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ORIGIN AND NATURE OF RADIATION
CONCEPT OF ELEMENT, COMPOUND,
ATOM AND MOLECULE
Elements - A substance that cannot be decomposed into simpler
substances.
- eg. carbon, oxygen, iron, etc.
Compounds - combinations of few elements
- eg. stainless steel, mild steel, cast iron
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Continued.
Atoms - smallest part of an element
- consists of :a) protons
b) neutrons
c) electrons
Molecules - smallest portion of a material that can exist
independently and able to retain the properties of the
original compound
eg. water
ORIGIN AND NATURE OF RADIATION
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ORIGIN AND NATURE OF RADIATION
STRUCTURE OF ATOM
a) Protons - positive electrical charge (+ve)
- mass = 1.673 x 10-24 gram
- located in the nucleus
b) Neutrons - neutral electrical charge
- mass = 1.675 x 10-24
gram (slightly heavier than protons)- also located in the nucleus
c) Electrons - negative electrical charge (-ve)
- mass = 9.11 x 10-28 gram (1/1840 of the mass of proton)
- located in its orbits
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ORIGIN AND NATURE OF RADIATION
ATOMIC NUMBER AND WEIGHT
Generally, all elements are written as:
ZXA
eg. 1H1, 2He
4, 8O16
where Z - atomic number
- denotes the number of protons in the nucleus
x - element
A - mass number
- sum of neutrons and protons in the nucleus
- approximately equal to the atomic weight
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ORIGIN AND NATURE OF RADIATION
ISOTOPES AND RADIOISOTOPESIsotopes - elements with a same atomic number (Z) and different mass
number (A)- eg. 1H
1, 1H2, 1H
3, 92U235, 92U
238
- stable when the extra neutron does not upset the balance in
the nucleus
Radioisotopes - unstable neutron (the extra neutron disturb the balancein the nucleus)
- it will disintegrate or decay to be stable
- natural radioisotopes : radium, radon
- artificial radioisotopes : U-238, Co-60, etc.: produced in the nuclear reactor
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ORIGIN AND NATURE OF RADIATION
DISCOVERY OF RADIATION
1985 - Roentgen found that rays caused certain chemicals to fluoresceand can penetrate solid substances.
- X - RAYS
1896 - Bacquerel discovered that certain radiation from certain
material could fog the photographic film
- material that contains uranium give off radiation that couldpenetrate materials
Years later - Rutherford classified those radiation into three types:
1) alpha particle (+ve charged)
2) beta particle (-ve charged)
3) gamma rays (no charged)
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ORIGIN AND NATURE OF RADIATION
TYPES OF RADIATION
1) Particulate Radiation - tiny particles that move in space
- eg. alpha, beta and neutron- the characteristics :
a) have mass
b) have an electrical charge
c) able to transfer energy from one point
to anotherd) travel at high speed
e) may strike and be deflected by other
particles
f) maybe stopped and captured by nucleig)may cause ionisation
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ORIGIN AND NATURE OF RADIATION
Continued.2) electromagnetic radiation - the characteristics:
a. have no mass or weightb. travel at speed of light
c. travel in straight line
d.not affected by magnetic field
- parameters involve:a. frequency
b. wavelength
c. speed of radiation through space
frequency X wavelength = speed
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ORIGIN AND NATURE OF RADIATION
RADIOACTIVITY
Definition - disintegration of unstable isotopes / radioisotopes emittingtiny particles travelling at high speeds and/or energy in the
form waves.
- types of particles and energies that are released from the
nuclei of radioactive atom :
a) alpha particles - largest particulate radiation(2 protons and 2 neutrons)
b) beta particles - very light high speed electron
c) gamma ray - energy waves
- widely used in industrial radiography eg.Ir-192
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ORIGIN AND NATURE OF RADIATION
Continued
Unit of radioactivity - old unit : curie: 1Ci = 3.7 x 1010 disintegration
per second
- new (S.I) unit : Becquerel (Bq)
: 1 Bq is defined as 1 integration
per second: 1Ci = 3.7 x 1010 Bq
Specific activity - defined as disintegration per second per unit mass
of the source- unit : Ci / gm
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v!
2
101
Tt0.693expNN
ORIGIN AND NATURE OF RADIATION
PRINCIPLE OF RADIOACTIVE DECAY
Radioactive decay - the activity of a radioactive material dies outwith the passage of time
# IMPORTANT!!!! Pg 141
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ORIGIN AND NATURE OF RADIATION
Continued...
where,N0 = number of radioactive atoms present at some
reference time t0
Nt = number of radioactive atoms left after time t has passed
T1/2 = half-life of the radioisotope
* T1/2 - TIME AFTER WHICH THE NUMBER OF RADIOACTIVE
ATOMS REDUCE TO HALF (50%) FROM ITS INITIAL
VALUE
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ORIGIN AND NATURE OF RADIATION
Continued.
Eg. 1. The source strength of radioisotope Ir-192 40 days ago was 9 Ci.What is the present strength? (half life of Ir-192 is 75 days)
Solution :
Given t = 40 days
T1/2 = 75 days
N0 = 9 Ci
v
!
2
1
0
T
t693.0expNN
v
!
21T
t693.0expCi9N
= 9 Ci x 0.691
= 6.22 Ci
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ORIGIN AND NATURE OF RADIATION
INTERACTION OF RADIATION WITH MATTER
1. IONISATION
- any action which disturbs the electrical balance of the atom
- this process may change the number of electrons
2. IONS- atoms, molecules and various subatomic particles that carry either
positive or negative electrical charge
- eg. Fe2+, Cl-
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ORIGIN AND NATURE OF RADIATION
MODE OF INTERACTION
1. Photoelectric absorption- an interaction with orbital electrons in which a photon of
electromagnetic radiation is consumed in breaking the bond and
imparts kinetic energy to the electron
- it occurs at low energy (
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ORIGIN AND NATURE OF RADIATION
Continued4. the electron becomes negative ions and the atom
becomes positive ions.
2. Compton Scattering
- interaction between an incident photon and a loosely held orbital
electron
- only a portion of kinetic energy of the photon is consumed- this interaction occurs at energy about 0.1 to 3.0 MeV
3. Pair Production
- photon interaction with the electric field surrounding or in the
vicinity of the nucleus atom
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Pg 14
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ORIGIN AND NATURE OF RADIATION
Continued
- the energy from the incident atom is completely absorbed increation of a positron electron pair
- it happens at very energy which the photon energy must be above
1.02 MeV
- the positron that is created will combine with electron and emit two
gamma photons with low energy that is 0.51 MeV each- the low energy gamma photons will be the incident photon and
may cause ionisation by the photoelectric effect or by Compton
scattering
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ORIGIN AND NATURE OF RADIATION
ATTENUATION OF X- AND GAMMA
RADIATION
- attenuation happens due to
a. transmission
b. absorption
c. scattering
- when X or gamma ray (with uniform energy) pass through a materialtheir intensity will decrease exponentially to an extent which is
definite function of the thickness of the absorbing material
- the photons will decrease by a constant percentage of the whole
for a given thickness of a absorber material
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ORIGIN AND NATURE OF RADIATION
Continued-how much the radiations are absorbed or scattered gives information
about the materials that the radiation pass through-the formula for intensity is given as below:
t
O
eIIQ
!
where I = intensity of the transmitted radiation
I0 = initial intensity
t = thickness of material
ffgdb= attenuation coefficient (cm-1)
# ideal case :
- thin material
- exam !!!!
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ORIGIN AND NATURE OF RADIATION
Continued
Eg. 1) If the dose rate from a radiographic source of Ir-192 is 100 mR/hr
What thickness of lead shielding is required to reduce the doserate at 1.0 mR/hr?
Solution: I = 1.0 mR/hr , I0 = 100 mR/hr , x = ?
1 = 100 e (-1.435 x)
1 / 100 = e (-1.435x)
0.01 = e (-1.435x)
ln 0.01 = ln e (-1.435x) (check back)
- 4.6052 = - 1.435 t
t = 4.6052 / 1.435 = 3.209
t0 eII
Q!
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ORIGIN AND NATURE OF RADIATION
BUILD - UP FACTOR
- build up intensity radiation happens due to multiple scattering
- applicable for thick material
x
0 eBIIQ
!
where B is the build up factor
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ORIGIN AND NATURE OF RADIATION
Continued
-build up factor is defined as :* the ratio of the actual radiation flux and the calculated flux *
fluxCalculated
fluxTrue
B!
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