NUCLEAR PHYSICS & RADIOACTIVITY

PHYSICS - UNIT ONE

ATOMIC STRUCTUREAtoms consist of:

Nucleus - The solid centre of the atom, containing protons and neutrons.

- Most of the mass of an atom is in the nucleus.

- The collective name for all particles found

in the nucleus is nucleons.

Protons are positively charged, with relative charge +1.

Neutrons are neutral and have no charge.

ATOMIC STRUCTUREAtoms consist of:

Electrons - Negatively charged particles, with relative charge of -1

- Found around the nucleus of an atom in ‘shells’ or ‘clouds’.

- These shells are arranged in energy levels – so the innermost

shell has the lowest energy level and is filled first.

ATOMIC STRUCTURE

The amount of protons, neutrons, electrons in an atom are represented as:

𝑋𝑍𝐴

Mass Number –The total number of

nucleons in the nucleus

(Neutrons + Protons)

Atomic Number –The total number of

protons in the nucleus

Chemical Symbol for the

element

The number of electrons in an

atom is equal to the number of

protons

ATOMIC STRUCTURE

𝐿𝑖36

𝐶612

𝑋𝑍𝐴

Mass Number –The total number of

nucleons in the nucleus

(Neutrons + Protons)

Atomic Number –The total number of

protons in the nucleus

Chemical Symbol for the

element

ISOTOPES

An atoms name is based on how many protons it has.

Not all atoms of the same name have the

same number of neutrons.

We call atoms with variations in number of

neutrons, isotopes of that element.

The isotopes therefore have the same Atomic Number

(number of protons), but different mass numbers.

ATOMIC STRUCTURE - QUESTIONS1. How many protons and neutrons are in the following atoms?

2. For the following elements, represent them in the form

a) 2 neutrons and 2 protons

b) 7 protons and 13 Nucleons

c) 91 Protons and 143 Neutrons

3. Explain why it is possible to have 2 atoms of different element with the

same number of nucleons

TYPES OF NUCLEAR RADIATION

Unstable isotopes can emit various types of radiation while striving to

become stable. The radiation that is emitted is ionising radiation, which

has the ability to knock electrons from atoms they come into contact with.

The 3 naturally occurring forms of nuclear radiation are:

Alpha Radiation α

Beta Radiation β

Gamma Radiation γ

ALPHA DECAY

An unstable nucleus ejects a relatively large particle known as an

α particle.

The α particle consists of 2 protons and 2 neutrons (a Helium

nucleus).

The remaining daughter nucleus is more stable & is now a

different element.

ALPHA DECAY

𝑈92238 → h𝑇90

234 + 𝐻𝑒24 +𝑒𝑛𝑒𝑟𝑔𝑦

𝑈92238 → h𝑇90

234 +𝛼+𝑒𝑛𝑒𝑟𝑔𝑦Or can be

written as…

BETA DECAY

Two types - β+ (produced under laboratory conditions only )

- β- (occurs naturally)

β- particle is a fast moving electron that is ejected from an unstable

nucleus.

In fact it’s true! The outer shell electrons don’t change, instead

other interesting changes take place in the nucleus……

Wait! An electron emitted from the

nucleus??? Nuclei doesn’t

contain electrons!

BETA DECAY One neutron transforms into a proton and an electron. The proton remains in the nucleus, the electron is emitted and is

called the β- particle.

The resulting daughter nucleus has the same number of nucleons as the parent, but one less neutron and one more proton.

𝑛01 → 𝑝1

1 + 𝑒− 10

h𝑇90234 → 𝑃𝑎91

234 + 𝑒−10 +𝑒𝑛𝑒𝑟𝑔𝑦

h𝑇90234 → 𝑃𝑎91

234 +𝛽−+𝑒𝑛𝑒𝑟𝑔𝑦Or can be

written

as…

GAMMA DECAY The nucleus is unstable after α or β decay and may need to

release energy.

Gamma emission occurs after another form of nuclear decay has

taken place, when the nucleus is ‘excited’.

A small packet of excess electromagnetic energy called a gamma

ray is emitted.

No mass, No charge, Travels at the speed of light.𝐵𝑖∗83

210 → 𝐵𝑖83210 +𝛾

PENETRATION POWERThe various forms of nuclear radiation are so different, so they of course react differently when coming into contact with matter after being ejected from the nucleus. They can each be absorbed by:α particles - a few centimetres of air

- a piece of paper

- a layer of dead skinβ particles - about 100 centimetres of air

- a few centimetres of Aluminiumγ rays - barely affected by air

- absorbed in many centimetres of Lead

IONISING POWER

An ion is an atom with an overall positive or negative charge.

Positive ions form when electrons are removed from a neutral atom.

Negative ions form when electrons are added to a neutral atom.

Alpha, Beta & Gamma radiation have the ability to ionise atoms they come into contact with.

IONISING POWER

α particles - Are slow-moving particles

- They have time to interact with most atoms in their path

- When they interact with an atom the α particles positive charge

attracts electrons from the atoms

- These atoms are ionised – they are no longer neutral

- With each ionisation, the α particle slows down and lose energy

- Poor penetrating power

- High ionising power

IONISING POWER

β- particles - Are repelled by the electrons in atoms

- The repulsion causes the β- particles to bounce between atoms.

- The collisions may cause some electrons to be ejected from the atom, ionising the atom

- Each β collision loses less energy than α collisions, so β particles have higher penetrating ability than α particles but less ionising power.

IONISING POWER

γ rays - May interact with electrons or nuclei they collide with as

they move through a substance.

- As γ rays have no charge, collisions are infrequent.

- Collisions occur only when a nucleus or electron is directly in the path of the γ ray (unlikely due to the large amount of empty space in an atom).

- Very low ionising power.

- Very high penetrating power.

HALF-LIFE

A half-life is the time taken for half a group of unstable nuclei to decay.

It’s impossible to know exactly when an unstable atom will decay.

We can however predict how many will decay in a period of time.

Half-lives vary according to the isotope that is decaying – these can range from microseconds, to thousands of millions of years.

HALF-LIFE

We will look at these in

greater detail later

in the course

HALF-LIFE

1st Half life –the time it takes for 50% of the nuclei to

decay

2nd Half life –the time it takes for

50% of the remaining nuclei to

decay

3rd Half life –the time it takes for

50% of the remaining nuclei to

decay4th Half life –

the time it takes for 50% of the

remaining nuclei to decay

The Half-life of an atom can be represented on a graph, known as a decay curve.

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.

X

The y-axis shows the number of Californium-

252 atoms as a percentage

~2.645 yrs

To find the half-life, find 50% on the y-axis, ruling a line to the plot and match this up to the corresponding value

on the x-axis

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.X

This tells us that the half-life of Californium-

252 is approx. 2.65 years

2.65

The second half-life (when only 25% remain un-

decayed – ie. Half of the remaining 50%) in this case, occurs in another

2.65 years, at approximately 5.3 years.

X

5.33

HALF-LIFE

X

2.65

The third half-life (when only 12.5% remain un-

decayed – ie. Half of the remaining 25%) in this case, occurs in another 2.65 years, at approximately

7.95 years.

X

5.33

7.95

X

The fourth half-life (when only 6.25% remain un-

decayed – ie. Half of the remaining 12.5%)

in this case, occurs in another 2.65 years, at approximately

10.6 years.

When does the fourth half-life occur?

X10.6

HALF-LIFE

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

710 million years

1420 million years

6.25%

1.5625%

NOW DO

HALF-LIFE ~ SIMULATION TASK

CHAPTER ONE - Q 17; 23-26

NOW TRY

HALF-LIFE

A half-life is the time taken for half a group of unstable nuclei to decay.

It’s impossible to know exactly when an unstable atom will decay.

We can however predict how many will decay in a period of time.

Half-lives vary according to the isotope that is decaying – these can range from microseconds, to thousands of millions of years.

HALF-LIFE

HALF-LIFE

1st Half life –the time it takes for 50% of the nuclei to

decay

2nd Half life –the time it takes for

50% of the remaining nuclei to

decay

3rd Half life –the time it takes for

50% of the remaining nuclei to

decay4th Half life –

the time it takes for 50% of the

remaining nuclei to decay

The Half-life of an atom can be represented on a graph, known as a decay curve.

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.

X

The y-axis shows the number of Californium-

252 atoms as a percentage

~2.645 yrs

To find the half-life, find 50% on the y-axis, ruling a line to the plot and match this up to the corresponding value

on the x-axis

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.X

This tells us that the half-life of Californium-

252 is approx. 2.65 years

2.65

The second half-life (when only 25% remain un-

decayed – ie. Half of the remaining 50%) in this case, occurs in another

2.65 years, at approximately 5.3 years.

X

5.33

HALF-LIFE

X

2.65

The third half-life (when only 12.5% remain un-

decayed – ie. Half of the remaining 25%) in this case, occurs in another 2.65 years, at approximately

7.95 years.

X

5.33

7.95

X

The fourth half-life (when only 6.25% remain un-

decayed – ie. Half of the remaining 12.5%)

in this case, occurs in another 2.65 years, at approximately

10.6 years.

When does the fourth half-life occur?

X10.6

HALF-LIFE

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

710 million years

1420 million years

6.25%

1.5625%

NOW DO

HALF-LIFE ~ SIMULATION TASK

CHAPTER ONE - Q 17; 23-29

NOW TRY

MEASURING DECAY

We can measure the ionising radiation of a

radioactive source using a Geiger counter.

• A Geiger counter detects Alpha, Beta and Gamma radiation.

• The common unit for measuring radioactive decay is Becquerel (Bq).

• Bq = number of decay’s per second.

http://atomic.lindahall.org/what-is-a-geiger-counter.html

MEASURING DECAY

Refer to the graph below, showing the decay curve of Thorium-234.

At the beginning when the decay is at large, the Geiger counter would of course be the most active, recording a

high count rate

Gradually decreasing over

time

So, if we measured the decay of a radioactive source as graphed it, it would be the same as

the decay curve

MEASURING DECAY

eg. A radioactive material is measured to have 600,000 decays per

second.

a) What is this equivalent to in Bq?

b) After 3 half-lives, what will the activity be in Bq?

600,000 Bq

One Half-life

Bq

Two Half-lifes

Bq

Three Half-lifes

Bq

RADIOACTIVE SERIES

We have been studying the decay (α, β, γ) of radioactive isotopes and

writing equations to represent the decay.

eg. α decay

The resulting daughter nucleus in this case is still not stable and as such

is still radioactive and will undergo additional decay in it’s pursuit to

reach stability.

The sequence of radioisotopes along this journey is called a decay chain. (Or decay series). Note: Not all radioactive isotopes go through a series of decay’s.

+

RADIOACTIVE SERIES

We can represent the decay graphically.

eg. α decay +

RADIOACTIVE SERIES

What happens to the daughter ?

β decay:

+

RADIOACTIVE SERIES

Lets look at what happens next...

β decay:

β decay:

α decay:

α decay:

α decay:

+

RADIOACTIVE SERIES

Which continues to decay to become Lead-206

RADIOACTIVE SERIES

Now you try:

For the radioisotope

• Write a series of equations to show it undergo the series of

decay: β, α, α, β.

• Represent the transformations using a Radioactive Series

graph

EFFECTS OF RADIATIONIONISING RADIATION

As covered previously, Alpha, Beta and Gamma radiation are Ionising

Radiation – high energy radiation that has the ability to change atom by

removing electrons and therefore giving the atom an overall charge (ions).

Overall positive charge - more protons than electrons. Can be represented as ,

Overall negative charge - more electrons than protons. Can be represented as

Recall – List the 3 types of decay in order of their ionising power……

Alpha, Beta, Gamma Do you remember why??

X-Rays are also a form of Ionising Radiation

EFFECTS OF RADIATIONNON-IONISING RADIATION

Other forms of electromagnetic

radiation, such as Radiowaves,

Microwaves & visible light,

have lower energies and don’t

interact with matter the same

way. These are Non-ionising

Radiation.

EFFECTS OF RADIATION

Sometimes when the electron that is knocked from the atom is part of a bond between one atom and another.

This causes the bond to be broken – this may result in a molecule being split in two. The two pieces have an overall charge and are known at free radicals.

Both ions and free radicals are very reactive – this may result in new chemical reactions taking place inside the substance that was exposed to the ionising radiation.

EFFECTS OF RADIATION

Video: The effects of radiation on our health

Source https://www.youtube.com/watch?v=tq6FDyFeCN0

more – textbook page 16

HOW MUCH RADIATION IS TOO MUCH?

Video: The most radioactive places on earth

Source https://www.youtube.com/watch?v=TRL7o2kPqw0

HOW MUCH RADIATION IS TOO MUCH?

The answer to this depends on many factors including:

• The type of radiation

• The part of the body exposed to the radiation

• The general health of the individual

ABSORBED DOSE

The amount of radiation received is called the Absorbed Dose.

This is the amount of energy absorbed by each kilogram of the tissue being exposed.

Absorbed Dose =

Units: Energy (Joules), Mass (kg), Absorbed Dose (Gray (Gy))

1 Gy = 1 joule per kilogram

ABSORBED DOSE

Absorbed Dose =

Example One: A 60kg person absorbs 0.06 Joules of energy due to ionising radiation.

Calculate the Absorbed Dose.

Absorbed Dose =

=

= 0.001 or Gy

ABSORBED DOSE

Unfortunately, the number of grays absorbed by a person does not provide much information about the extent of the damage to that person. We need to take into account the penetrating power of the type of radiation too. Why???

Alpha Particles are stopped in a short distance, passing on all energy in a short space. This causes much localised damage.Beta Particles are more penetrating, so the damage is less severe in any one area but is more widespread.Gamma rays (and X-rays) are far more penetrating than either α or β particles. They spread their energy over a large range.

DOSE EQUIVALENT

Units: Dose Equivalent (Sv), Absorbed Dose (Gy), Quality Factor (No units)

As this new equation takes into account the type of radiation, we can now make a true measure for comparision of the biological damage caused by

the radiation exposure.

DOSE EQUIVALENT

Source: From page 17 in your text book

DOSE EQUIVALENT

Eg1 (from before) : A 60kg person absorbs 0.06 Joules of energy due to ionising radiation.

Now calculate the Dose equivalent if the energy was delivered by Gamma Rays.

Absorbed Dose =

=

= 0.001 or Gy = 1 mGy

= 0.001 x 1= 0.001 Sv = 1 mSv

DOSE EQUIVALENT

Eg 2: An 80kg person absorbs 20 mJ of energy due to ionising radiation.

a) Calculate the Absorbed Dose.

b) Calculate the Dose equivalent if the energy was delivered by Alpha Particles (QF 20).

Absorbed Dose =

=

= 0.00025 or Gy = 250 Gy

= 0.00025 x 20= 0.005 Sv = 5 mSv

CHAPTER ONE - Q 30-43NOW TRY