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CHAPTER 5: RADIOACTIVITY

5.1 Understanding the Nucleus of an Atom.


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The structure of an Atom

An atom consists of a nucleus which made of protonsand neutrons.It also has electronsorbiting the nucleus.


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2. Protons and neutrons are called nucleons.

Particle

Charge Value of

charge / C

Mass/kg Relativ

emass

Proton(p) Positive +1.6 x 10-19 1.67 x 10-27 1

Neutron(

n)

neutral 0 1.67 x 10-27 1

Electron(

e)

negative -1.6 x 10-19 9.11x 10-31 1/1840


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Rutherfords Explanation


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The result of the experiment shows that:i) Most of the alpha particle passed through the gold foil in their original

position.

ii) A very small number of alpha particles were deflected by the gold foil.

iii) A very small number of alpha particles were bounced back by the gold foil.


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3. The conclusion of the experiment:

i) The atomic mass is placed at the center of the atom and is

positively charged.

ii) The atom has a lot of empty space.

iii) The electrons orbiting the nucleus make up most of the atomic volume


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Proton number and nucleon

number

1. The atoms of different elements are given different symbols.

2. 2. The nuclide notation of an atom gives the symbol of the element:

3. a) The proton number (atomic number), Z. For a neutral atom, the number of

protons equals the number of electrons.

4. b) The nucleon number, A is a total number of protons and neutrons.


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Absorption of Radiation


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Radioactive decay.

A) Alpha Decay

A A - 4 4X Y + He

Z Z - 2 2

Parent daughter particle

nucleus nucleus (alpha)

In alpha decay, the proton number is reduced by 2 while

the nucleon number is reduced by 4.


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Beta DecayA A 0

X Y + eZ Z + 1 -1


nucleus nucleus (beta)

In beta decay, the proton number is increased by 1 because the new

proton formed but the nucleon number is unchanged because of the

loss of 1 neutron.


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C) Gamma Decay

A A

X Y + Z Z


nucleus nucleus (gamma

Example :

210 206 4Po Pb + He +

84 82 2

There is no change in the proton numberand nucleon number for a nuclide

that emits a gamma ray photon.


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Half-life.

1. Radioactive decay is a random and spontaneous process. This

means that all the unstable nuclei do not decay at the same

time, some decay earlier while others will decay at a much later

time.

2. As the time progresses the number of atoms undecayed orremaining decreases while the number of atoms decayed or

disintegrated increases.

3. The half-life, T1/2of a radioactive sample is the time taken for the

number of radioactive atoms in the sample to be reduced to halfof its original number.

4. After one half-life the activity, mass and the number of atoms

remaining of any radioactive substance is halved.


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1

8

1

8

7

8 ()3N=N

N =

Half-life T0 1T1/2 2T1/2 3T1/2

Number of

undecayed

atoms.

N ()1N = N ()2N = N N

% atoms

undecayed

100 % 50 % 25 % 12.5 %

Mass 64 g 32 g 16 g 8 g

Activity(s-1) 120 s-1 60 s-1 30 s-1 15 s-1

Number of

atoms decayed

NN = 0 NN = N NN = N N

Mass have

decayed

0 g 32 g 48 g 56 g

% atoms

decayed

0 % 50 % 75 % 87.5 %


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Decrease in mass = initial mass final mass

= 80 g -2.5 g

= 77.5 g

U f H lf lif


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Usage of Half-life


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Usage of Half-life

Half-Life in Archeology

Carbon-14 has a half-life of 5600 years.

1. Humans, animal and plants absorb Carbon-14 through

carbon dioxide gas in the atmosphere. A small amount of carbon

in CO2 exist as Carbon-14.

2. Living animals and vegetables have a constant amount of

Carbon-14 because the C-14 decayed will always be replaced.

However, for dead beings the amount of C-14 in it will decrease

because new C-14 will not be absorbed causing its reactivity to

decrease.

3. When an antique or human skull are found, their age can be determine by :

Measuring the activity of C-14 in it.

Determine the ratio of decay C-14 against intact C-14


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Example 1 :

An old human skull is found buried in the sand. The skull produces

radioactive ray with reactivity of 30 pulses per second. A new skull

produces reactivity of 100 pulses per second. If the background pulse is

20 pulses per second and the half-life of C-14 is 5700 years, how oldis the skull?

Original reactivity = 100

20 = 80Reactivity of oldskull = 3020 = 10Decay process :

T1/2 T1/2 T1/21. 40 20 10Decay has passed through 3 half-lives ( 3T1/2 )

The age of the old skull = 3 x 5700

= 17100 years.


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Radioactive Detectors

1. Most method of detecting nuclear radiation are based on these two

properties

(a) The ionizing effect of the radiation. When a radioactive ray is

emitted into the air, the air molecules around it will turn into ions.(charged molecules). The alpha particle has the highest ionization

power, followed by beta and gamma ray.

(b) The ability of the radiation to blacken a photographic-plate

or film.


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1. Photographic plate or film.

1. A photographic plate works on the principle that radioactive

radiation can cause a chemical change on the plate and

produce a dark trace.

2. The photographic plate can detect , and.3. It can be used as a special badge or tag to record the dosage

of radiation a person (worker) is exposed to.


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2. Scintillation detector

1. A scintillation detector is used to detect gamma radiation.

2. 2. The detector has sodium iodide which flashes when it is hit by

gamma.

3. 3. The light flash (scintillation) causes photoelectron to be ejected

from the photocathode.

4. These electrons are pulled towards anode, X ( photomultiplier)

causing more electrons to be emitted. The electrons emoted are

pulled to other anode Y, Z etc which multiply the number of electrons

emitted successively.

5. A pulse is therefore produced and will be measured by an

electronic counter.


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3. Gold Leaf Electroscope

1. The electroscope is positively charged. The

gold leaf is deflected because same charges

repel.

2. When the charged plate of the electroscope

is exposed to the sourceof alpha particles, the gold leaf will collapse.

3. This due to the ionizing effect of alpha

particles, both positive and

negative ions are produced in the air.

4. negative ions are attracted towards the

positive ions on the surface

of the disc and positive charges are neutralized.

5. The deflection of the gold leaf decreases.

This indicates the

presence of the alpha particles.

4 S k C t


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4. Spark Counter

1.A high voltage supply between the wire and the wire gauze is increase slowly until spark is

observed.2.The voltage is then reduced slowly until no spark is observed (4 kV).3.A source of alpha particles is then brought closer to the wire gauze.

4.The air molecules in the counter is become ionized.

5.The positive ions and negative ions produced will be attracted to the wire gauze and thin

wire respectively.

6.As the ions moves towards their respective terminals more collisions with air moleculesoccur causing secondary ionization.

7.The heat and light energies generated from these collisions produce

the spark.

1.The intensity of the radiation is indicated by the number of

sparks produced per unit time.

It cannot detect beta particles and gamma rays


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5. Geiger-Muller Tube (G-M Tube)

1.A Geiger-Muller (G-M) tube consist of an aluminium tube. It is filled

with Argon gas at low pressure.

2.The central tungsten wire acts as anode(+ve electrode) andthe aluminium wall of the tube acts as cathode ( -ve electrode).

1.G-M is connected to high voltage before being used.

2.When a radioactive particle enters the tube through mica window

and the argon gas atoms are ionized.

3.The positively charged ions and electrons produced will accelerate

towards the cathode and anode respectively.

1.As the ions accelerate, they collide with other atoms, henceproducing more ions(secondary ions)

1.The anode will collect all these electrons to produce an electric

pulse in the circuit. The strength of the pulse is increased by

an amplifier and then transfer to a recorder.

1.The recorder are also record all readings when there are radioactive

brought closer to G-M. This called the background pulse

1.The background pulse are produced by

a)radioactive material from earth and the surroundings such as

stones, soil, sand, etc.

a)cosmic rays and sun rays.

Actual reading = Reading recorded - Background reading.

10.


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A cloud chamber is a detector which displays the tracks of charged

particles.

The felt strip round the top is soaked with alcohol. This causes the air in

chamber to be saturated.

The lower part is filled with solid Carbon dioxide, CO(dry ice).

Sponge is used to push CO towards the metal plate. As a result,the air

space becomes supersaturated.

When the radioactive rays enter the upper part, the ionization

of air will occur. Saturated alcohol vapour will move above the ions and

condensed. Droplets of liquid alcohol on the ions will causethe formation of misty tracks.

Steps to ensure the clear tracks :

the transparent perspex cover is rubbed with a soft cloth to

produce charges which will remove all ions in the chamber before

radioactive rays enter.The cloud chamber must be placed horizontally

to ensure smooth flow particle in it.The light must shine on the area supersaturated with vapour

and not the black base of the chamber in order to avoid

heating it.


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Radiation Track Characteristics and explanation.

Alpha

particle

- thick because have strongest ionizing power.A lot of alcohol droplets are formed alongthe ions produced along the track.

- straight because not easily deflected by airmolecules with its greater mass.

- same length because each particle has equal

amount of energy.

Beta

particle

- thin because ionizing power is weak.- curvy because the particles are light and

deflected by air molecules.

- different length because each particle hasdifferent amount of energy.

Gamma

ray

- thin, short and scattered because it has thelowest ionizing power.


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