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Background

Around the time cathode rays were being discovered by Thomson and other scientists, Henry Becquerel accidentally discovered another radiation. Evidence collected on the atom suggested it could be sub-divided while others showed certain atoms disintegrated by themselves. Becquerel discovered the phenomenon while investigating properties of fluorescent materials as he used photographic plates to record it. The photographic plates were kept in same drawer with mineral uranium while covered in opaque paper. On developing the films they were found to be blackened or fogged. Becquerel concluded that Uranium material must have emitted some radiations that might have penetrated heavy paper and affected photographic plates.

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Objectives

By the end of the lesson the you should be able to:

-Define radioactive decay and half-life

-Describe the types of radioactive emissions in neutral radioactivity

-Explain the detection of radioactive emissions

-Define nuclear fission and fusion

-Write balanced nuclear equations

-Explain the dangers of radioactive emissions

-State the applications of radioactivity


-
Solve numerical problems involving half-life.















Introduction

An atom consists of a central nucleus having protons and neutrons called nucleons with electrons orbiting around the nucleus. The sum of protons and neutrons is called the atomic mass and the number of protons called the atomic number. All natural elements with atomic numbers greater than 83 are radioactive. However, there are a few isotopes of elements with atomic number less than 83 were discovered to be radioactive. If the ratio of neutrons to protons is more than 1 (one) the nucleus is usually unstable. Play the animation below to observe a visualization of an atom with protons (+) and neutrons while electrons, e, are orbiting around the nucleus.

Definition of radioactivity

Due to the imbalance between neutrons and protons in the nucleus, there is instability. To achieve stability the nucleus will tend to break (disintegrate) with emission of energy and particles. This disintegration of nucleus is known as Radioactivity. Click on the play button of the animation below to observe this.

Observations

Disintegration is likely to continue further as long as the resultant nuclei are still unstable. Therefore, Radioactivity is the spontaneous and random disintegration of an unstable nuclide (an atomic nucleus with a specific number of protons and neutrons) with the emission of particles and rays.

Radioactive decay

Radioactive decay is a spontaneous and random process in which one cannot predict the nuclide that will disintegrate next and at what instant. To visualize this, play the animation presented below.

Half - life

There is a time period associated with decay of a radioactive substance when the original value reduces to a half. The length of time during which, half of the original number of atoms of a radioactive substance/nuclei decays is known as half-life. Different nuclides have different half-lifes. For example, the half-life of radium is 1600 years. This means that 2g of radium will take 1600 years to decay to 1g, similarly 1g of radium will take the same period (1600 years) to decay to 1/2g. The animation below shows how determination of half - life can be done, click on its play button and make your observations.

Types of radiations

There are three types of radiations that can be emitted during a radioactive decay. These are: alpha (A) beta (B) and gamma (G) radiations.

 

Alpha Particle

Beta Particle

Gamma rays

Gamma rays is energy emitted in form of electromagnetic radiation. They have neither mass nor charge. The main difference between X-rays and G-rays is that G-rays originate from energy changes in the nucleus of atoms while X-rays originate from energy changes associated with the electron structure of atoms.

Detection of radioactive emissions

Photographic emulsions

All the three emissions (A,b and G) affect photographic emulsions or plate. They darken or blacken photographic films. The emission's effect is observed when such plate is developed. Play the animation below to observe how this takes place.

Cloud Chamber

Common types of cloud chambers are expansion and diffusion cloud chambers. Click on the play button of the animation below to observe how it works.

Explanation

When air is cooled until the vapor it contains reaches saturation, it can be cooled further without condensing until it reaches super-saturation. Dust should not be present since it would act as centre of condensation. The radioactive emissions ionize molecules in a cloud chamber to different degrees depending on the type of emission.

The tracks obtained vary according to the type of radiation.

(i) Alpha particles cause heavy ionization, rapidly losing energy hence the tracks will have a short range. Because of their larger mass they do not change their path in air and as they pass, they cause more ionization on their path by knocking off many electrons.

(ii) Tracks formed by beta particles are thin and irregular in direction. This is because beta-particles are lighter and faster hence cause less ionization of air molecules. Also the particles are repelled by electrons of atoms in their path.

(iii) Gamma rays produce scanty disjointed tracks. The radiation ejects electrons from molecules and these electrons behave like weak beta-particles.

The Geiger-Muller tube

It is a type of ionization chamber. To visualize how this device works, play the animation below by clicking on its play button.

Observation
When there is no source in front of the tube the scale records background radiation which occurs naturally. An increase in the scale reading when radioactive source is introduced is noted.
Explanation

When the source is placed in front of the window, emitted radiations or particles enter the tube through mica window and ionize Argon gas. The negative ions move towards the anode while positive ones towards the wall or cathode. These ions collide with other gas molecules causing further ionization. This avalanche effect causes a pulse of voltage corresponding to current flowing and it is registered on the scale. An amplifier can be used to produce audible clicks through a speaker. It can also be used to amplify small currents from B-particles and G-rays emissions in the scale. Bromine gas is used as a quencher to absorb secondary electrons which could ionize molecules and also prevents sparking.


Spark Counter

It consists of wire gauze placed a few millimeters from a thin wire which has an adjustable voltage from an E.H.T source. Play the video below to observe how it works.


Explanation

When source of emission(A-particles) is brought near the wire gauze, air in its neighborhood is ionized and ions make the gap between the gauze and wire conduct more easily. The positively charged ions will flow towards the wire gauze causing flow of current which produces the sparks seen. The Counter is more sensitive to A-particles than B-particles or G-rays due to their lower ionization power.

Cloud Chamber

Common types of cloud chambers are expansion and diffusion cloud chambers. Click on the play button of the animation below to observe how it works.

Explanation

When air is cooled until the vapor it contains reaches saturation, it can be cooled further without condensing until it reaches super-saturation. Dust should not be present since it would act as centre of condensation. The radioactive emissions ionize molecules in a cloud chamber to different degrees depending on the type of emission.

The tracks obtained vary according to the type of radiation.

(i) Alpha particles cause heavy ionization, rapidly losing energy hence the tracks will have a short range. Because of their larger mass they do not change their path in air and as they pass, they cause more ionization on their path by knocking off many electrons.

(ii) Tracks formed by beta particles are thin and irregular in direction. This is because beta-particles are lighter and faster hence cause less ionization of air molecules. Also the particles are repelled by electrons of atoms in their path.

(iii) Gamma rays produce scanty disjointed tracks. The radiation ejects electrons from molecules and these electrons behave like weak beta-particles.


b. Cloud Chamber

When air is cooled until the vapor it contains reaches saturation, it can be cooled further without condensing until it reaches super-saturation. Dust should not be present since it would act as centre of condensation. The radioactive emissions ionize molecules in a cloud chamber to different degrees depending on the type of emission. Common types of cloud chambers are expansion and diffusion cloud chambers. In both types saturated vapor of either water or alcohol is made to condense on air ions caused by the emissions producing whitish drops of tiny liquid droplets to show tracks when illuminated. To visualize this, Click on the play button of the animation below to observe this.

Explanation:

The tracks obtained vary according to the type of radiation.

(i) Alpha particles cause heavy ionization, rapidly losing energy hence the tracks will have a short range. Because of their larger mass they do not change their path in air and as they pass, they cause more ionization on their path by knocking off many electrons.

(ii) Tracks formed by beta particles are thin and irregular in direction. This is because β-particles are lighter and faster hence cause less ionization of air molecules. Also the particles are repelled by electrons of atoms in their path.

(iii) Gamma rays produce scanty disjointed tracks. The radiation ejects electrons from molecules and these electrons behave like weak β-particles.

c. The pulse electroscope or pulse electrometer

It is used to measure the rate of decay or activity of a sample element and works on similar principles as a gold-leaf electroscope with a specially designed leaf system. Play the animation below and observe what happens.


Observations
The Leaf falls and rises repeatedly when E.H.T. is on.
Explanation

Radiations from the source ionize the air in the chamber. Due to the high electric field between central electrode and the walls of the chamber, positive ions move towards the chamber wall which is negative while negative ions towards the central electrode causing the leaf to diverge and touch the side electrode. Ions of opposite charge on the central electrode through the leaf causes discharge and leaf falls back. The process is repeated as air in the chamber is continually ionized, while the leaf pulses or beats at a rate depending on value of ionization current which depends on activity.

d. The Geiger-Muller tube


It is a type of ionization chamber. To visualize how this device works, play the animation below by clicking on its play button.

Observation
When there is no source in front of the tube the scale records background radiation which occurs naturally. An increase is the scale reading when radioactive source is introduced is noted.
Explanation

When the source is placed in front of the window, emitted radiations or particles enter the tube through mica window and ionize Argon gas. The negative ions move towards the anode while positive ones towards the wall or cathode. These ions collide with other gas molecules causing further ionization. This avalanche effect causes a pulse of voltage corresponding to current flowing through R and registered on the scale. An amplifier can be used to produce audible clicks through a speaker. It can also be used to amplify small currents from B-particles and G-rays emissions in the scale. Bromine gas is used as a quencher to absorb secondary electrons which could ionize molecules and also prevents sparking.

e. Spark Counter


It consists of wire gauze placed a few millimeters from a thin wire which has an adjustable voltage from an E.H.T source. Play the video below to observe how it works.

Explanation

When source of emission (α-particles) is brought near the wire gauze, air in its neighborhood is ionized and ions make the gap between the gauze and wire conduct more easily. The positively charged ions will flow towards the wire gauze causing flow of current which produces the sparks seen. The Counter is more sensitive to α-particles than β-particles or γ-rays due to their lower ionization.

Background radiation

In the absence of a radiation source, a GM tube records a background count due to the following:

(i) Cosmic rays which enter the atmosphere from outer space. They produce radioactive nuclides by collision with atoms in the atmosphere

(ii) Radiations from materials in the Earth's crust

(iii) Radiations from 'fall out' atomic bomb tests

(iv) Radioactivity in our own bodies

(v) Impurities with radioactive substances present in apparatus and surrounding.

This count rate should always be recorded before hand and subtracted from subsequent readings to get the correct count rate.

Nuclear fission

Nuclear fission is the splitting of a nucleus of a radioactive element due to bombardment with a neutron. For example Uranium-235 is bombardment with a neutron and it becomes Uranium-236 which is more active. The Uranium-236 then splits into Barium-144 and Krypton-90 with production of more neutrons and energy. The emitted neutrons may encounter other Uranium nuclides resulting in more splitting with further release of energy. Play the animation below to observe what happens when a moving neutron knocks a Uranium nuclide.

Explanation

The produced neutrons are fission neutrons. The energy produced is called nuclear energy and the reaction resulting with further bombardments is referred to as a chain reaction which produces a lot of energy. Such uncontrolled reaction may result to an explosion.

Nuclear Fusion

The fusing or combining of lighter nuclides to form a heavier one is called nuclear fusion. This is accompanied by enormous energy production. An example of nuclear fusion is formation of alpha particles when two heavy hydrogen isotopes fuse. The animation below shows two heavy isotope nuclides combining. Click on the play button and make your observations.

Nuclear equations

In the same way if a nuclide undergoes B-decay the atomic number increases by 1 but there is no change in the mass number. This leads to an isotope of same nuclide as shown in the illustration below. During A and B- decay, G-rays are produced in form of energy. Note that the production of gamma rays does not affect the mass or atomic numbers of the nuclide since they are not particulate. It is important to note that during B- decay the number of protons increases and occurs mostly when neutrons are more than protons in the nucleus as in the illustration below. On the other hand if a nuclide has excess protons it emits alpha particles thus reducing the atomic number by 2 and mass number by 4. Therefore the General formulae is given in the Illustration below.

Hazards of radioactivity

When the human body is exposed to radiation, the effect of the radiation can be hazardous because of the penetration and ionizing effect of the emissions on the cells. Gamma rays present the main radiation hazard. This is because they penetrate deeply into the body, causing damage to body cells and tissues. This may lead to skin burns, blisters and sores, delayed effects such as cancer, leukemia and hereditary defects. In 1945 during the World War II, atomic bombs were dropped at Hiroshima and Nagasaki resulting to instant widespread destruction felt due to the blast. Some of the damage caused by the emitted radiation became evident within days. other devastating effects became evident much later. The most recent accident was the Chernobyl disaster in Ukrainian republic of the USSR on April 26, 1986. The disaster occurred at the Chernobyl nuclear power plant that produced plume of radioactive debris that drifted over parts of the Western USSR, Eastern Europe, and Scandinavia. Animals feeding on the infected grass had to be destroyed so that their products do not reach people in other areas. It is important to note that, the harm caused by radioactive substance is likely to take a long time because they decay slowly due to some having long half-lives.

Precautions

Due to the hazardous nature of radioactive substances, schools in Kenya have been prohibited from storing them. However, the following measures are recommended as precautions when handling radioactive substances:


1. Radioactive elements should never be handled with bare hands but with forceps or well protected tongs. Click on the play button to observe how this is done.


2. For the safety of the users, radioactive materials should be kept in thick Lead boxes.

 


3. Persons using or working with the radioactive sources should avoid long exposure times and be checked regularly by a doctor for safety. A badge with photographic plate which can be developed is used to show the amount of exposure every day.

Applications of radioactivity

Though radioactivity is hazardous it has uses that are beneficial industrially, medically, agriculturally and in carbon dating.

Detecting defects and bubbles in metals

The source of radioactive radiation (preferably gamma-rays) is put on one side of a piece of forged metal while a photographic film is placed on the other end. Play the animation below and make your observations.

Observation

When the film is developed we see the equivalent of an X-ray picture of metal. Bubbles and defects inside the metal can be clearly detected.


b. Radioactive tracers

Radioactive tracers are used to detect leaks in underground pipes. The tracer is fed into the pipe and then a Geiger Muller (GM) tube is run above ground along the pipeline to detect any increase in the radiation level and hence the leaking spot. Play the animation below to observe how radioactivity helps in detecting leaking spots in pipes.

c.Determining the thickness of paper

In industries which manufacture paper and plastics, radioactive radiations can be used to determine thickness. If a beta source is placed on one side of the paper and a GM tube on the other side, the count-rate will be a measure of the thickness of the paper. Click on the play button to observe how radioactive materials can be used to control the thickness a sheet of paper.

d. Control of static electricity

In textile industry, the presence of static charges can be a nuisance since they can attract dust and even cause fire. When a radioactive element is placed in such industries, the radiations emitted will ionize air and the ions formed will attract the static charge. This will neutralize the charges and hence solving the problem due to static charges to a great extent.

e. Medical uses

Gamma rays, like X-rays, are used in the treatment of cancerous body growths. They kill cancerous cells when the tumor is subjected to them. Gamma rays are also used in sterilization of medical equipment. To determine the position of a blood clot in the body, a patient swallows a small amount of radioactive sodium. After a while, the radioactive sodium flows in the bloodstream to all parts of the body. The radioactive sodium stops flowing at the place where a blood clot develops in a blood vessel. A detector is then used to find out where the blood flow stops. Click "next" to view a visualisation on this.

Click on the play button of the animation below to observe these three uses.

f. Agricultural uses

The movement of traces of a weak radioisotope introduced into an organism can be monitored using a radiation detector. In agriculture, this method is applied to study the plant uptake of fertilizers and other chemicals. Sometimes gamma radiation is used to prolong the shelf-life of pre-packaged foods. Gamma rays are used to kill bacteria present in food to reduce the risk of food poisoning. Play the animation below to observe how some of these things can be done.

g. Carbon dating

Living organisms take in small quantities of radioactive carbon-14 in addition to the ordinary carbon-12. The ratio of carbon-12 to carbon-14 in the organisms remains fairly constant. When the organisms die, there is no more intake of carbon. The ratio of carbon-14 to carbon-12 therefore changes due to the decay of carbon 14. The new ratio is then used to determine the age of the fossil.

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