RADIOACTIVITY [PHYSICS]

 

DATE: -

CLASS: - S. S. 3                                                                   DURATION: - 2 HRS 40MINS

TOPIC: -                                                                RADIOACTIVITY              

INSTRUCTIONAL MATERIALS: -                 

REFERENCES BOOK: -

1.       FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.       M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.  

3.       INTERNET           

PREVIOUS KNOWLEDGE: - Students have been familiar with gadgets uses electromagnetic principles.

 OBJECTIVES: -  At the end of the lesson students should be able to: -

1.       Define radioactivity

2.       State methods of detection of radiation

3.       List the types of radioactivity

4.       Difference between nuclear fusion and fission

CONTENT: -

RADIOACTIVITY

Radioactivity is the decay or disintegration of the nucleus of an unstable nucleus or atom (of an element) with the release of any one, two or all of α. Β and ϒ radiation. It can also be define as the process by which the atom of a natural occurring substance emits particles or radiation and energy as a result of spontaneous disintegration of its atomic nucleus. E.g. radium, radon, ionium, polonium.  The process is said to be spontaneous because it is not influenced by any physical factors such as temperature, pressure, time, etc.

Methods of Detection of Radiation.

i.                     Photographic film/plate                                        ii. Geiger muller tube                     iii. Cloud Chamber

i.                     Semi-conductor detector                                     v. Gold-leaf electroscope             vi. Solid state detector

vii.                Dekatron Counter                                                   viii. Crooke’s Sprinthariscope (Scintillation)

TYPES OF RADIOACTIVITY

There are two basically types of radioactivity: Natural and Artificial Radioactivity.

NATURAL RADIOACTIVITY: This occurs when a substance absorbs heat energy from its surroundings. When it is buried under the soil or exposed to air, its nucleus becomes unstable and may split or break up by itself.

Types of natural radioactivity

i.                     Alpha emission- when radioactive element emits only alpha particles.

 E.g.           +  + Energy

ii.                   Beta emission- when radioactive element emits only beta particles.

E.g.           +  + Energy

     iii.           Gamma emission- when radioactive element or substance emits only a gamma radiation. Ina balance        nuclear equation the emission of gamma particles is not reflected as in the case of other particles, rather it occurs in certain nuclear reaction.  E.g.           +  + ϒ

ARTIFICIAL RADIOACTIVITY: This radioactivity occurs when a non-radioactive and a radioactive element or substance are bombarded with fast moving particles such as proton, neutrons, electrons etc. As a result, the nucleus becomes unstable and may split or break up. 
e.g.  +                     +     +Energy

                                Unstable

This is sometimes refers to as induced radioactivity.

Differences between Natural and Artificial Radioactivity

Natural Radioactivity	Artificial Radioactivity
1. spontaneous disintegration of nucleus	Nucleus must be bombarded by fast-moving particles
2. unstable nucleus	Stable nucleus
3. heavy  nucleus	Light nucleus

 TYPES OF RADIATION

There are three different types of radiation.

Alpha particle- a

Beta particle- B

Gamma ray-r

 Table below shows the characteristics of alpha particle, beta particle, and gamma particle.

Characteristic	Alpha particle	Beta particle	Gamma ray
Nature	Positively charged helium nucleus, He	Negatively charged electron, e	Neutral electromagnet ray
In an electric field	Bends to the negative plate	Bends to the positive plate	Does not bend, showing that it is neutral.
In magnetic field	Bends a little showing that it has a big mass. Direction of the bend indicates that it is positively charged.	Bends a lot showing that it has a small mass. Direction of the bend indicates that it is negatively charged.	Does not bend showing that it is neutral.
Ionising power	Strongest	Intermediate	Weakest
Penetrating power	low	Intermediate	High
Stopped by	A thin sheet of paper	A few millimeters of aluminium	A few centimeters of lead or concrete
Range in air	A few centimeters	A few metres	A few hundred metres
Speed	1/20 X the speed of light, c	3%-99% of the speed of light, c	The speed of light, c

HALF-LIFE

Half-life of a radioactive element is the time taken  for half of the atoms initially present in the element to decay.

For instance, if the half-life of an element is 2 years, it means that after 2 years,10g of an element will have a mass of 5g, after 4 years , the mass of the element will be 2.5g and so on.

If the original number of atoms = N_o, when t = 0

The remaining number after time t = N and the half life = t^1/2

Then the relation below can be applied for easy calculation

                                                 = ( )^t/T

DECAY CONSTANT, is defined as the ratio of the number of atoms disintegrating or breaking down per atom to the number of atoms remaining. i.e.

Decay constant  =

                                  =  where T= ½ life

Example

 The half life of a radioactive material is 6 hours. What quantity of 1Kg of the material would decay in 24 hours?

Solution

After 6 hrs, Kg decays; Kg remains

After 6 hrs,   Kg decays; Kg remains

After 6 hrs, Kg decays; Kg remains

After 6  hrs, Kg decays; Kg remains

Therefore, the material decayed would be: 1-  =

 

 

Mass defect

The difference between the mass of an atom and the sum of mass of its constituent parts is called its mass defect.

Mass defect can be explained by Einstein’s mass-energy equivalence: As energy required to break apart a nucleus, the sum of energy contained in the constituent nucleons is higher than that of the combined nucleus. Energy is related to mass.

ȡm = mass of the initial nucleus – total mass of the split apart.

Atomic energy

This is the energy released as a result of the splitting of a radioactive nucleus by fusion or fission.

 

Nuclear binding energy

The nuclear binding energy of a nucleus is the amount of work required to separate the nucleons inside the nucleus.

Binding energy per nucleon = binding energy of nucleus / number of nucleons in nucleus.

where E is energy in J, m is mass in kg, and c is the speed of light in m/s.

Problems

Calculate in joules the binding energy for  [atomic mass of  = 58.9332u][mass of proton = 1.00783u  ; mass of neutron = 1.00867u; unified atomic mass unit, u=931Mev; 1ev = 1.6x10^-19J, 1Mev = 1.6x10^-13J; electron mass = 9.11x10^-31J].

Solution

No of nucleon = 59; atomic no or no of proton = 27; no of neutron = 59-27= 32.

Total mass of the nucleons = (27x1.00783) + (32x1.00867) = (27.21141+32.27744) = 59.48885u

Atomic mass of  = 58.9332u

Binding energy = (59.48885 -  58.9332)u = 0.55565u

                                = 0.55565 x 931Mev = 517.31Mev

                                =517.31 x 1.6 x10^-13J = 8.27x10^-11J

Nuclear fission

·         Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei.

·         A great amount of energy is released in nuclear fission due to the greater binding energy of the daughter nuclei.

·         Nuclear fission is used in nuclear power plants.

 +                                         +   + 2  + Energy

Nuclear fusion

·         Nuclear fusion is the joining of two light nuclei to form a heavy nucleus.

·         A great amount of energy is released in nuclear fusion due to the greater binding energy of the daughter nucleus.

·         Nuclear fusion yields more energy than nuclear fission.

·         Nuclear fusion is the main source of the sun’s energy.

 +                + energy

NUCLEAR ENERGY

Component	Function / Explanation
Graphite Moderator	Fast moving neutrons are slowed down by collisions with nuclei in the moderator so that they can cause further fissions. In some nuclear power plant, the moderator is water.
Uranium rod (Fuel)	Fission reactions take place in the uranium rod to create nuclear energy. The uranium used is often ‘enriched’ by increasing the proportion of the isotope uranium-235 above the natural value of 0.7% to 3%.
Control Rod	The rate of the fission reaction is controlled by inserting or withdrawing these rods. The nuclei in the rods absorb neutrons without undergoing any reaction. Sometimes the rod is made of cadmium.
Coolant	To take away heat from the nuclear reactor. Substances with high specific heat capacity such as ‘heavy’ water and carbon dioxide are used.
Thick Concrete Wall	To avoid the run off of harmful radiations.
Steam generator	Water in the generator is heated and changed into steam. The steam then drives the turbines.
Turbines	To revolve the dynamo in the electrical generator to generate electricity

 Nuclear reactors are used in the production of:
a) High-intensity neutron beams for research
b) Artificial Radioactive Isotopes for medical research
c) Fissionable transuranic elements such as plutonium from uranium-238
Reasons for the use of Nuclear Energy
1. Production of nuclear energy from nuclear fuels involves a decreased cost. A small amount of nuclear fuel can provide a large amount of energy.
2. Nuclear reactors are relatively safe especially with the sophisticated technology constantly developed and improved.
3. The decreasing supply of fossil fuels make it essential for the use of alternative sources of energy.
4. The use of nuclear energy does not release greenhouse gases such as carbon dioxide.
Reasons against the use of Nuclear Energy
1. Radioactive residues from nuclear stations have quite a long half-lives.
2. There is a chance of leakage in the radioactive waste containers placed underground or underwater.
3. High cost of constructing a nuclear power station.
4. Accidents could happen due to human error no matter how sophisticated the technology is and this should be put into consideration.

PRESENTATION

Step I: The teacher revises the previous topic.

 Step II: The teacher explains radioactivity.

Step III: The teacher explains methods of detecting radiation.

Step IV: The teacher explains characteristics of alpha, beta and gamma emission.

Step V: The students differentiate alpha, beta and gamma by their characteristics.

Step VI: The teacher explains nuclear energy.

Step VII: The teacher leads the students to solve problems on binding energy and radioactivity

 EVALUATION

The teacher evaluates the lessons by asking these questions:

1.       Define radioactivity

2.       State methods of detection of radiation

3.       List the types of radioactivity

4.       Difference between nuclear fusion and fission

  ASSIGNMENT