with the same number of protons and different number of
neutrons== Isotopes
• Nuclides with the same number of neutrons but different number of
protons== Isotones
• Nuclides with the same number of nucleons == Isobars
• Identical nuclides with different energy states == Isomers

What is Radiation?
•Radiation:
Emission and propagation of energy in the form of
waves or particles

Classification of radiation
Radiation is classified into two main categories:
• Non-ionizing radiation (cannot ionize matter).
• Ionizing radiation (can ionize matter).
• Directly ionizing radiation (charged particles)
electron, proton, alpha particle, heavy ion
• Indirectly ionizing radiation (neutral particles)
photon (x ray, gamma ray), neutron

The process by which a neutral atom acquires a positive
or negative charge
Ionization
electron is
stripped from
atom
-
-
-
-
The neutral atom
gains a + charge
= an ion

• Ionization: Removal of electrons
• Excitation: Shifting of an electron to a higher energy level

Binding Energy
• Energy required to remove an electron completely from an
atom
• By convention, binding energies are negative with increasing
magnitude for electrons in shells closer to the nucleus
• Binding energy of electrons in a particular orbit increases with
the number of protons in the nucleus (i.e., atomic number, Z)

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