Transcript Radioactivity - resources.teachnet.ie

Radioactivity
Physics and Chemistry
Radioactivity in Radium Killed Marie Curie

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Marie and Pierre Curie isolated 1/30 ounce of
radium from one ton of uranium ore.
Marie died from radiation-induced leukemia.
The pages of her lab notebook were later
found to be contaminated with radioactive
fingerprints.
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Radioactivity
Radioactivity has become a matter of serious public
concern. Ionising radiation emitted by radioactive matter
cannot be detected by any of the senses but excess
exposure to it can cause serious health problems. It can
cause cancer which might only express itself in years later.
It can produce defects in unborn children and can possibly
lead to death.
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Radioactivity
Nevertheless, ionising radiation is used in the service of
man in electric power generation, in medicine, in scientific
research, in video display units and in industrial radiography.
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What is radioactivity?
Matter is made of elements and the smallest part of an
element that can exist independently is the atom. Atoms of
certain elements tend to be unstable; they tend to
disintegrate spontaneously. These elements are said to be
radioactive. Each disintegration is accompanied by the
emission of high energy waves or particles.
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What is radioactivity?
These emissions are called ionising radiation. As the radiation is
emitted, the atoms change their nature from one element to a ‘daughter’
element. This may also be radioactive leading to a second generation
radioactive daughter and so the process will continue until eventually a
stable atom is reached.
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Ionising Radiation

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A general word for any form of radiation that will knock off outer electrons of
atoms, forming ions. Such radiation will cause ionisation when they are
absorbed by the human body.
a – radiation, b – radiation, g – radiation, x – rays and neutrons are examples
of ionising radiation.
All ionising radiation are harmful to the human body.
(There are also non – ionising radiations examples of these are uv, ir,
radiowaves and microwaves)
Ionising ability is the ability to knock electrons off atoms to create ions.
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Ionising Radiation Can Cause:
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Skin burns similar to intense sunburn.
Cataracts, leukaemia and other cancers.
Genetic defects in children of parents exposed to the radiation.
Death.
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Background Radiation
We are all exposed all the time to some radiation, called background
radiation. Background radiation is natural radiation and comes from the
following:
 Cosmic Radiation.
• Radiation coming from outer space.

Rocks in the Earth’s crust.
• Rocks in the Earth’s crust contain traces of uranium and its
decay products, one of which is radon gas. In Ireland,
regions of granite rock release radon gas, which can
accumulate in houses to levels that increase risk of lung
cancer.

Man-made radioactive materials.
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Natural Radiation
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Natural sources account for about 87% of background radiation.
Some natural radioactive substance are uranium, radon and thorium.
They produce natural radioactivity.
Relatively few naturally occurring atoms are radioactive.
Many radioactive atoms that once existed have now emitted radiation and
become non – reactive.
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Experiment: To investigate the relative ionising power
Method:
1. Charge the electroscope
negatively.
2. Hold an alpha source near
the cap.
3. Note the time taken for the
electroscope to discharge.
4. Repeat the experiment
using other sources
Results:
The alpha discharges the
electroscope the quickest, then
beta, then gamma.
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Isotopes


Same number of protons, but different
numbers of neutrons.
Electrical and chemical properties are the same, but nuclear
properties are different.
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Radioactivity
Radioactivity is the spontaneous disintegration
of unstable nuclei with the emission of one or
more types of radiation
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The three main types of radiation are:

Alpha Radiation
(a)

Beta Radiation
(b)

Gamma Radiation
(g)
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Uranium Decays via Alpha-Particle Emission

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The first particle ejected from an unstable nucleus was called an alpha
particle because alpha is the first letter of the Greek alphabet.
It's now known to consist of two protons and two neutrons, which is the same
as a helium nucleus.
When an alpha particle is emitted from an unstable radioactive nucleus, the
atom is transmuted (changed) into a different element.
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Alpha Emission
Parent nucleus  Daughter nucleus  Alpha Particle
A
Z

X
A 4
Z 2
X

4
2
He
Example :
Radium  Radon  a particle
226
88
Ra

222
86
Rn  42 He
Note:
A – stands for the atomic mass number. (No. of Protons & neutrons)
Z – stands for the atomic number. (No. of Protons)
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Carbon-14 Decays by Beta Emission
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The beta particle is now known to be just an electron, traveling at
high speeds.
They are emitted by atoms whose nuclei contain too many neutrons
to be stable.
A neutron is split into a proton (remains in nucleus) and an electron
(which escapes)
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Beta Emission
Parent nucleus  Daughter nucleus  Beta Particle
A
Z

X
A
Z 1
X

0
1
e
Example :
Radium  Radon  b particle
228
88
Ra

228
89
Rn  01e
Note:
A – stands for the atomic mass number. (No. of Protons & neutrons)
Z – stands for the atomic number. (No. of Protons)
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Reaching Stability Through Gamma Ray Emission
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Nuclei with excess energy emit gamma-rays, which are extremely shortwavelength electro-magnetic waves, i.e. very high energy photons.
The energy of the gamma ray accounts for the difference in energy between
the original nucleus and the decay products.
Gamma rays typically have about the same energy as a high – energy x- ray.
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Nature of the Radiation
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a – particles are identical to helium nuclei 42 He
b – particles are identical to an electron 01e
g – radiation is electromagnetic radiation, which travels at the same speed as
light (in a vacuum)
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Penetrating Ability
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Alpha particles are 8,000 times
as heavy as beta particles.
Paper or clothing will block
alpha particles, while beta
particles require a few sheets
of aluminum foil.
Gamma radiation is extremely
dangerous - a thousand times
more potent than x-rays.
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Experiment: To demonstrate the penetrating power
Method:
1. Set up the apparatus and take a read
on the ratemeter due to back ground
radiation.
2. Place an alpha source in front of the
G-M tube. Take the reading.
3. Slowly move the alpha source away
from the G-M- tube until the reading is
the same as the background count.
Measure the distance.
4. Repeat the above steps for a beta
source.
5. Repeat step 2 but place sheets of lead
of varying thickness in front of a
gamma source.
Results:
Beta more penetrating than alpha.
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Summary Table
Properties
Alpha particle (α)
Beta particle (β)
Gamma Rays (γ)
Nature
Helium nucleus
Electron
Electromagnetic radiation
Deflection in electric
& magnetic fields
Yes
(Slightly)
Yes
(strongly)
No
Poor
Medium
Good
(6 cm of air / Stopped by paper)
(5 m of air / Stopped by 3mm Al)
(up to 10 cm of lead)
Ionising Ability
Good
(Strong)
Medium
(Weak)
Poor
(Very weak)
Speed
10 % speed of light
95 % speed of light
Speed of light
Detectors
Photographic film
Cloud chamber
G-M tube
Photographic film
Cloud chamber
G-M tube
Photographic film
Cloud chamber
G-M tube
+2
-1
0
4
1/1840
0
Penetrating Ability
Charge
Mass (a.m.u.)
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Examining Reactions
Rembember the following when looking at nuclear reactions:
The atomic nos. on L.H.S  The atomic nos. on R.H.S.
The atomic mass nos. on L.H.S  The atomic mass nos. on R.H.S.
a  particles are represented by 42 He
b  particles are respresented by -10 e
g  particles do not have another representation.
Note: When looking up isotopes in
the PTE only go by the atomic
number (smaller of the two) and
NOT the atomic mass number as
this can vary for each isotope.
Also:
Neutrons are represented by 01n
Pr otons are represented by 11H
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Example 1:
Identify the missing isotope in each of the following:
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(i) 13
Al  01n 
Na  ?
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11
1
7
(ii) 10
5 B  0 n  3 Li  ?
(iii)
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Cu  01n  ? 11H
Solution:
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4
(i) 13
Al  01n  24
11Na  2 He
1
7
4
(ii) 10
5 B  0 n  3 Li  2 He
(iii)
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Cu  01n 
65
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Ni  11H
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Example 2:
Uranium  238 decays to Uranium  234 by emitting a and b particles.
How many of each does it emit?
Solution:
Let x be the no. of a  particles it emits and y the no. of b  particles.
238
92
238
92
U  x  a   y b  
234
92
U
U  x  42 He   y  01e  
234
92
U
Looking at atomic mass numbers we have:
238  4x  0y  234
238  234  4x
4  4x
1  x  1 a  particle present.
Looking at the atomic num bers we have:
92  2x  y  92
0  2x  y
0  2(1)  y
[x  1 got from other part]
2  y  2 b  particles present.
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Precautions When Using Radiations
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Minimise the time spend using sources of radiation.
Use proper protective clothing, e.g. gloves, glasses, coat etc.
Make sure sources are properly shielded from you.
Keep as far away from the source as possible.
Use tongs for handling sources.
Store radioactive sources inside metal containers
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Uses of Radioactive Isotopes
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Medicine.
• Image of an organ can be seen by radiation given off. Radiation
can kill cancer cells.
Smoke detectors.
Food irradiation.
• Gamma rays can be used to sterilise food.
Carbon dating.
• The age of archaeological specimens can be determined by the
activity of the isotope C - 14 contained in them.
In Industry.
• To check the fullness of containers, thickness of objects, to find
leaks and to detect wear in components.
Nuclear energy.
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Radioactive Decay
The half life T½ is the time take for half the number of atoms of a
radioactive isotope to decay.
After 1 half - life:
After 2 half - lifes:
After 3 half - lifes:
After 4 half - lifes:
1
remains.
2
1 1 1
  remains.
2 2 4
1 1 1 1
   remains.
2 2 2 8
1 1 1 1 1
   
remains.
2 2 2 2 16
In general :
Fraction remaining 
1
(where n is the number of half - lives)
2n
Half – lives vary over a very wide range, for example the half – life of
polonium – 212 is 3 x 10-7 seconds, and the half – live of uranium –
236 is 4.5 x 109 years.
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Half life Calculations – Example 1
The half – life of a radioactive sample is 15 minutes. What fraction of the
sample will remain after 1 hour.
Soln:
T1  15 mins  1hour = 4 half - lives
2
Fraction remaining 
1
1
1


2n 24 16
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Half life Calculations – Example 2
One – sixty fourth of the original quantity of a radioactive isotope was left after 1
year. Calculate the half – life of the radioactive isotope.
Soln:
1
1
Fraction remaining  n 
2
64
 2n  64
n6
Therefore the half - life is 60.83 days.
(Using the fact that there are 365 days in a year)
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Nuclear Fission
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Nuclear fission is the splitting up of a large nucleus into two smaller nuclei of
similar size with the release of energy.
Fission is produced in a large nucleus by bombarding it with neutrons.
During fission very large amounts of energy are given off.
More neutrons are produced in the fission reaction. These can produce
further fission.
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Nuclear Fission
235
92 U
+
1
0n

144
56 Ba
+
90
36 Kr
1
+ 2 0n
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A Fission Chain Reaction
A fission chain
reaction in U
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Nuclear Fission
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100,000,000 times more
energy than is released when the
same quantity of coal is burned.
Slow neutrons are required.
A chain reaction occurs if
more than one neutron
goes on to cause another
fission.
Neutrons can be slowed by
bouncing them off of small
objects, such as carbon
nuclei.
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Uses of Fission
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Nuclear Reactors produces energy by fission in uranium fuel rods. (controlled
reactions)
Nuclear Weapons.
• Atomic bombs – an uncontrolled chain reaction. (Using plutonium – 239
or uranium – 235)
• Hiroshima in Japan 1945,  of the city was devastated. 75, 000 people
killed.
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Dangers of Fission Reactors.
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Mining Uranium ore.
• The mining of uranium ore releases radon gas, which can cause lung cancer
in miners. The area around the mine may contain radioactive material.
Containment of radioactive material within the reactor.
• Accidents have happened – Chernobyl 1986.
Removal and treatment of spend fuel rods.
Radioactive waste
• Remaining waste products must be stored securely for a very long time. This
is likely to be a very big problem for future generations.
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Nuclear Fusion
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Nuclear fusion is the joining together of two small nuclei to form one
larger nucleus with the release of energy.
Fusion can only occur if the two reacting nuclei are forced together with
sufficient force to overcome the coulomb repulsion between them. This
is done by heating them to extremely high temperatures, typically
greater than 108 K.
When fusion starts, energy is released which can help keep the reaction
going.
No one has yet managed to achieve a sustained controlled fusion
reaction. A great deal of effort is currently being put into this project.
The hydrogen bomb is an uncontrolled fusion reaction. The initial high
temperatures are produced by a small fission bomb exploding in the
deuterium. (One tested in 1952 - whole island disappeared)
Nuclear fusion is in the interior of the Sun is the principle source of the
Sun’s energy. In a series of reactions hydrogen fuses to form helium,
releasing energy in the process.
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Nuclear Fusion
Fusion is the opposite of fission. Deuterium must be moving extremely
fast to fuse.
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Examples of Fusion Reactions

An important example is the fusion of two heavy hydrogen atoms
(Deuterium) to form helium.
2
1H

+
2
1H

3
2 He
+
1
0n
Another is the fusion of deuterium and tritium.
2
1H
+
3
1H

4
2 He
+
1
0n
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Advantages of Fusion over Fission
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There is less radioactive waste produced.
The reaction produces far more energy from a given
mass of material than any fission reaction.
There is an abundance of deuterium in sea water, so the
fuel is plentiful.
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Albert Einstein and Mass-Energy Equivalence
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When a uranium nucleus
splits, the mass of the
remnants is less than the
original mass.
The difference
appears as light, heat, and
kinetic energy.
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Mass – Energy Conservation
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In 1905 Einstein in his Special Theory of Relativity concluded that mass
and energy are not independent.
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He stated that mass can be converted into energy and energy converted into
mass.
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Principle of mass – energy conservation
For any nuclear reaction, the mass – energy of the reactants equals the mass
energy of the products. [Loss in mass = gain in energy]

E = mc2 is the equation that governs this.
(where c = 3.0 x 108 ms-1).

Because of the large value of c, the speed of light, a tiny decrease in mass can
cause an enormous release of energy.

It has been estimated that 1 gram of matter was converted into energy in the
atomic bomb that was dropped on Hiroshima in 1945.
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Problem based on E = mc2
The difference between the masses of the reactants and
products in a nuclear reaction is 1.2 x 10-29 kg. How much
energy is released in the reaction.
E  mc 2
E  (1.2  1029 )(3  108 )2
[Only square the 3  108 ]
E  1.08  1012 J
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Cockcroft & Waltons Experiments
In a series of experiments they showed that when lithium
is bombared by protons, two a  particles are emitted in opposite
directions.
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3
Li  11H  42He  42He  energy
The mass of the lithium nucleus and the proton that hits it
is greater than that of the two a  particles produced.
The kinetic energy of the two a  particles equals the loss
in mass (calculated using Einstein's mass  energy equation
E=mc 2 ) in the reaction plus the kinetic energy of the proton.
Cockcroft & Walton's experiment was the first direct confirmation
of Einstein's prediction of the equivalence of mass and energy.
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2003 Q 1 (o)
The atom was first ‘split’ in 1932 by Cockcroft and Walton in
the reaction:
7
3
Li  11H  42He  42He  energy
Explain why energy is released in this reaction.
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Solution
Loss in mass is converted to energy by the equation
E  mc 2
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Albert Einstein
Albert Einstein 1879 – 1955
was probably the greatest theoretical
physicist of the twentieth century. He
developed mathematical models to
explain physical phenomena. Einstein
developed the mass – energy equation
E = mc2 , which predicted the
possibility of nuclear fission. He
explained the nature of space
and the time in the general theory of relativity, and predicted that light would
‘bend’ near a large mass. This was later verified experimentally, and is the reason
why light does not escape ‘black holes’. Einstein was awarded the Nobel Prize for
physics in 1921.
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