Radioactivity

• This is the emission of ionising radiation from the nucleus of an unstable atom
• Radio active atoms are uranium, polonium and radium, Astatine , francium,
Americium etc
• Radio active decay is the process where by the nucleus isotopes split and produces
energy in the form of radiation and such atoms of elements are said to be
radioactive
• The process of emitting radiation is called radioactivity
• These radio active emission occurs randomly over a space of time since there is no
way of telling which atom will decay at a particular time. The process is not affected
by temperature, pressure or a chemical change

BACKGROUND RADIATION

This is a radiation from radioactive isotope present in the environment. It is a low level
radiation that id produced at all time by radioactive isotopes and which we are all exposed
to

BACK GROUND RADIATION COMES FROM

• Radioactive elements present in rocks in the Earth crust as they decay

• Cosmic RADIATION: nuclear radiation from the sun and stars produces radioactivity
in the air we breath
• Radio active elements present in food e.g potassium _40 in tomatoes which our
body absorb
• Radio active radiation from some of radioactive atoms that make up our body e.g
carbon 14 we all breath in the radioactive element carbon_14 present in the
environment
• Radiation leaking form nuclear power stations into the environment
• Radioactive tracers used in industry and medicine
• Nuclear fall out from the testing of nuclear weapons in the atmosphere

Types of nuclear radiation

There are three principal kinds of radiation given off by radioactive substances namely

• Alpha particles (symbol)

• Beta particles (symbol)
• Gamma rays(symbol)
DETECTION OF THE THREE TYPES OF RADIATION

Radioactivity from a radioactive source may be detected by means of

• Ionisation effect in a Geiger Muller tube
• Its ionization effect in a cloud chamber

DETECTION OF ALPHA PARTICLES, BETA PARTICLES AND GAMMA RADIATION USING A

GWIGER MULLER TUBE

• THESE METHOD ENABLES the particles emitted in radioactive decay to be counted

as pulse of electric current
• The tube contains two electrodes with long central wire filled with a gas such as
Argon or Helium under reduced pressure
• The two electrodes are at a voltage just less than the value which will allow an
electric current to pass or ionise the gas
• Each time radiation enters the Geiger MULLER TUBE through the thin Window or at
one end , it removes an electron from the atoms of the gas .
• The gas atoms becomes positively charged ions
• The electrons move through the positively charged ions to the central wire in the
tube, setting up an electric current
• The current which is magnified and fed into the counter or rate metre linked to the
Geiger MULLER TUBE, produces a flashing light and a clicking sound.
• A counter attached to the wire measuring the amount of radioactivity per second by
measuring the amount of current
• The number of flashes and clicks per unit of time indicates the strength of the
radiation

EXPERIMENT TO DETECT ALPHA PARTICLES, BETA PARTICLES and GAMMA radiation using
a Geiger Muller tube

Procedure

• Set up the apparatus as shown

• Record the count rate without the sources. Since this gives the background
radiation
• Then place the source confront of the counter
• Place a sheet of paper between GM tube and the source and records the count rate
• Insert a 3mm thick aluminium sheet behind the paper and record the count rate
• Insert a 4cm thick lead behind the aluminium and record the count rate

RESULTS TABULATED IN THE TABLE BELOW

Count Count Count Count
With no with a with with
absorber paper aluminium lead
absorber absorber
6765 2827 1442 82

OBSERVATIONS

• The count rate falls when a sheet of paper is placed between GM tube and the
source. These particles stopped by then paper are called alpha particles
• The count rate falls further when a e mm thick aluminium sheet is placed
between the GM tube and the source. These particles absorbed by aluminium
are called beta particles
• The count rate falls when a 4 mm thick lead is placed between the Geiger
MULLER TUBE and the source . The radiation is reduced but not stopped by lead
is called gamma rays
CONCLUSION
There are three different kinds of radiation from a particular source

PENETRATING ABILITY OF ALPHA BETA AND GAMMARAYS

The set up in in the last experiment shows that these three type of radiation have
different penetrating abilities
• Alpha particles have the weakest penetrating ability . It can penetrate no more
than 0.01 mm of metal but it is absorbed by 3mm aluminium
• Gamma rays have the strongest penetrating ability. It is able to pass through
0.1m og of metal . It is absorbed but not completely by a few centimetres of
lead

DETECTING ALPHA PARTICLES , BETA PARTICLES AND GAMMA PARTICLES USING CLOUD
CHAMBER

• A cloud chamber actually allows us to see the tracks of charged particles .

• A felt trip around the top of the chambers is soaked in alcohol vapour that has been
cooled below its condensation point
• Under the base of the chamber is a dry ice(solid carbon dioxide ) that has been
cooled to about negative 80 degrees Celsius
• Alcohol vapour spreads downwards through the chamber and its cooled below the
point where its normally condense
• When nuclear radiation enters the chamber it ionises the air inside
 • Alpha particles from the source produces a trail of ions along their path
• and the alcohol condense more rapidly around these air ions forming white line of
tiny liquid drops which shows as tracks when illuminated

• A narrow cloud forms along these tracks of alpha just like the vapour trail of a high
flying aircraft
• Alpha particle tracks are straight and thick because are strongly ionizing
• Diagram here
• Fast moving beta particles produces faint straight tracks and slow moving Bata
particles produces short twisted tracks because of the frequent collision between
the beta particles and the electrons in the air
• Diagram here
• Gamma rays do not leave tracks in the cloud chamber but the electrons they knock
out of atoms leave short tracks which can be seen
• Diagram here

DEFLECTION OF ALPHA PARTICLES AND GAMMA RADIATION , BETA PARTICLES IN

ELECTRIC FIELD

EXPERIMENT TO INVESTIGATE THE DEFLECTION OF ALPHA PARTICLES AND GAMMA

RADIATION AND BETA PARTICLES

• An experiment must be done in a vacuum since they have very short short range , so
can lose energy quickly or absorbed by the air
• Diagram here
• Subject a narrow beam of radiation containing alpha particles, beta particles and
gamma rays from a radioactive source
OBSERVATION
• Alpha particles are deflected towards the negative plate by the field. This shows that
alpha particles carries are positive charge since unlike charges attract
• Beta particles are deflection towards the positively charged plate by the field. This
shows that BETA PARTICLES carries a negative charge since unlike charges attract
• Gamma rays are not deflected by the electric field. This indicates that gamma rays
carry no electric charge

DEFLE TION OF ALPHA BETA AND GAMMA RADIATION IN ELECTRIC FIELDS

INVESTIGATION
Subject a narrow beam of radiation e.g radium226 containing alpha particles and gamma
rays and BETA PARTICLES from a radioactive source

Diagram here

Observation

• Alpha particles are deflected towards the negative plate by the field . This shows
that alpha particles carries a positive charge since unlike charges attract
• BETA particles are deflection towards the positive plate by the field This suggests
that beta particles carries a negative charge since unlike charges attract
• Gamma rays are not deflected by the electric fields this shows us that GAMMA
RADIATION carries no electric charge

Deflection of alpha particles, BETA PARTICLES AND GAMMA RADIATION in magnetic fields

INVESTIGATION

Subject a narrow beam of radiation e.g radium-226 containing alpha particles, beta
particles and gamma rays from a radio active source to a uniform magnetic fields as shown
below

Diagram here page 200

Observation

• Alpha and beta particles are deflected in the opposite directions. This shows that
they carry opposite charges
• Alpha particles are deflected much less than BETA particles. This suggests that
alpha particles have greater mass than beta particles. A very much stronger magnet
is needed in the deflection of alpha than with the betas
• Gamma rays are not deflected by magnetic field this shows that gamma rays
carries no electric charge

IONISING EFFECT OF ALPHA PARTICLES BETA PARTICLES AND GAMMA RADIATION

Ions are charged atoms or group of atoms formed by gaining or losing electrons. Alpha
particles, beta particles and gamma rays causes neutral atoms in their path to form ions.
These effect is called ionisation

• Alpha are massive and have a greater charge so exert more force and knock
electrons off nearby atoms or atoms in their path. Hence alpha have the strongest
ionising effect
 • Betas are light with negligible mass and charge so exert less force on electrons of
atoms in their path consequently , betas have a medium ionising effect
• Gamma rays: have no mass and charge. They exert least force on electrons of
atoms in their path so have the weakest ionising effect

Properties of alpha particles

• They are Helium nucleus made up of 2 protons and 2 neutrons

• They carry a positive charge 2+
• They are stopped by thin sheet of paper or metal foil or 6cm of air
• They have the weakest penetrating ability
• They are massive as they have a mass of 4unit
• Since they are massive
1. They travel quite slow in air 1/20 the speed of light
2. They are not deflected much in electric and magnetic field
3. They have a short range in air . Can not travel far because they rapidly lose their
initial energy when they collide with nearby atoms
• They have the strongest ionising power than Bata and gamma since they
carry a highest charges of 2+

Properties of beta particles

• They are electrons

• They carry a negative charge of 1-
• They are absorbed or stopped by a 3mm aluminium plate or perpex
• They have a medium penetrating ability. They can pass through paper and
aluminium foil
• They ae lighter than alpha because they have a negligible mass of 1/1840
units
• Since they are lighter
1. They travel fast in the air than alpha
2. They are deflection much greater than alpha in electric and magnetic
fields
3. They have a longer range in air (3m) can travel far than alphas
• They have medium ionising power as they have very small mass and carry a less
charge of -1

Properties of gamma rays

• They are not particles but an electromagnetic wave

 • They carry no electrical charge
• They are stopped by 2cm of lead or concrete
• They have the strongest penetrating ability. They can pass through paper and
aluminium
• THEIR high penetrating ability makes gamma to be very dangerous as they can
penetrate deep into humans, animal plants tissue and destroy living cells
• They have no mass
• They are not deflected in electric and magnetic fields because they have no charge
• They have the greatest range in air (100m): can travel far than alphas and betas
• They have the weakest penetrating ability because they have no mass

Type of Nature Charge Mass Relative Relative Deflection

radiation ionising penetrating in electric
effect abilities and
magnetic
field
Alpha Nucleus of +2 4units Strongest Weakest Least
particles helium
Beta Electron -1 1/1840 Medium Medium Most
particles
Gamma Electromagnetic 0 0 Weakest Strongest No
rays wave