Lecture no 1

FRCR Physics
ESLAM KAMAL
Outlines

1 - Basic science
• Atomic structure
• Radioactive decay
• Electromagnetic radiation
1.1 - Atomic structure
The Rutherford-Bohr model of an atom
• Where is the central mass of the atom ?
Nucleus
Describing an atom
 Electrons
Types of electrons
Electrons are either bound or free.
• Bound electrons: These are the electrons that are held in orbit around the nucleus in the electron shells by
the attractive force of the positive nucleus.

• The binding energy is the positive energy required to overcome the pull of the nucleus
and release the electron from the shell. This is of the same magnitude as the actual
(negative) energy of the electron that is released if the electron is freed.
• Free electrons: These are the electrons that are not bound in an electron shell around
a nucleus. They have a kinetic energy of:
 • Binding energy for k-shell is greater than L-shell
Ek>El>EM.
increasing the atomic number , increase the
binding energy .

Never greater than 100 keV.

􀀸 The binding energy depends on
1. The shell (EK > EL > EM …).
2. The element (↑ Atomic number → ↑ binding energy)
 Free and valence electrons
Free Electrons: Free electrons are electrons that are not attached to an atom.
Ionization & Excitation:
• Ionized atom → if one of its electrons has been completely
removed → ion pair
"electron + positive ion"
• Excited atom → if an electron is raised from one shell to a farther
one with the
absorption of energy → the atom has more energy than normal.
When it falls back → energy is re-emitted as a single 'packet' of
energy or light photon.
 1.2 Nuclear stability
• The nucleus is composed of protons and neutrons. The protons repel each
other (electrostatic force) but the nucleus is kept held together by the
strong nuclear force.
• Strong nuclear force: There is a strong force of attraction at distances
between nucleons of <10-15 m which changes to a repulsive force at <10-
16 m. The nucleons are kept apart at a distance of ~ 5 x10-16 m, the
distance at which there is the greatest attraction.
• Electrostatic force: this is the force of repulsion between protons. At
distances of 10-15 to 10-16 m the strong attractive interaction (strong
nuclear force) is much greater than the repulsive electrostatic force and
the nucleus is held together.
As the atomic number increases (i.e. the number of protons)
more neutrons are required to prevent the electrostatic
forces pushing the protons apart and to keep the nucleus
stable.
If an atom has too many or too few neutrons and does not
lie upon the "line of stability", it becomes unstable and
decays to a more stable form. This is the basis of
radioactivity and is discussed next in the "electromagnetic
radiation" chapter.
• 1.2 - Radioactive decay
Radioactive decay generally involves the emission
of a charged particle or the capture of an
electron by the nucleus to form stable nuclides.
The amount
of decay = the radioactivity = the number of
nuclear transformations per second.
In the chapter on "Atomic
structure" we covered
nuclear stability and
referred to the Segré chart.
What the line of stability
shows is that as the number
of protons increases, the
proportion of neutrons
needed to keep the nucleus
stable increases. When the
nuclide doesn't lie on the
line of stability it becomes
unstable and radioactive.
Nuclear Reactor : atoms can’t be separated chemically as ,all have the
same Z

Cyclotron : atoms can be separated chemically as ,all have different Z

Decay model of nuclides
Types of radiation
When a nuclide undergoes radioactive decay it breaks
down to fall into a lower energy state expending the excess
energy as radiation. The radioactivity released can be:
1. Alpha particles
2. Beta particles
3. Gamma particles (or photons)
4. Others
Penetration of different types of radiation
1. Alpha particles
• Symbol: α
• Formed of 2 protons and 2 neutrons (i.e. a helium atom)
• Positively charged
• Relatively heavy
• Short range of travel
2. Beta particles
• Symbol: β
• Electrons emitted from radioactive nuclei
• Carry negative charge
• Split into β- (negatron) and an antimatter equivalent β+
(positron)
• Lighter and smaller than α
3. Gamma particles
• Symbol: γ
• Identical to x-rays except for the origin (x-rays originate from electron
• bombardment, gamma particles from radioactive atoms)
• Result of transition between nuclear energy levels
• Very high energy and range of travel
4. Others
• X-rays
• Internal conversion: γ ray energy transferred to inner shell electron which is then
emitted from the nucleus
• Auger electron: ejected from electron shells as a result of same radioactive decay
processes that create electron shell vacancies. Competes with emission of x-rays.
• Neutrinos and anti-neutrinos: electrically neutral particles with very
• little mass emitted from atomic nuclei during β+ and β- decay respectively.
• Spontaneous fission: very heavy nuclides are so unstable they split into
• two smaller nuclides emitting neutrons in the process.
Natural source of Radiation
An alpha particle is a helium nucleus.
•It has a relative charge of +2.
•Its penetration power is the
lowest among the three types
of particles and can be blocked
by a piece of paper or a few cm
of air.
•Its ionizing power is the
highest among the three types
of particles.
(Z=53) I-131 Xe-131(Z=54)
A beta particle is an
electron or a positron.
•It has a relative charge of -1
or +1.
•Its penetration power is in
the middle among the three
types of particles and can be
blocked by a thin sheet of
aluminum.
•Its ionizing power is in the
middle among the three types
of particles.
The electron capture
99mTc & 99Tc are isomers
that have different energy
states and different half-
lives
•Gamma rays are photons.
•It does not have a charge.
•Its penetration power is
the highest among the
three types of particles
and can be blocked by
several cm of lead.
•Its ionizing power is the
lowest among the three
types of particles.
1.3 Electromagnetic radiation
• Electromagnetic (EM) radiation arises from oscillating electric and magnetic fields. It can be considered either as a stream
of quanta (photons, particles) or waves.
• EM radiation as waves Concerning the wave aspect, it is a sinusoidally varying electric and magnetic field vector pointing
at right angles to one another and to the direction of the travel of the wave
EM radiation as particles
When considering EM radiation as particles, the particles are small packets, or quanta, of energy called photons that travel in
straight lines. The energy of the photon packet is measured in joules but this is inconveniently small when describing EM
radiation so the unit of electron-volt is used.
 Inverse Square Law
• the total amount of
radiation does not change
but its concentration
decreases

• We can conclude that the

concentration of radiation is inversely
related to the square of the distance
from the source. This is commonly
known as the inverse-square law.
Beam intensity =energy fluence rate is
the total amount of energy per unit area
traveling per unit time (strength of the
beam )
• Electromagnetic rays originate from a point source and diverge out, traveling in a
straight line, unless attenuated.
• The inverse square law states that the intensity of radiation emitted from a point
source will reduce in intensity, proportional to the square of the distance from that
point, i.e. the area covered by the beam increases as the rays are diverging from a
point; however, the number of photons remains the same and hence their intensity
reduces.

• Important points to remember regarding the inverse square law are:

i. Radiation comes from a point source.
ii. There is no absorption or scatter of radiation between a source and point of
measurement.
Thank You