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Nuclear Physics G J Z A H R A B . E D ( H O N S ) Y E A R 1 0 C H A P T E R 1 4

L E A R N I N G

O U T C O M E S

What is matter

made up of?

What are atoms?

Are atoms un-

breakable?

What distin-

guishes particles

of different ma-

terials?

What is a nucle-

ar reaction?

Is uranium the

only radioactive

element?

What are the

uses of radioac-

tivity, apart from

military and

electricity pro-

duction?

What are the

types of radioac-

tivity? What are

the differences

between them?

How can radio-

activity be de-

tected and meas-

ured?

Which materials

can stop radia-

tion?

What is back-

ground radia-

tion?

What is a half-

life?

How can radio-

active com-

pounds be han-

dled safely?

Name and Surname:

______________________

Class:

______________________

G J Z A H R A B . E D ( H O N S )

P A G E 2

N U C L E A R P H Y S I C S

Atoms consist of negative-

ly charged electrons orbit-

ing a positive charge nu-

cleus.

14.1 Atomic Structure

Nearly all the matter in the universe is made up of atoms.

Elements consist of one type of atom only, while compounds

consist of different types of atoms chemically bonded to-

gether. There are 118 (since March 2010) elements of which

about 92 (disputed value) are naturally occurring. The word

atom means ‘indivisible’, because it was postulated (by John

Dalton) that this particle is the smallest building block of na-

ture. However, this hypothesis has been falsified.

Atoms consist of a nucleus and electrons orbiting the nucleus in energy levels.

The nucleus then consists of two other subatomic particles which are the proton

and neutron.

Protons are positively charged and the number of protons within a nucleus

determines the identity of an atom. This means that if an atom has one pro-

ton it will be a hydrogen atom while if it has 8 it will be an oxygen atom. The

number of protons in an atom is called the atomic number.

Since protons are positively charged, they repel each other within the nucle-

us. Neutrons are found within the nucleus and have approximately the same

mass as protons but no electrical charge. The role of neutrons is to keep the

nucleus intact. Too little or too much neutrons will cause a nucleus to be un-

stable.

Electrons are much smaller than protons and neutrons and are found or-

biting the nucleus in energy levels. Electrons are negatively charged and are

thus attracted towards the positive nucleus by electrostatic interactions.

Electrons determine chemical reactivity and chemistry deals mainly with

these particles.

The number of

protons deter-

mins the identity

of an atom.

John Dalton postulated that

all matter is made up of at-

oms. He also studied colour

blindness


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The Standard Model of particle physics is currently the best theory explaining how the universe works in its most fundamental level. The Higgs boson was confirmed in July 2012 and was the final missing link of the puzzle. The search for the Higgs boson was more than forty years long.

In the standard model, protons and neutrons are Baryons, or triquarks, which are particles made up of three quarks. Quarks and electrons are

elementary particles, which means that they are not made up of anything else.

In simple terms, string theory is a hypothesis that postulates that elementary particles are made up of tiny, vibrating strings. It has great implications on the forces in the universe, but has not yet been confirmed by falsifiable, ob-servable, novel predictions.

Particle Location Relative

Mass Importance

Relative Charge

Proton 1

1p

Neutron 1

0n

Electron 0

-1e 0


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14.2 Timeline of Atomic Theory and Energy

1803 John Dalton proposes that matter consists of atoms, indivisible units that are different for each element, but that can be combined to form compounds. This is called the billiard ball model of the atom.

1832 Michael Faraday shows how electricity can be used to separate a compound into its constituent elements (electrolysis)

1896 Henri Becquerel discovers radioactivity

1897 JJ Thomson discovered the electron. By doing so he disproved Dalton’s billiard ball model. Thomson believed the atom to be made up of a positively charged cloud with electrons embedded in it. This is called the plum pudding model.

1898 Marie and Pierre Curie discover two naturally occurring radioactive elements. They coin the term ‘radioactive’.

1900 Frederick Soddy observes the radioactive decay of elements and coins the term half-life

1909 After seven years of experiments, Robert Andrew Millikan discovers the exact mass and change of the elec-tron

1911 Through the gold leaf experiment, Ernest Rutherford shows that the atom consists mainly of empty space with a dense, positive nucleus and orbiting electrons. He falsifies the plum pudding model and adopts a plan-etary model

1922 Niels Bohr shows that electrons do not orbit haphazardly about a nucleus, but along a number of energy lev-els

1932 James Chadwick discovers the neutron

1941-51 Glenn Seaborg artificially produces six new elements

1942 Enrico Fermi produced energy from an atomic nucleus - the first nuclear reactor

1945 First nuclear bombs are dropped on Hiroshima and Nagasaki

1952 First nuclear fusion explosion (hydrogen bomb)

1954 USSR’s Obninsk Nuclear Power Plant – the first nuclear power plant. It was decommissioned in 2002

1961 USS Enterprise – the first nuclear powered aircraft carrier

1968 The discovery of quarks – subatomic particles that constitute the nucleons of an atom

1970s First protest movements against nuclear energy

1986 A nuclear disaster occurred at Chernobyl nuclear power station in Ukraine. Most radioactive fumes went on the town of Pripyat which at present is still evacuated

1995 The sixth and last type of quark was discovered at Fermi Lab

4 July 2012

CERN announces the discovery of the Higgs Boson, the subatomic particle used to explain why particles have mass. It was predicted in 1964 and was the last elementary particle which was to be discovered

Today 14% of the world’s energy comes from nuclear fission plants

Today Research on controlling nuclear fusion for energy continues

Today

String theory proposes that fundamental particles (electrons, quarks, etc) are made up of resonating strings. This theory could solve the inconsistencies between present quantum mechanics and general relativity. The theory has not yet made testable, observable, novel predictions and thus is not yet considered a part of sci-ence


P A G E 5


14.3 Mass Number and Isotopes Subatomic particles have a very small mass (mass of proton is 1.67 x 10-27kg). In order to avoid using compli-

cated figures, we usually speak of relative mass. Here the mass of a proton is taken to be 1. As a neutron

weighs as much as a proton, it also has a relative mass of 1. Electrons have a relative mass of 1/1836, which is

approximately 0. Relative masses have no units because they are relative (compared to) quantities.

The relative mass of atoms can be found by adding the number of protons and neutrons in the nucleus. In

the periodic table, elements are represented showing their symbol, atomic number (usually at the bottom)

and mass number (usually on top).

Not all atoms of the same element have the same number of neutrons in their nuclei. Different atoms of the

same element with different mass number (or number of neutrons) are called isotopes. For example, Car-

bon has three naturally occurring isotopes, with 6, 7 and 8 number of neutrons respectively. Carbon-12 and

carbon-13 are stable, while carbon-14 is unstable to radioactive decay.

Element Number of Protons (Atomic Number)

Number of Neu-trons

Mass Number Symbol

Fluorine 9 10 19

Oxygen 8 8 O

Sodium

Calcium 20 40 Ca

Lead 125 207 Pb

Uranium-235 92 U

Chlorine-37 17 Cl

Chlorine-35


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The energy needed to start

the fission of uranium-235 is

large. An fission atomic

bomb has a traditional TNT

bomb to trigger the nuclear

reaction.

14.4 Stability and Types of Decay Not all nuclei are stable: some have too many neutrons while others have too

little neutrons. There are three main types of radiation by which a nucleus can

undergo radioactive decay.

Alpha decay is the emission of alpha particles (a

helium nucleus with no orbiting electrons), repre-

sented as 42α. This is undergone by nuclei that

have too many neutrons. Alpha particles are rela-

tively heavy and have a highly positive charge

which gives them a large ionising power (ability to

strip electrons and ionise other substances). An

example is the decay of uranium-238 into thorium.

Beta decay is the emission of a beta particle (an

electron) from the unstable nucleus. The elec-

tron from the nucleus is produced from the de-

cay of a neutron into a proton and an electron.

This thus results in an increase in the proton

number and no change in the mass number and

is represented as 0-1β.

Beta particles are thus light and negatively charged. They have a small ionising

power. An example is the decay of caesium-137 into barium-137.

Gamma decay is the emission of gamma radiation, 00γ as a means of releasing

excess energy. This type of radiation accompanies most radioactive decay and

results in no change in the proton or mass number of the radioactive nucleus.

Gamma radiation has no charge and thus no ionising power.

Although alpha, beta and gamma are the most common types of radioactivity,

there are other types of radiation, for example electron capture and proton

emission:

The Manhattan Project in

World War II resulted in two

atomic bombs being dropped

on Nagasaki and Hiroshima

on 6 August 1945. The

bombs were named Fat Man

and Little Boy (pictured)

respectively. The little boy

contained 64kg of uranium,

of which less than 1kg under-

went nuclear fission. The

mass decreased by 0.6g,

which was converted into

pure energy according to the

equation E=mc2. The blast

was 3.2km in diameter. This

bomb only killed about

140,000 people by the end of

December 1945.

“since wars begin in the

minds of men, it is in the

minds of men that the de-

fences of peace must be

constructed”

UNESCO (1945)


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G J Z A H R A B . E D ( H O N S ) , S A I N T A L B E R T T H E G R E A T C O L L E G E

1. The atomic number of americium-241 is 95 while its symbol is Am. It decays by losing an alpha particle to

form neptunium, Np

2. An atom of americium with mass number 242 can be produced in industry starting from plutonium, 23994Pu.

Americium-242 is itself radioactive. Write nuclear equations for the steps below, clearly identifying X, Y

and Z:

3. Uranium-239 (atomic number 92) decays by beta emission to give an isotope of neptunium, Np. This neptu-

nium decays by further beta emission to give an isotope of plutonium, Pu. The plutonium formed decays by

alpha emission to give A. Write nuclear equations for all the reactions and hence identify element A.

(10)

4. Astatine is one of the rarest elements with less than 30g estimated to be contained in the whole Earth’s

crust. Astatine-218 (atomic number 85, symbol At) undergoes the steps below to give a stable element.

Identify the atomic and mass number of all atoms formed, and hence identify element X. (7)

Practice Exercise 14A

a. How many protons, neutrons and electrons are there in a neutral atom of americium-241 (3)

b. Another radioactive atom of americium has a mass number of 243. This is an example of another

(i)____________ of americium. Both atoms have the same number of (ii)_____________ but different

number of (iii)______________. (3)

c. Write a nuclear equation for the decay of americium-241 into neptunium and an alpha particle and hence

find the atomic and mass number of the neptunium formed. (3)

a. Plutonium-239 absorbs one neutron to give X (3)

b. X absorbs another neutron to give Y (3)

c. Y decays by beta emission to give Z (3)

d. Z absorbs a neutron to give americium-242 (3)

e. Americium-242 decays to give californium-242 (24296Cm) (3)


P A G E 8


14.5 Types of Radiation: Properties and Uses Alpha particles travel only a few centimetres in air and can be stopped by a thin

sheet of paper. This is because of their high ionising power – they powerfully

strip electrons from air particles and thus lose their energy quickly. Since they are

positively charged, alpha particles are deviated towards the negative end of an

electric field.

Alpha emitting sources are

used in smoke detectors.

The alpha particles

emitted ionise the air in

the detector which gener-

ates an electric current. If

smoke particles enter the

alarm, they absorb the

alpha particles and the

circuit breaks. The alarm

thus goes off.

Beta particles can travel a few metres in air. They are less ionising and damaging

than alpha particles. They can be stopped by aluminium. Due to their negative

charge, they are deflected towards the positive side of an electric field. Since they

are much lighter than alpha particles, they have less momentum and deflection is

larger.

Beta radiation is used to check the

thickness of materials (eg. aluminium

sheets or paper) in the manufacturing

industry. Beta radiation from a radio-

active source is passed through the

material and the amount passing

through is measured with a Geiger

Müller (G-M) tube.

Gamma rays travel long distances in air and are stopped by thick sheets of lead,

for example 10cm thick. They are uncharged and not effected by electric fields.

Highly powered gamma rays can be used to sterilize equipment since they kill bac-

teria without the need of high temperatures and chemicals, and to kill cancer cells

by being concentrated in one area of the body (some radiotherapy).

A Geiger-Müller counter

measures radiation as this

ionizes a low pressure gas

inside the instrument.

The first type was invented in

1908 by Hans Geiger and Ern-

est Rutherford who was the

first person to split the atom

and discoverer of the atomic

nucleus and the proton. This

could detect only alpha radia-

ton and was called the Geiger

counter.

In 1928 Geiger and his PhD

student Walther Müller im-

proved the counter so that it

could detect all three types of

radiation. The current ver-

sion, which was invented in

1947 by Sidney H Liebson, has

a longer life and uses a lower

voltage. It is called the Halo-

gen Counter.


P A G E 9



1. Copy and complete the diagram by labelling the three types of radiation and the three materials. (6)

2. The diagram shows alpha, beta and gamma radiation be-

ing emitted through an electric field.

3. Copy and complete the table below

Practice Exercise 14B

a. Which type of radiation passes unchanged through the

field? Why? (2)

b. Which type of radiation is deflected towards the positive

side? Why? (2)

c. Which type of radiation is deflected towards the negative

side? Why? (2)

d. Which type of radiation is deflected the most? Why? (2)

Type Symbol Relative

Mass Charge

Ionising

Power

Penetrating

Power

n/a High

Alpha Positive


P A G E 1 0


14.6 Detectors for Radiation Radiation can be detected using a variety of detectors:

A photographic film darkens on exposure to radiation. This is used in dosime-

ters, which are badges given to workers who work with radioactivity to moni-

tor how much and which type of radiation was incident on the workers. These

badges (dosimeters) must always be worn by workers as a safety measure.

A Geiger-Müller tube (G-M tube) is the most commonly used detector. It

must be thin windowed in order to detect alpha radiation.

A gold-leaf electroscope and a spark counter can only detect alpha radiation

since they detect strongly ionising sources

A G-M tube will always show a small reading and a photographic film will always

darken (slowly) even if there is no known radioactive source. This is due to back-

ground radiation from the sources shown in the chart.

Prac

tice

Exe

rcis

e 14

C

The dosimeter shown has a photographic

film covered with three types of filters:

metal filters, plastic filters and open win-

dow (only filters light). What type of radi-

ation would be incident on the worker if:

i. Only the film under the open window

darkens?

ii. The film under the plastic window

darkens less than the film under the

open window?

iii. The film under all filters darkens to

the same extent?

Henri Becquerel is known for

discovering radioactivity. He

did so by chance after wrap-

ping a piece of uranium in

photographic plate. His dis-

covery was followed by in-

tense research on radioactive

elements and the discovery of

radioactive elements thorium,

poloniuim and radium. The

latter two were discovered by

Bequerel’s student Marie Cu-

rie and her husband Pierre.

Marie Curie remains one of

the most well known female

scientists. In 1903 Becquerel

and the Curies shared the

Nobel Prize in Physics.


P A G E 1 1


14.7 Half Life As nuclei of atoms undergo radioactive decay, the activity of a sample decreases

with time. The decay of nuclei with time is quite random however, and on

plotting the activity of a sample against time, a graph as shown is obtained. One

can note that the time taken for the activity of the sample to decrease by half is

constant. The half life is the time taken for half the radioactive nuclei to decay.

For example, Strontium-90 has a half life of about 28 years. This means that a

sample that shows an activity of 20,000 counts per second will show 10,000

counts per second after 28 years, 5,000 counters per second after 56 years, 2,500

counts per second after 84 years etc. This means it will take 224 years for the ac-

tivity to drop below 100 counts per second.

Different substances have different half lives. For example:

In medicine, radioactive isotopes are chosen with a half life of a few days. This

gives the medical team enough time to track the sample, but does not leave the

patient radioactive for years!

Nuclear fusion is the process

by which atoms are joined to

form larger atoms. This is

the opposite of nuclear fission

whereby large nuclei such as

that uranium decay to form

smaller nuclei.

Nuclear fusion is many times

more powerful than nuclear

fission. The notorious hydro-

gen bomb works by nuclear

fusion of hydrogen nuclei.

The first successful hydrogen

bomb, the Ivy Mike, was deto-

nated in 1952.

The ITER is an international

fusion engineering and re-

search project. They are cur-

rently building the world’s

most advanced experimental

fusion reactor in southern

France.

Radioactive Isotope Half-life

Polonium-215 0.0018 seconds

Bismuth-212 60.5 seconds

Sodium-24 15 hours

Iodine-131 8.07 days

Cobalt-60 5.26 years

Radium-226 1600 years

Uranium-238 4.5 billion years


P A G E 1 2



1. Look at the graph to the right and answer the fol-

lowing questions

2. The half life of carbon-14 is 5760 years. When liv-

ing things are alive, they absorb carbon-14 from

their food and keep a constant concentration of carbon-14 in their bodies. However, when objects die no

carbon-14 is being taken and this starts to decrease. This principle is used in carbon dating. Find the age

of a tree which shows an activity of 12.5%. (2)

3. Iodine-131 is ingested by some hospital patients for tracing disease. If it has a half-life of 8days and 5mg are

ingested by a patient, what mass of the sample will still be emitting radiation after 32 days? (2)

4. Sodium, 2311Na, used as a coolant in nuclear reactors as it can absorb a neutron to form a heavier isotope of

sodium.

Practice Exercise 14D

a. Find the half-life of the sample (2)

b. What will be the activity of the sample (in counts

per minute) after 12 hours? (2)

c. How much time will it take for the activity of a

sample to drop from 100% to 6.25%? (2)

a. Write a nuclear equation for the reaction (3)

b. The so formed isotope undergoes beta decay to form an isotope of magnesium, Mg. Write an equation

for the reaction. (3)

c. This makes the coolant radioactive. If it has a half life of 15 hours, what mass of a 1kg radioactive sample

would still be active after 5 days? (2)


P A G E 1 3


A tank containing radioac-

tive waste from a nuclear

test site

14.8 Handling and Storage The symbol shown is used whenever radioac-

tive substances are being used. It must be

displayed in every site containing radioactive

sources.

Workers working with radioactive sources

wear a badge (or dosimeter) as shown previ-

ously. This is then analysed to carefully as-

sess the amount and type of radiation that

workers are being exposed to.

Radioactive sources are carefully selected giv-

ing special attention to the amount used and

the half-life of the sample. Thus, in hospitals

small amounts of radioactive substances with

short half-lives are used.

Nuclear waste may have a very long half-life. These are stored deep under-

ground in concrete-covered tanks. Some countries recycle this waste to pro-

duced glass once it is no longer radioactive.

Workers working with dangerously radioactive sources (either due to high ac-

tivity or due to high exposure times) can work with samples behind screens

sometimes using robotic arms.

Handling of a radioactive

source at a distance using

robotic arms

In 2010, 13.5% of the

world’s electricity produc-

tion came from nuclear

power. In 2011, France

was generating about 78%

of its electricity needs us-

ing nuclear power. In Feb-

ruary 2012 there were 439

operating nuclear power

stations

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