Radioactivity

Physics and Chemistry

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Radioactivity in Radium Killed Marie Curie

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

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.

– radiation, – radiation, – 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:

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

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 differentnumbers of neutrons.

Electrical and chemical properties are the same, but nuclear

properties are different.

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Radioactivity is the spontaneous disintegration of unstable nuclei with the emission of one or more types of radiation

Radioactivity

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The three main types of radiation are:

Alpha Radiation ()

Beta Radiation ()

Gamma Radiation ()

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Uranium Decays via Alpha-Particle Emission

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

226 22

A

2 48

A 4Z

8 6

4

8

Z 2 2

2

Daughter nucleus

X

Example :

Radium Radon parti

Parent nucleus

X

Alpha Partic

cle

Ra R

H

n

le

e

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

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

0AZ 1

A

228 22

1

8 01

Z

88 89

Beta Particle

e

Example :

Radium Radon parti

DaughteParen

cle

Ra

r t nucleus

Rn

nucleus

XX

e

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

Nuclei with excess energy emit gamma-rays, which are extremely short-wavelength 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

– particles are identical to helium nuclei

– particles are identical to an electron

– radiation is electromagnetic radiation, which travels at the same speed

as light (in a vacuum)

42He

01e

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Penetrating Ability

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 fieldsYes

(Slightly)

Yes

(strongly)No

Penetrating Ability Poor(6 cm of air / Stopped by paper)

Medium(5 m of air / Stopped by 3mm Al)

Good(up to 10 cm of lead)

Ionising AbilityGood

(Strong)

Medium

(Weak)

Poor

(Very weak)

Speed 10 % speed of light 95 % speed of light Speed of light

DetectorsPhotographic film

Cloud chamber

G-M tube

Photographic film

Cloud chamber

G-M tube

Photographic film

Cloud chamber

G-M tube

Charge+2 -1 0

Mass (a.m.u.) 4 1/1840 0

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Examining Reactions

2

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 ma

particles

ss nos. on R.H.S.

are represented by

4

0-1

10

11

are respresented by

do not have another representation.

Also:

are represented by

He

particles e

particles

Neutrons n

Pr oto

are representedns H by

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.

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Example 1:

27 1 2413 0 11

10 1 75 0 3

65 1 129 0 1

Identify the missing isotope in each of the following:

(i) Al n Na ?

(ii) B n Li ?

(iii) Cu n ? H

Solution:42

42

6

27 1 2413 0 11

10 1 75 0 3

65 1 12

529 80 1

(i) Al n Na

(ii) B n Li

(iii) Cu

He

He

in HN

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Example 2:

Uranium 238 decays to Uranium 234 by emitting and particles.

How many of each does it emit?

Solution:

238 23492 92

92 2238 4 0 234

1 92

Let x be the no. of particles it emits and y the no. of particles.

U x y U

U x He y e U

Looking at we have:

238 4x

a

0y 234

238 234 4x

4 4x

1 x 1 particl

tomic mass numb

e p

e

r

rs

e

sent.

Looking at the we have:

92 2x y 92

0 2x y

0 2(1) y [x 1 got from other part]

2 y 2 particles presen

atomic num

t.

bers

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Precautions When Using Radiations

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

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.

n

1After 1 half - life: remains.

21 1 1

After 2 half - lifes: remains.2 2 41 1 1 1

After 3 half - lifes: remains.2 2 2 81 1 1 1 1

After 4 half - lifes: remains.2 2 2 2 16

1Fraction remaining (where n i

2

In general :

s the number of half - lives)

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:

12

n 4

T 15 mins 1hour = 4 half - lives

1 1 1Fraction remaining

2 2 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:

n

n

1 1Fraction remaining

2 64

2 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

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

U23592

10n+ 144

56Ba 9036Kr+ + 2 1

0n

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A Fission Chain Reaction

A fission chain reaction in U

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Nuclear Fission

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 neutrongoes on to cause anotherfission.

Neutrons can be slowed bybouncing them off of smallobjects, such as carbonnuclei.

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Uses of Fission

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.

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

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.

Another is the fusion of deuterium and tritium.

H21

21H+ 3

2He 10n+

H21

31H+ 4

2He 10n+

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Advantages of Fusion over Fission

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

When a uranium nucleussplits, the mass of theremnants is less than theoriginal mass.

The differenceappears as light, heat, and kinetic energy.

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Mass – Energy Conservation

In 1905 Einstein in his Special Theory of Relativity concluded that mass and energy are not independent.

He stated that mass can be converted into energy and energy converted into mass.

Principle of mass – energy conservationFor 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.

2

29 88 2

12

[Only square the 3

E mc

E (1.2 10 )(3 10 10 ])

E 1.08 10 J

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Cockcroft & Waltons Experiments

In a series of experiments they showed that when lithium

is bombared by protons, two particles are emitted in opposite

directions.

7 1 4 43 1 2 2Li H He He energy

The mass of the lithium nucleus and the proton that hits it

is greater than that of the two particles produced.

The kinetic energy of the two particles equals the loss

in mass (calculated using Eins

2

tein's mass energy equation

E=mc ) 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:

Explain why energy is released in this reaction.

7 1 4 43 1 2 2Li H He He energy

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Solution

Loss in mass is converted to energy by the equation

2E mc

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Albert Einstein

Albert Einstein 1879 – 1955was 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.