Topic 6 - Radioactivity

Question 1

6.1 - What particles make up atoms?

Answer 1

A positively charged nucleus with protons and neutrons, surrounded by negatively charged electron.
Nucleus contains most of the atom’s mass.

Question 2

6.2 - What is the typical size of aoms?

Answer 2

The radius of a nucleus is 110^-15.
The radius of an atom is 110^-10.
An atom is 100,000x bigger than its nucleus.

Question 3

6.3 - How are isotopes represented by symbols?

Answer 3

Atoms of the same element that contain the same number of protons but different number of neutrons are called isotopes.
Carbon has three isotopes, these are carbon-12, 13, 14. Their mass number ( nucleons ) is the in the name.
As carbon has a mass number of 12 and its atomic mass is 6, it has 6 neutrons.

Question 4

6.4 - How do isotopes differ from its element?

Answer 4

The nucleus of each element has a positive charge, but in isotopes, the mass number changes as there are a different number of neutrons.

Question 5

6.5 - What are the relative masses and charges of subatomic particles in an atoms?

Answer 5

Protons, which are found in the nucleus have charge of +1 and a mass of 1.
Neutrons, which are found in the nucleus have charge of 0 and a mass of 1.
Electrons, which are found around the nucleus have charge of -1 and a mass of 1/2000.

Question 6

6.6 - How are atoms neutral?

Answer 6

Atoms of an element have the same number of protons which is the atomic number.
The nucleus has a positive charge, this is due to protons, neutrons have no charge.
Atoms have the same number of protons and electrons to stay electrically neutral.

Question 7

6.7 - How are electrons arranged in atoms?

Answer 7

Electrons in an atom only exist in orbits around the nucleus, called electron shells.
Each electron shell has different energy level.

Question 8

6.8 - What happens to atoms when they absorb or emit EM radiation?

Answer 8

If an atom absorbs energy, an electron moves into higher orbit/shell, away from the nucleus.
When an atom emits energy as EM radiation, an electron returns to a lower orbit/shell.
This EM radiation can be emitted as colour, each colour is a different wavelength and the wavelength can be measured using an emission spectrum.

Gases can also absorb energy transferred by EM radiation, like visible light.
The measure of wavelengths of light that are absorbed when radiation passes through it is called the absorption spectrum.

Question 9

6.9 - Explain how atoms form positive ions by losing outer electrons

Answer 9

Atoms can gain so much energy that one or two electrons can escape the electron shells and the atom.
The energy is radiation and when it forms ions, it is called ionising radiation.
An atom that has lost or gained electrons is an ion.
An atoms has the same number of protons and electrons so has no charge, in ions which lose electrons, there are more protons forming a positive ion.

Question 10

6.10 - What are the types of ionising radiation?

Answer 10

Radioactive substances are unstable, so it can change and decay
Decay causes radiation to be emitted, which causes the nucleus to lose energy and become more stable.
The decay of a nucleus is a random process.

Alpha particles (α) have two protons and neutrons like a helium atom
It has a relative mass of 4 and charge of 2+ because there are no electrons.

Beta particles (β-) are high energy, high speed electrons. They come from the nuclei of an atom when a neutron becomes a proton, they don’t cause ionisation to the atom.
They have a relative mass of 1/2000 and a charge of -1.

Positrons (β+) are high energy, high speed particles with the same mass as an electron by charge of +1.

Gamma rays (γ) are high frequency EM waves which don’t have an electric charge.

Neutrons can also be emitted from an unstable nucleus, they have a relative mass of 1 and no electric charge.

Question 11

6.12 - What is background radiation?

Answer 11

Ionising radiation that is around us all the time from many sources. Some sources are natural while some come from human activities.

Question 12

6.13 - What are the sources of background radiation?

Answer 12

The main source is radon, this radioactive gas is produced by rocks with small amounts of uranium.
It can diffuse into air and build up in houses.
Radon amounts also depends on the type of rock in the area in buildings and its amount uranium content, this varies across the country.

Some foods can increase background radiation exposure as they naturally contain small amounts of radioactive substances.
Hospital treatments like X-rays and gamma-ray scans can also contribute.

Cosmic rays are high energy, charged particles from the Sun and other stars, it is a form of radiation.
Most are stopped in the upper atmosphere but some still reach Earth’s surface.

Question 13

6.14 - Describe methods to detect and measure radioactivity

Answer 13

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tub

Question 14

6.14 - Describe methods to detect and measure radioactivity

Answer 14

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tub

Question 15

6.14 - Describe methods to detect and measure radioactivity

Answer 15

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tub

Question 16

6.14 - Describe methods to detect and measure radioactivity

Answer 16

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tub

Question 16

6.14 - Describe methods to detect and measure radioactivity

Answer 16

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tub

Question 16

6.14 - Describe methods to detect and measure radioactivity

Answer 16

Photographic film becomes darker as more radiation reaches it.
But it has to be developed in stages of radioactivity to measure radiation amount.
People who deal with radiation wear film badges to check radiation.

A Geiger-Muller tube or GM tube measures radiation by passing it through a tube that ionises the gas and allows a short pulse of current to flow.
The GM tube can be connected to a counter, which counts the current pulses or the GM tube gives a click every time radiation is detected.
The count rate is the clicks per minute.

When measuring radioactivity of a source, scientists measure the background radiation by taking several readings and finding the mean.
This mean value is subtracted from the measurement of the source.

Question 17

6.15 - What are types of radiation compared to?

Answer 17

Alpha particle is equal to a helium nucleus.
A beta particle is an electron emitted from the nucleus.
Gamma rays are EM radiation.

Question 18

6.16 - Compare types of radiation in their abilities to penetrate and ionise

Answer 18

Alpha particles are emitted at high speeds with high relative mass, so they transfer a lot of energy, highly ionising atoms.
Although when they ionise, they lose energy, therefore they highly ionise in a short penetration distance.
These particles can be stopped by paper.

Beta particles are moderately ionising compared alpha particles, so they can penetrate much further, as they don’t lose as much energy.
These particles can be stopped by 3mm thick aluminium.

Gamma rays are weakly ionising, so has high energy to penetrate particles easily.
These particles can be stopped by thick lead.

Question 19

6.17 - Describe how the atomic model has changed over time

Answer 19

In 1897, J.J Thomson showed that atoms contain smaller subatomic particles called electrons with a negative charge and negligible mass.
In 1900s, he supported this using a new model for atoms called the plum pudding model.
The pudding was positively charged material with negatively charged electrons scattered through it as the plums.

Between 1909-1913, Ernest Rutherford experimented what would happen when positively charged subatomic particles or alpha particles pass through gold foil.
The experiment showed most of the alpha particles passed through but a few bounced back and were deflected.
Rutherford suggested that atoms were mostly empty space with most their mass in a central nucleus with a positive charge with electrons in the empty space.

Niels Bohr amended Rutherford’s model, suggesting that electrons are in fixed orbits around the nucleus, this explains absorption and emission spectra.

Question 20

6.18 - Describe the process of β– decay

Answer 20

A neutron changes into a proton and an electron.
The electron is emitted from the atom.
Atomic number increases by one, because there is one less electron.

Question 21

6.19 - Describe the process of β+ decay

Answer 21

A proton becomes a neutron and a positron.
Atomic number goes down by one because a positron is emitted.

Question 22

6.20 - Explain the effect of decays on the atomic and mass number

Answer 22

α decay changes the mass number by 4 and atomic number goes down by 2.
β decay increases atomic number by 1.
γ decay causes nuclei to lose energy to make itself stable.
Neutron emission decreases mass number by 1.

Question 23

6.21 - How does radioactive decay affect the nucleus?

Answer 23

Radioactive decays causes nucleus to undergo rearrangement of its subatomic particles with a loss gamma radiation to make it more stable.

Question 24

6.22 - How do you balance nuclear equations?

Answer 24

Nuclear equations show what happens during radioactive decay, the total mass on both sides must be the same.

If the atomic number (protons) is 53 and a beta minus particle is emitted (-1), then the nucleus would have 54 as its atomic number.
53 = -1 + 54

Question 25

6.23 - How does the activity of a radioactive substance change over time?

Answer 25

When an unstable nucleus undergoes decay, its nucleus changes to become more stable.
Activity is initially high as there are more undecayed nuclei.
The activity decreases exponentially as more undecayed nuclei decay.
After a nucleus decays, it may be more stable, as it gets more stable the activity decreases.

Question 26

6.24 - What is the unit for activity of a radioactive isotope?

Answer 26

The activity of a radioactive substance is measured as the number of nuclear decays per second and is measured in becquerels.

Question 27

6.25/26 - What is half-life?

Answer 27

It is the time taken for half the undecayed nuclei to decay.
We cannot predict when decay of individual nuclei happen as it is a random process.
Half-life allows predictions of a large number of nucleus’ activity.

Question 28

6.27 - How can graphs showing radioactive activity help calculate when it will decay?

Answer 28

Activity at 3 mins is 800Bq
To find when its half life is find time at 400Bq.

Question 29

6.28 - Describe the uses of radioactivity

Answer 29

Killing microorganisms - Food can be irradiated with gamma rays to kill bacteria.
This makes food safer to eat and can be stored for longer.
Surgical instruments need to be sterilised by heating them or sealing them in bags and irradiating it with gamma rays, penetrating the instruments and bag.

Radioactive detecting - Radioactive isotopes can be used as tracers.
Gamma sources could be added to water to detect a leak. When there is a leak, water flows out into the Earth.
A GM tube on top of the pipe can detect the higher levels of radiation.

Cancer - Radioactivity can be used to help diagnose cancer using tracer.
It can also treat cancer.

Checking thickness - Paper can be made in different thicknesses and roller need to squeeze with the right force for thickness.
Detectors can count the rate at which beta particles go through the paper emitted by a radiation source.
If paper is too thin, more beta particles penetrate the paper and detector records higher count rate.
A computer then reduces the force applied to make paper thicker.

Smoke alarms - It has a source of alpha particles usually a radioactive isotope called americium-241.
The source gives a constant stream of alpha particles which ionises molecules.
These ions are attracted to the detector and allows an electrical current to flow.
As long as current flows, alarm doesn’t sound, but when smoke gets into the air gap, the ions slow down and current flown decreases.
This sounds the alarm.

Question 30

6.29 - Describe the dangers of ionising radiation

Answer 30

Large amounts of ionising radiation can cause tissue damage.
Small amounts over a long time can damage cell DNA, this is a mutation.
Mutations can make a cell malfunction leading to cancer.
Not all mutations are harmful and cells can usually repair this damage if the radiation dose is low.

Question 31

6.30P - Explain how dangers of ionising radiation depend on half-life

Answer 31

Patients may be exposed to a dose of radiation for treatment, this is done when benefits are greater than the radiation’s harm.
The source used has a short half-life to minimise the exposure time.
Longer half-life sources are a greater threat, as the effects will last longer.

Question 32

6.31 - Explain the precautions taken to ensure the safety of people exposed to radiation

Answer 32

Limiting the treatment dose so less exposure.
Intensity of radiation decreases with distance from the source, so sources are handled with tongs, not pointed at people and stored in lead lined containers.
Medical staff also limit exposure by keeping distance from the source, shielding it and minimising exposure time.
Exposure levels are monitored with dosimeter badges.

Question 33

6.32 - Describe the differences between contamination and irradiation

Answer 33

Irradiation is when someone is exposed to alpha, beta or gamma radiation from a nearby radioactive source.
Once the person moves away the irradiation stops.
Someone becomes contaminated if they get particles of radioactive material on their skin or in the body.
They will be exposed to radiation as the isotopes decay from the material, this stops when all the source decayed or it has been removed.
Contamination can spread in things like water.

Question 34

6.33P - Compare and contrast the treatment of tumours using radiation internally and externally

Answer 34

Internal radiotherapy uses a beta emitter like iodine-131 and is placed in the body close to the tumour.
This doesn’t always need surgery and the patient stays in the room alone.

External radiotherapy, which is more common, uses beams of gamma rays, X-rays and protons directed at the tumour from outside the body.
Several lower energy beams may be directed at the tumour so the tumour absorbs most of the energy and so the surrounding tissue isn’t harmed.

Question 35

6.34 - Explain some uses of radioactive substances in diagnosis

Answer 35

Radioactive materials can be used to diagnose conditions without performing surgery.
A radioactive tracer which emits gamma rays can be injected into the body.
Tracers have a radioactive isotope that attaches to molecules that are taken up by an organ.
The tracers location can be found using gamma cameras.
Tracers help find sources of internal bleeding, where the cameras detect the highest gamma rays, is the source of the bleeding.
Tracers can also detect tumours, when they are made or radioactive glucose, as very active cells like cancer cells take in glucose quicker.
Where there is more glucose there is the tumour.

Tracers that emit positrons can help diagnose conditions.
When the positron that is emitted meets and electron, they are destroyed and two gamma rays are emitted in opposite directions.
The PET scanner moves around the patient and builds up an image of the brain from where the gamma radiation moves.

Question 36

6.35 - Why do isotopes used in PET need to be produced nearby?

Answer 36

Radioactive isotopes used in all medical tracers have short half-lives so that the body is affected as little as possible.
This means that they lose their radioactivity quickly, so they are made closer to the hospital.
They are used within hours of production.

Question 37

6.36P - Evaluate the advantages and disadvantages of nuclear energy

Answer 37

Nuclear energy stores more energy per kilogram, which is useful in big machinery.
They don’t need to burn and don’t release CO2.
Most nuclear energy is used in power stations to generate electricity.
Conventional power stations burn fossil fuels, to produce CO2, soot and acidic gases. This can contribute to climate change.
Although uranium is non-renewable, it will last longer than other non-renewables at this rate.

Nuclear power stations don’t emit gases, but they do produce waste that will be radioactive for a million years.
The waste needs to be sealed in concrete safely.
Nuclear power station can become radioactive but are very expensive to decommision.
Radioactive materials could leak and nuclear reactors could explode.

Question 38

6.37P/44P - What are some nuclear reactions?

Answer 38

In radioactive decay, radiation emitted by unstable nuclei transfers energy.

Nuclear fission - large nuclei like uranium-235 break up to form smaller nuclei and release energy. This is used in power stations.
Nuclear fusion - two small nuclei fuse to form a larger nucleus and energy. This happens in the Sun.

Question 39

6.38P/42P - Explain what are the products of the fission of U-235

Answer 39

When uranium-235 absorbs a neutron it splits into two smaller daughter nuclei which are also radioactive.
Two or more neutrons are also released and energy is transferred by heating.

Question 40

6.39P - Explain the principle of a controlled nuclear chain reaction

Answer 40

If the neutrons emitted from one fission are absorbed by other uranium-235 nuclei, they become unstable and release more neutrons when split, which are again absorbed.
This is an uncontrolled nuclear chain reaction.
The reaction can be controlled if there are other materials to absorb some neutrons.

Question 41

6.40P - How are chain reactions controlled in a nuclear reactor?

Answer 41

Nuclear reactor is made into fuel rods.
As fission reactions happen, neutrons leave the fuel rods at high speeds, they are slowed by a moderator.
They are slowed in order to increase the chance of being absorbed by another U-235 nucleus.

Control rods control the chain reaction by absorbing neutrons.
They are placed between fuel rods, if the rate of fission needs to be increase, control rods are moved out of the core so fewer neutrons are absorbed.
When the control rods are fully lowered into the core, the chain reaction stops.

Question 42

6.41P - Describe how thermal energy from chain reactions are used for electricity

Answer 42

Energy released from the core is transferred to a coolant like water, which is pumped through the reactor.
When the water gets hot and turns into steam, it drives a turbine which turns a generator to produce electricity.

Question 43

6.43P - What is nuclear fusion?

Answer 43

It is when small nuclei combine to form larger ones. The new nuclei has less mass as some energy is emitted.
Fusion reactions where hydrogen nuclei combine to form helium are the main energy source of stars as it produced a lot of energy.

Question 44

6.45P - Explain why nuclear fusion doesn’t happen at low pressures and temperatures

Answer 44

The protons in the nuclei are positively charged and like charges repel, this is electrostatic repulsion and it takes lots of pressure to overcome.
The nuclei need to get very close, and the Sun has a strong gravitational field, creating a high pressure at its centre, this makes them more likely to collide.
Nuclei are more likely to collide at high temperatures when they are moving faster, as they overcome electrostatic repulsion and fuse.

Question 45

6.46P - Relate conditions for fusion to the difficulty of being practical and economical

Answer 45

A fusion reactor on Earth needs to produce very high pressures and temperatures.
Fusion reactors can produce more energy than fission reactors.
The helium produced in fusion isn’t radioactive while the reactor would be radioactive.
In fusion reactions, there are less problems with disposal of radioactive waste.
However, fusion requires extreme temperatures and pressures and it is hard to sustain it.