Topic 40 - RADIOACTIVITY

 GO HOME

SPECIFIC OBJECTIVES
By the end of this topic, the learner should be able to:
a) define radioactive decay and half-life;
b) describe the three types of radiations emitted in natural radioactivity;
c) explain the detection of radioactive emissions;
d) define nuclear fission and fusion;
e) write balanced nuclear equations;
f) explain the dangers of radioactive emissions;
g) state the applications of radioactivity;
h) solve numerical problems involving half-life.

INTRODUCTION
This is the emission of high penetrating particles from the nucleus.
This is the disintegration of nucleus with the emission of radiations which are in form of energy.

The atomic structure
The atom consists of:
a) Nucleus.
b) Protons.
c) Neutrons
Nucleus has both protons and neutrons. Electrons move around the nucleus in energy shells. Protons and neutrons are collectively called nucleons. A nuclide is an atom with a specified number of neutrons and protons. Number of protons in the atom is called its atomic number Z. Total number of neutrons and protons is collectively called mass number or nucleon number. An atom with atomic number Z and having N neutrons and mass number A, can be symbolized by

Where A = Z + N

Nuclear Stability
The nucleus is said to be stable if the ratio of neutron to protons is 1:1. If the ratio deviates from this ratio, the nucleus become unstable. If it’s is unstable the nucleus disintegrates in trying to achieve stability. Graph of number against number of protons is shown.

Stability curve 

From the graph, most of the unstable nuclides are outside the stability line. The nuclides above the line decay in order to increase the proton number and the nuclide is brought nearer to the stability line. Those nuclides below the line will decay in such a way that their proton number decreases (increases neutron proton ratio)

Types of radiations
In order to identify the types of radiations produced, radium was put in a thick lead box with a small opening. When a strong magnetic field was introduced perpendicular to the path of radiations, some were deflected. Using Fleming's left hand rule, it was shown that some radiations carried positive charge while others were negatively charged. Other radiations were not deflected by the magnetic field.
The positively charged radiations were called alpha(α) particles.
The negatively charged radiations were called Beta(β) particles.
The uncharged were called gamma rays(γ). (refer to the diagram below)
Radiations from radium source 

• P- positively charged (α) particles.
• R- negatively charged(β) particles.
• Q – carries no charge (γ) radiations.

Alpha Particles
• They are positively charged.
• Has a mass equal to that of helium atom (two protons and two neutrons)
• It is helium nuclei.
• When a nuclide decay by releasing an alpha particle, it loses two protons and two neutrons. (see the equation below)

Example
Uranium decays by emitting alpha particles to become thorium.

Penetrating power of alpha particles. 

The penetrating power of alpha particles is least. They are absorbed by papers of few mm thick. Alpha particles penetrate air to a few centimetres (about 5 cm)

Beta Particles 

Their mass and charge was found to be equal to that of an electron. Thus Beta particle is an electron. When a nuclide decays by emitting a beta particle, it loses an electron. This electron results fro the decay of a neutron to proton. A general equation for a beta decay is as shown below.

For example, sodium undergoes beta decay to become magnesium. The equation is written as follows:

When a nuclide undergoes beta decay,
a) Its atomic number increases by one.
b) Its mass number does not change.

Penetrating power of beta decay
It was observed that when radioactive source was placed near the counter:
a) When the distance between the source and the tube is increased, the count rate is not affected until the distance between them was a few metres.
b) When aluminium sheets are placed between the source and the tube, the count rate decreases.
The above observations showed that beta particles are able to penetrate a few metres of air but are absorbed by aluminium.

Gamma Rays  

During the decay, energy in form of electromagnetic radiation is released. This radiation is called gamma rays.  They have neither mass nor charge. They are similar to X- rays but have shorter wavelength. Have much penetrating power than alpha and beta particles. Gamma penetrate most materials and are only stopped by a block of lead about 5 cm thick or very thick concrete wall.
Penetrating power of radioactive particle. 

Ionizing Effect of Radiations
•Radiations knock electrons from air molecules, resulting in the formation of positive ions the process called ionization.
•Alpha particles is heavy and slow, it takes quite sometimes to pass through air hence causes more ionization as compared to beta and gamma radiations.

RADIATIONS DETECTORS
Some of the detectors include:
a) Photographic emulsions
b) Cloud chambers.
c)Geiger-Muller tube.
d)The pulse electroscope (pulse electrometer).

a) Photographic emulsions 

The three radiations affect photographic emulsion or plate. Workers who handle radioactive materials wear clothes with photographic emulsions i.e. when they darken shows that the worker needs more protection.

b) The Cloud Chambers
In cloud chambers saturated vapour (water or alcohol) is made to condense on air ions caused by ionization by radiations.
There are two types. Namely:
a) Expansion cloud chamber
b) Diffusion cloud chamber.

Expansion Cloud Chamber

 

When a radioactive element emit radiations into the chamber, the air inside the is ionized. If the piston is moved down suddenly, air in the chambers will expand causing cooling. The ions formed act as nuclei on which the saturated alcohol or water vapour condenses forming tracks. The nature of these tracks identifies the type of radiation.

Diffusion Cloud Chamber  

Made up transparent(cylindrical) container partitioned into two compartment. The upper compartment is fitted with a transparent Perspex lid and its top is lined with a thin strip of felt soaked in alcohol or water. The bottom compartment contains air at room temperature at the top. The air at the bottom is at a temperature of about -780 C due to a layer of dry ice placed in lower compartment. The alcohol vaporises in the upper warm region, diffuses down and is then cooled. At a region when air contains a layer of saturated vapour, alcohol droplets form on the air ions produced by radiation. The droplets are seen as tracks along the paths of radiations. Traces are well defined if an electric fields are created by frequently rubbing the Perspex lid of the chamber with a cloth. Tracks obtained vary according to the type of radiation.
Alpha Tracks
Tracks are short, straight and thick. This is because:
a) They are heavy ionization, rapidly losing energy (short range)
b) The are massive, their path cannot be changed by air molecules.

Beta Tracks
Tracks are thin and irregular in direction. This is because the beta particles are lighter and faster, causes less ionization of air molecules.

Gamma Tracks
Produce scanty. They behave like weak beta particles which are responsible for tracks seen.

c) The Geiger -Müller tube

Consists of a thin mica or aluminium window at one end of a closed glass tube. Argon gas and a little bromine gas at low pressure. A thin wire runs through the centre of the tube and is connected to the positive terminal of a high voltage supply. When a radioactive substance is placed is placed in front of the window, the radiation enters the tube(window) and ionizes the argon gas. The negative ion moves towards the central wire(anode) and positive ions move towards the walls(cathode) A pulse current flows. A corresponding pulse voltage is registered across the high resistance R. This current is amplified and passed through a loudspeaker and clicks are heard. The exact count rate of the radiation is registered if the loudspeaker is replaced by a rate-meter. Amplification increases the sensitivity of the tube and it is possible to register very small currents from beta and gamma radiations
Bromine gas helps to absorb any secondary electrons, a process called quenching the tube. Without bromine, secondary electrons will ionize more molecules so that the pulse current that is measured will not be due to the radiations only.

Background Radiations
These are the radiations that a counter registers some readings in the absence of radioactive source. The count registered due to background radiations is called background count. Some sources of background radiations include:
a) Cosmic rays from outer space.
b) Radiations from the sun.
c) Rocks with traces of radioactive materials(granite)
d) Natural and artificial radioisotopes.

Artificial Radioactivity
These are naturally occurring nuclides which can be made radioactive artificially by bombarding them with neutrons, protons or α-particles.

Decay Law
The choice of the nuclide that decays is governed by chance. Decay involves large number of atoms. Decay law states that: The rate of disintegration at a given time is directly proportional to the number of nuclide present at that time.

Half- life
This is the time taken for half the number of nuclides initially present in a radioactive sample to decay. Different nuclides have different half-lives e.g. Radium half-life = 16oo years. It will take 1600 years 2g of Radium to decay to 1g.
The number of nuclides remaining undecayed, N, after a time T is given by:  

Where No is the original number of nuclides and t1/2 the half – life.
Example 1
Half-life of a certain radioactive element is 16years.
a) What fraction of the element will be remaining after 48 years.
b) What fraction of the elements will have decayed after 64 years.

Half-life from the graph
The following graph was obtained from the reading of a counter connected to Geiger Muller tube placed in front of radioactive source.

Half-life of the source is 6.6 minutes.

Applications of Radioactivity
a) In medicine
•Gamma rays are used in control of cancerous body growth.
•Gamma rays are also used in sterilization of surgical equipment; they kill pests or make them sterile.
b) Detecting pipe bursts
•Water or oil mixed with radioactive substances can be used to detect leakages in the underground pipes. This is because they seep out hence the radiations can be detected from the surface.
c)Determining the thickness of metal foil.
•Radioactive radiations can be used to determine and maintain the thickness of metal foils, paper, plastics in manufacturing industries.
d)Detection of flaws.
•In welded joints, gamma radiations can be used to detect cracks and air spaces.

Hazards of Radiations
•Gamma rays causes damage to body cells and tissues if exposed on them for a longer time.
•Extreme heavy dose of gamma radiations may lead to death. Also extended exposure to gamma rays may lead to cancer, leukaemia, delayed and hereditary effects.

Nuclear Fission
•This is the splitting up of nucleus when bombarded with neutron.
•The splitting results into equal two nuclei with release in energy.
•This energy released is called nuclear energy.

Nuclear fusion
•This is the fusion of light elements to form a heavier nucleus.
•This fusion is accompanied by release of a lot energy that can be harnessed in nuclear power.

GO HOME