Origin and Nature of Radiation (English)

8/3/2019 Origin and Nature of Radiation (English)

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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


Pg. 136

Pg 136


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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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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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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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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)

Pg 138


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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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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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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

Pg 139


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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


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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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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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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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-

Pg 142


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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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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

Pg 143

Pg 14


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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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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

Pg 144


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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 !!!!

Pg 145


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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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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

Pg 145


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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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THANK YOU FOR ATTENTION

GOOD LUCK

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Origin and Nature of Radiation (English)

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