Nuclear Physics

What was proven by the rutherford scattering experiment?

The existence of the nucleus

Describe JJ Thomson’s plum pudding model.

A sphere of positive charge with small areas of negative charge evenly distributed throughout

What is the name of the atomic model that arose from the scattering experiment?

Nuclear Model

Describe the process of the rutherford scattering experiment.

Alpha particles were fired at a thin piece of gold foil and the path of the alpha particles was observed on the walls of the vacuum chamber

What results were expected from the experiment?

Most particles would be deflected with only few passing through the foil

What results were actually observed from the experiment? 

Most alpha particles passed through with no deflection

A small amount of particles deflected at a large angle

Very few particles were deflected back by more than 90*

What did most particles passing though the gold foil suggest?

Most of the atom is empty space

What did the deflections at large angles by the particles suggest?

The centre of the atom is positively charged

What did the fact that few particles were deflected more than 90* suggest about the atom?

The nucleus of the atom is very small and very dense (as it deflects fast moving alpha particles)

What is radiation?

When an unstable nucleus emits energy in the form of EM waves or subatomic particles in order to become more stable

Name the 3 types of radiation?

Alpha

Beta 

Gamma 

State the ranges of the 3 types of radiation.

Alpha - 2-10cm

Beta - 1m

Gamma - infinite 

State the ionising power of the 3 types of radition.

Alpha - Highly

Beta - Weakly

Gamma - Very weakly 

What are all types of radiation deflected by?

Electric and magnetic fields

What is alpha radiation absorbed by?

Paper

What is beta radiation absorbed by?

Aluminium foil

What is gamma radiation absorbed by?

Several metres of concrete of several inches of lead 

Describe a simple experiment to determine the type of radiation produced by a source.

1. Use a GM counter to find the background radiation 

2. Place the source next to the counter and measure the count rate 

3. Place a sheet of paper between the source and counter and measure the count rate (if it decreases significantly, the source is an alpha emitter)

4. Repeat using foil and lead 

How can beta radiation be used during the production of aluminium foil?

A beta source and counter are placed either side of the foil, if the foil become too thick, less beta radiation is detected and the rollers will press the foil more 

Why is gamma radiation appropriate for use in medicine?

It is very weakly ionising so does far less damage then alpha and beta radiation

Describe 3 ways in which gamma radiation is used in medicine.

As a detector - gamma sources can be injected into a patient and cameras can be used to identify it and diagnose them

To sterilise surgical equipment - gamma radiation will kill any bacteria 

Radiotherapy - gamma radiation can be used to kill cancerous cells in a targeted region of the body 

What safety measures can be taken to reduce risk of radiation poisoning during gamma radiation therapy? 

Reduced exposure times 

Use of shielding 

What law does the intensity of gamma radiation in air follow?

I = k/x²

Describe an experiment to verify that gamma radiation obeys the inverse square law.

Measure the count rate at different distances from the GM tube, making sure to account for background radiation

Plot a graph of corrected count against 1/x² - a straight line verifies the equation 

Why is alpha radiation dangerous to humans?

It is highly ionising, so can damage and kill cells

Why is gamma radiation dangerous to humans?

Prolonged exposure can cause mutations and damage to body cells

What safety measures should be taken when handling radiation in a lab?

• Use long handled tongs to move the source

• Store in a lead-lined container

• Keep as far as possible away from people 

• Do not point the source at others 

What is background radiation?

Low-level radiation present in the atmosphere at all times

Where does background radiation come from?

Cosmic rays, radioactive elements in rocks and soil, medical X-rays, power plants, radon gas

State how to calculate the corrected count in an experiment.

Corrected count = total count rate - background count

Why is radioactive decay a random process?

It cannot be precited when the next decay will occur and which nucleus will decay 

What is meant by the decay constant of a sample?

The probability of a nucleus decaying per unit time

State the equation for the decay constant.

change in N / change in t = -decay constant x N

State the formula for exponential radioactive decay.

N = N₀e^{-^t}

What is meant by half-life?

The time taken for the number of radioactive nuclei in a sample to halve

Describe how to determine half-life from a graph.

Find the time where the nuclei halves and then repeat, calculating a mean

What is a more accurate way of calculating half life from an experiment?

Plot a graph of ln(N) against time and calculate the modulus of the gradient

State the equation for half-life.

T_1/2 = ln2 / decay constant

What is the activity of a sample?

The number of nuclei that decay per second

State the equation for activity.

A= N x decay constant

Why does activity follow the same exponential decay constant?

It is directly proportional to the number of nuclei

Why is activity normally used to calculate half-life rather than number of nuclei?

It is much easier to measure

How is half-life used in carbon dating?

Nuclei with a long half-life (eg C-14), is measured in a sample and compared to the initial amount (% is approx equal in all living things) 

How is half-life used in medical diagnosis?

Short half-life nuclei is used in medicine as the half-life is short enough to limit exposure but long enough to carry out tests 

How does half-life effect how a source is stored?

Long half-lives must be stored suitably to prevent nuclei from damaging the environment and the people around it

What force holds nuclei together?

The strong force

Why do protons experience repulsion?

Due to electromagnetic force between them

State 4 reasons why a nucleus will decay.

1. Too many neutrons

2. Too many protons

3. Too many nucleons

4. Too much energy 

How does a nucleus with too many neutrons decay?

Beta minus emission, one of the neutrons turns into a proton and a beta minus particle and an antineutrino are released

What happens to the nucleon and proton number during beta minus emission?

Nucleon number stays constant

Proton number increases by 1 

How does a nucleus with too many protons decay?

Beta plus emission - a proton changes into a neutron and a beta plus particle and a neutrino are released

Electron capture - an orbiting electron is combined with a proton and a neutron and a neutrino are formed

What happens to the nucleon and proton number during beta plus emission?

The nucleon number stays constant

Proton number increases by 1 

How does a nucleus with too many nucleons decay?

Alpha emission

What happens to nucleon and proton number during alpha emission?

Nucleon number decreases by 4

Proton number decreases by 2

How does a nucleus with too much energy decay?

Gamma emission - usually occurs after a different type of decay

Why does gamma emission normally occur after another type of decay?

The nucleus becomes excited and has excess energy

Why does the number of protons and neutrons not increase uniformly for larger elements?

The electromagnetic repulsion becomes larger than the strong force, so more neutrons are needed to increase the distance between the protons and therefore decrease the EM force and keep the nucleus stable

How does the energy of the nucleus change during alpha emission?

It decreases

How is technetium-99m used in medical diagnosis?

Technetium is formed in an excited state, and will emit a gamma photon in order to reach ground state, which can be used for imaging 

How can the nuclear radius of an atom be estimated?

Calculating the closest approach of a charged particle

Electron diffraction

How is closest approach to a nucleus calculated?

E_elec = 1/4piE₀ x Q₁Q₂/r

Explain how electron diffraction is used to estimate nuclear radius.

The electrons are accelerated towards a thin film of material in front of a scree, causing them to diffract through gaps between nuclei

Plot a graph of intensity against diffraction angle and find the first min angle

Use the formula sinX = 0.61 x wavelength / R 

What is the typical value of nuclear radius?

1 × 10^-15m

How is nuclear radius dependent on nucleon number?

The higher the nucleon number, the wider the atomic radius

How can the relationship between nuclear radius and nucleon number be found from a graph of nuclear radius against nucleon number? 

Log the relationship and plot a graph of ln(R) against ln(A) 

The gradient gives the relationship and the y-intercept gives the value of ln(k) where k is a constant 

State the equation linking nuclear radius and nucleon number.

R = R₀A^1/3

Use R=R₀A^1/3 to show that nuclear density is constant for all nuclei.

Density = mass/volume

A x m / 4/3piR³

A x m / 4/3pi(R₀³A)

m / 4/3piR₀³ = constant 

State the calculated value of nuclear density.

1.45 × 10¹⁷ kgm^-3

Why is the density of a nucleus much higher than the density of an atom?

Most of the atom is empty space

State the equation for nuclear energy.

E = mc²

What is mass defect?

The mass lost as energy when a nucleus is formed from its constituents

What is binding energy?

The energy required to separate the nucleus into its constituents (or released when it’s formed)

What is the atomic mass unit defined as?

1/12th of the mass of a carbon-12 atom

Convert 1u to kg.

x 1.661×10^-27

A change of 1u means how much energy is released?

931.5MeV

How is mass defect calculated?

sum of nucleon masses - mass of nucleus

What is nuclear fission?

The splitting of a large nucleus into 2 smaller daughter nuclei

Why is energy released during nuclear fission?

The smaller daughter nuclei have a higher binding energy per nucleon than the larger nucleus

What is nuclear fusion?

When 2 smaller nuclei join to form one larger nucleus

Why is energy released during nuclear fusion?

The larger nucleus has a much higher binding energy per nucleon than the smaller nuclei

Which fission/fusion process releases more energy?

Fusion

Why can fusion only happen at high temperatures?

It takes large amounts of energy to overcome the electrostatic force of repulsion between the nuclei 

What element has the highest binding energy per nucleon? 

Iron 

What process do elements smaller than iron undergo?

Fusion

What process do elements bigger than iron undergo?

Fission

How can the energy released in fusion/fission be calculated?

Calculate the energy difference between the nuclei

What process is used in nuclear power plants?

Fission

What safety risks are associated with nuclear power plants?

Meltdowns, radioactive products

How can nuclear fission be induced?

Thermal neutrons are fired into the uranium nucleus causing it to become unstable and decay 

Why do faster moving neutrons rarely induce nuclear fission?

They collide with the nucleus and are bounced back rather than being absorbed

What are the products of nuclear fission?

2 daughter nuclei and at least one fission neutron

What happens to the fission neutron released from the uranium?

They go on to cause a chain reaction where each fission goes on to cause at least one more fission  

What is meant by critical mass?

The minimum mass of fuel required to maintain a steady chain reaction of fissions

Give 3 key features of a nuclear reactor.

Moderator

Control rods

Coolant 

Explain the purpose of a moderator in a nuclear reactor.

It slows down the neutrons released in fission reactions to thermal speeds

The fast-moving neutrons collide with moderator atoms and transfer some of their kinetic energy, slowing them down

The closer the size of the moderator atoms to the neutrons, the higher the proportion of momentum transferred, therefore the lower number of collision required to slow it to thermal speeds  

What materials are used as moderators and why? 

Graphite and water - inexpensive and not very reactive and close in size to neutrons 

Explain the purpose of control rods in a nuclear reactor.

They absorb neutrons to control the chain reaction, controlling the amount of energy produced so it doesn’t become out of control

What materials are used as control rods and why?

Boron or cadmium as they can absorb neutrons without undergoing nuclear fission 

Explain the purpose of a coolant in a nuclear reactor.

It absorbs the heat released during fission from the reactor, using it to make steam to power electricity-generating turbines

Why is water often used as a coolant in nuclear reactors?

It has a high specific heat capacity so can transfer large amounts of thermal energy