Section 8: Nuclear Physics

Question 1

Describe the apparatus used in the Rutherford scattering experiment

Answer 1

An alpha source and gold foil in an evacuated chamber (vacuum), which was covered in a fluorescent coating. The fluorescent coating meant you could see where the alpha particles hit the wall of the chamber. There was also a microscope which could be moved around the outside of the chamber to observe the path of alpha particles.

Question 2

What would be the expected result of the Rutherford scattering experiment if the plum pudding model was true?

Answer 2

The positively charged alpha particles would be deflected by a very small amount when passing through the foil.

Question 3

Explain the observations of the Rutherford scattering experiment

Answer 3

Most alpha particles passed straight through the foil with no deflection : the atom is mostly empty space.

A small number of particles were deflected by a large angle : the centre of an atom is positively charged, as the positive particles were repelled and deflected.

Very few particles were deflected back by more than 90° : the centre of an atom was very dense, as it could deflect fast moving alpha particles, but it was very small as very few particles were deflected by this amount.

Question 4

For ALPHA radiation: what is its range in air, how ionising is it, and what is it absorbed by?

Answer 4

Range in air: 2 - 10 cm
Ionising: Highly
Absorbed by: Paper

Question 5

For BETA radiation: what is its range in air, how ionising is it, and what is it absorbed by?

Answer 5

Range in air: ~ 1 m
Ionising: Weakly
Absorbed by: Around 3mm of aluminium foil

Question 6

For GAMMA radiation: what is its range in air, how ionising is it, and what is it absorbed by?

Answer 6

Range in air: Infinite, following inverse square law
Ionising: Very weakly
Absorbed by: Several metres of concrete of several inches of lead

Question 7

Describe 3 ways gamma radiation is used in medicine

Answer 7

As a detector: a gamma source with a short half-life can be injected into a patient and the gamma radiation can be detected to help diagnose patients.

To sterilise surgical equipment: gamma radiation kills any bacteria present on the equipment.

In radiation therapy: gamma radiation can kill cancerous cells in a targeted region of the body such as a tumour, however it will also kill any healthy cells in that region.

Question 8

State some safety precautions when handling radioactive sources

Answer 8

• Use long handled tongs to move the source
• Store the source in a lead-lined container when not in use
• Keep the source as far away from people as possible
• Never point the source towards others

Question 9

State some sources of background radiation

Answer 9

• Radon gas released from rocks
• Artificial sources such as nuclear weapons testing and nuclear meltdown
• Cosmic rays
• Rocks containing naturally occurring radioactive isotopes

Question 10

What is the decay constant?

Answer 10

The probability of a nucleus decaying per unit time, symbol λ.

It can be found by dividing the change in the number of nuclei over time, over the initial number of nuclei.

Question 11

What is the half-life of a radioactive sample?

Answer 11

The time taken for the number of nuclei to halve. It is a constant.

Question 12

What are the two ways to find the half-life of a radioactive sample graphically?

Answer 12

Take the negative exponential graph of N over t and read the time taken for N to halve across several half-lives and find a mean.

or

Plot the graph of ln(N) against time, which will be a straight line graph. The magnitude of the gradient will be the decay constant, which can then be used to find the half-life.

Question 13

What is the activity of a radioactive sample and how does it relate to the number of nuclei?

Answer 13

It is the number of nuclei that decay per second.

It is proportional to the number of nuclei in the sample, which means it also follows the same exponential decay equation, substituting N for A and N0 for A0.
This also means that the time for the activity to halve is equal to the half-life.

Question 14

When can the decay constant be used to model the decay of nuclei and why?

Answer 14

Only when there is a large number of nuclei in the sample, as the decay constant models the number of nuclei decayed by statistical means.

Question 15

Explain carbon dating

Answer 15

It is the process of dating organic objects. The percentage of carbon-14 is approximately equal in all living things, and carbon-14 has a half-life of 5730 years. Therefore, comparing the current amount of carbon-14 in an organic object with the initial amount allows us to find out how old the object is.

Question 16

What nucleus is best for medical diagnosis and why?

Answer 16

Technetium-99m, as it is a pure gamma emitter and has a half-life of 6 hours, which means that exposure is limited but gives enough time for tests to be carried out.

Question 17

Explain the 4 reasons why a nucleus might decay

Answer 17

1. It has too many neutrons: It will decay through beta-minus emission - a neutron turns into a proton and releases a beta-minus particle and antineutrino.
2. It has too many protons: It will decay through beta-plus emission or electron capture. In beta-plus, a protons turns into a neutron and releases a beta-plus particle and a neutrino. In electron capture, an orbiting electron is taken in by the nucleus and combines with a protons, causing the formation of a neutron and neutrino.
3. It has too many nucleons: It will decay through alpha emission - an alpha particle is released.
4. It has too much energy: It will decay through gamma emission. This situation usually occurs after a different type of decay because the nucleus becomes excited.

Question 18

Describe and explain the shape of the N-Z graph

Answer 18

The graph increases uniformly up to around 20 of each neutrons and protons. Beyond this the graph curves upwards, meaning there are more neutrons than protons at any given point.

This is because beyond around 20 of each nucleon, the electromagnetic force of repulsion becomes larger than the strong force keeping the nucleus together, and so more neutrons are required to increase the distance between protons, in order to reduce the magnitude of the electromagnetic force of repulsion.

Question 19

State the two methods for calculating nuclear radius

Answer 19

Distance of closest approach, and electron diffraction.

Question 20

Describe the distance of closest approach method for finding nuclear radius

Answer 20

Fire a positive particle at a nucleus. The initial kinetic energy of the particle can be measured and therefore is known, and as the particle approaches the nucleus it experiences an electrostatic force of repulsion that slows it down, converting kinetic energy to electric potential energy.

The point at which the particle stops is its distance of closest approach, and its electric potential energy is equal to its initial kinetic energy. Knowing this, you can find the distance by using the equation of electric potential. Since electric potential is potential energy per unit positive charge, multiplying it by the charge of the particle forms an equation for electric potential energy involving r, the distance from the centre.

Question 21

Describe the electron diffraction method for finding nuclear radius

Answer 21

Electrons are accelerated to very high speeds - so that their de Broglie wavelength is around 10^-15 m, and directed to a very thin film of material in front of a screen, causing them to diffract through the gaps between nuclei and form a diffraction pattern.

The pattern is a series of concentric circles that get dimmer as you move away from the centre, and you can plot a graph of intensity against diffraction angle. From this graph you can find the diffraction angle of the first minimum, and plug it into the following formula to find an estimate for nuclear radius:

sin θ = 0.61λ / R

Question 22

What is the mass defect?

Answer 22

The difference between the mass of a nucleus and the total mass of its constituent nucleons. The mass of the nucleus is always lower, since some energy is released when nucleons fuse, and mass and energy are interchangeable.

Question 23

What is binding energy?

Answer 23

The energy required to separate the nucleus into its constituent nucleons.

(or the energy released when a nucleus is formed from its constituent nucleons)

Question 24

Define one atomic mass unit

Answer 24

1/12th the mass of a carbon-12 atom: 1.661 x 10^-27 kg
Represented by 1u.

Question 25

What is nuclear fission?

Answer 25

The splitting of a large nucleus into two daughter nuclei.

Question 26

Why is energy released in nuclear fission?

Answer 26

Because the two smaller daughter nuclei have a higher binding energy per nucleon.

Question 27

What is nuclear fusion?

Answer 27

Where two smaller nuclei join together to form one larger nucleus, and only occurs in fairly small nuclei.

Question 28

Why is energy released in nuclear fusion?

Answer 28

Because the larger nucleus has a much higher binding energy per nucleon.

Question 29

What are the advantages and disadvantages of nuclear fusion?

Answer 29

It releases far more energy than fission, however it can only occur at extremely high temperatures (such as in stars), because a massive amount of energy is required to overcome the electrostatic force of repulsion between nuclei.

Question 30

What is the binding energy per nucleon?

Answer 30

The binding energy of a nucleus divided by the number of nucleons in the nucleus.

Question 31

Describe the shape of graph of binding energy per nucleon against nucleon number and what this means

Answer 31

For small nucleon numbers, there is a very sharp positive gradient up to around 15, trailing off to a maximum binding energy per nucleon at 56, otherwise known as iron. Beyond this there is a very gradual negative gradient.

The graph can tell you whether an element will undergo fission or fusion. Nuclei smaller than iron undergo fusion, while nuclei larger than iron undergo fission.

Question 32

What is induced fission?

Answer 32

When a nucleus is artificially caused to undergo fission by firing a thermal neutron into it, causing it to become extremely unstable.

Question 33

Why do thermal neutrons induce fission?

Answer 33

Because they are slow moving have a low energy, which means they are more easily absorbed by the nucleus, while neutrons that move faster and have higher energy rebound from the nucleus after the collision and do not cause a fission reaction.

Question 34

What are the products of induced fission?

Answer 34

Two daughter nuclei and at least one neutron.

Question 35

What is a chain reaction?

Answer 35

The neutrons released during fission go on to cause more fission reactions, forming a chain reaction, where each fission goes on to cause at least one more fission.

Question 36

What is critical mass?

Answer 36

The minimum mass required to maintain a steady chain reaction. Using exactly the critical mass of fuel will mean that a single fission reaction follows the last, while using less than the critical mass would lead the reaction to eventually stop.

Question 37

What are the three main features of a nuclear reactor?

Answer 37

The moderator, the control rods, and the coolant.

Question 38

Describe and explain the purpose of a moderator

Answer 38

It slows down the neutrons released in fission reactions to thermal speeds through elastic collisions between the nuclei of the moderator atoms and the fission neutrons. The closer the moderator atoms are in size to a neutron, the larger the proportion of momentum which is transferred, therefore the lower the number of collisions required to get the neutron to thermal speeds. Because of this, water is often used because it contains hydrogen, and it’s also inexpensive and not very reactive.

Question 39

Describe and explain the purpose of control rods

Answer 39

They absorb neutrons in the reactor in order to control chain reactions. Their height can be controlled in order to control the rate at which fission reactions occur, to control the amount of energy produced. They are made of materials which absorb neutrons without undergoing fission, such as boron and cadmium.

Question 40

Describe and explain the purpose of a coolant

Answer 40

It absorbs the heat released during fission reactions in the core of the reactor. This heat is then used to make steam, which powers electricity-generating turbines. Water is sometimes used as it has a high specific heat capacity, meaning it can transfer large amounts of thermal energy. Other materials such as molten salt or gases such as helium can be used.

Question 41

What fuel is used in nuclear reactors and where does it come from?

Answer 41

Enriched uranium, formed through the enrichment of mined uranium. Mined uranium is made up of around 99% uranium-238, which does not experience fission. Enriching mined uranium increases the percentage of uranium-235, which does undergo fission, to around 5%. The uranium-238 helps to control the rate of fission reactions by absorbing fission neutrons.

Question 42

Describe 3 safety precautions used in nuclear power stations

Answer 42

Fuel rods are inserted remotely to limit workers’ exposure to radiation.

Around the reactor is a very thick concrete shielding, which blocks radiation from escaping the reactor. It may become radioactive after long term use because neutrons which escape the reactor may enter the shielding nuclei, causing them to become unstable.

In an emergency, control rods are dropped into the reactor core entirely in order to stop fission reactions from occurring as soon as possible by absorbing all free neutrons. This is known as an emergency shut-down.

Question 43

Describe the process of high-level waste disposal

Answer 43

The waste is removed and handled remotely to limit exposure.

Material from the reactor is extremely hot due to the fission reactions in the reactor, so it is placed in cooling ponds on-site for up to a year.

At this point any plutonium or usable uranium in spent fuel rods is removed and recycled.

The waste is then vitrified (encased in glass) and placed in thick steel casks, and stored in deep caverns in geologically stable locations.

Question 44

Discuss the advantages and disadvantages of nuclear power

Answer 44

It produces no polluting gases and is reliable, and needs far less fuel - 1kg of uranium gives as much power as 25 tonnes of coal.

However, it produces radioactive waste, and a nuclear meltdown can have catastrophic consequences.

These risks and benefits must be balanced, therefore risks are minimised through various safety aspects, so that the benefits outweigh the risks.