Radioactivity

Question 1

Describe the structure of an atom

Answer 1

A positively charged nucleus made of protons and neutrons. Surrounded by negatively charged electrons which orbit the nucleus. The nucleus radius is much smaller than the atom’s radius. Almost all the mass of the atom lies in the nucleus.

Question 2

What is the relative charge and relative mass of protons, neutrons, electrons and positrons?

Answer 2

Particle: Relative Charge: Relative Mass
Protons: +1: 1
Neutrons: 0: 1
Electrons: -1: 1/1836
Positron: 1+ : 1/1860

Question 3

What is the size of an atom?

Answer 3

~0.1 nanometers

Question 4

Describe the term “isotope”

Answer 4

Atoms of the same element that have the same number of protons but different number of neutrons.

Question 5

Explain how the movements of electrons within an atom relates to the absorption and emission of electromagnetic radiation.

Answer 5

When electrons move closer to the nucleus, the atom emits electromagnetic radiation.
Conversely, when electrons move further away from the nucleus, the atom absorbs electromagnetic radiation.
If electrons gain enough energy, they can leave the atom to form an ion.

Question 6

What is background radiation?

Answer 6

Weak radiation that can be detected from natural/external sources.

Question 7

Give examples of background radiation

Answer 7

Examples of background radiation include cosmic rays, radiation from underground rocks, nuclear fallout, medical rays.

Question 8

Explain two methods of measuring radioactivity

Answer 8

1. Photographic film: The film is initially white, however the more radiation it absorbs the darker it gets. They are worn as badges by people who work with radiation, to check how much radiation exposure they’ve received.
2. Geiger-muller tube: Each time it absorbs radiation, it transmits an electrical pulse to the machine, which produces a clicking sound. The greater number of clicks per second (the frequency of clicks) the more radiation is present.

Question 9

What did John Dalton propose?

Answer 9

That everything was made of atoms.

Question 10

What did J.J Thomson discover and what model did he propose based on his discovery?

Answer 10

J.J Thomson discovered the electron and proposed the Plum Pudding Model, where electrons are dispersed throughout a positively charged “pudding”.

Question 11

What did Rutherford discover through his gold foil experiment?

Answer 11

Rutherford discovered the nucleus by firing a beam of alpha particles at thin metal foils.
Most alpha particles passed straight through the foil (this suggested that most of the atom is empty space), some particles bounced back (the large deflections suggest that some positively charged mass in the atom was repelling the particles).

Question 12

What did Bohr propose in his atomic model?

Answer 12

Bohr proposed that electrons exist in fixed orbits around the nucleus to prevent the atom from collapsing.

Question 13

Decay occurs in a random process. What are the forms of decay? (4)

Answer 13

1. Alpha
2. Beta minus
3. Beta plus
4. gamma

Question 14

What is an alpha particle?

Answer 14

a helium nucleus
charge: +2
range in air: few cm
penetration: stopped by paper
ionisation: high

Question 15

What is beta minus decay?

Answer 15

a neutron that becomes a proton and releases an electron.
charge: -1
range in air: few 10s of cm
penetration: stopped by a few mm aluminium
ionisation: medium

Question 16

What is beta plus decay?

Answer 16

a proton that becomes a neutron, and releases a positron.
charge: +1
range in air: few 10s of cm
penetration: stopped by a few mm aluminium
ionisation: medium

Question 17

What is a gamma ray?

Answer 17

electromagnetic radiation
charge: 0
range in air: infinite
penetration: reduced by a few mm lead
ionisation: low

Question 18

What happens to nuclei after decay?

Answer 18

After decay, nuclei often have excess energy which they release as gamma radiation when the atom undergoes nuclear arrangement.

Question 19

What is activity?

Answer 19

Activity is the number of decays in a sample per second. It is initially high and decreases exponentially over time. The units are Becquerel (Bq).

Question 20

What is the half-life of an isotope?

Answer 20

The time taken for half of the nuclei in a sample to decay.

Question 21

How to work out the net decline of radioactive nuclei after X half-lives?

Answer 21

Net decline = (initial number – number after X half lives)/initial number

Question 22

What are the uses of radioactivity? (5)

Answer 22

1. smoke alarms
2. irradiating food
3. sterilization of equipment
4. tracing and gauging thickness
5. diagnosis and treatment of cancer

Question 23

How do smoke alarms work?

Answer 23

Americium is used in smoke alarms and has a half-life of 432 years. In the smoke alarm it has an alpha emitter, which is stopped by a few centimeters of air (as it is weakly penetrating). The alpha particles ionise air particles and makes them charged therefore making a current. If smoke enters the air around the alarm, the current drops in the circuit, causing the alarm to make a sound.

Question 24

How does tracing and gauging thickness work?

Answer 24

Beta radius is mildly penetrating and can just pass through paper. How it works: A source and receiver are placed either side of the paper during its production if there is a drop or rise in received electrons, then that means the thickness of the paper has changed. It is also used inside pipes, with a detector placed externally to measure the thickness of walls of the pipe.

Question 25

How does the diagnosis and treatment of cancer work?

Answer 25

Consuming or injecting a gamma emitter, which passes through the body, the external detector can picture where the tracer has collected in the body, which can reveal tumours. Gamma rays are used on the tumour, killing the cancer cells, however exposing rays on healthy cells cause them to possibly mutate or causes damage.

Question 26

Explain how the dangers of ionising radiation depend on half-life and relate this to the precautions needed

Answer 26

A short half life – the source presents less of a risk, as it does not remain strongly radioactive. This means initially it is very radioactive, but quickly dies down so presents less of a long term risk.
A long half life: the source remains weakly radioactive for a long period of time.

Question 27

What are some precautions to take place to ensure safety to those exposed to radioactivity?

Answer 27

Limiting patient dose: Only use radioactive tracers with a short enough half life, to quickly be removed over a day or so, but long enough to still be detectable after the time taken for it to pass through the body. Common medical tracers used have a half life of 6 hours.
Limiting risks to medical personnel: They leave during radioactive tests.

Question 28

What is contamination?

Answer 28

Lasts for a long period of time
The source of radiation is transferred to an object e.g radioactive dust settling on your skin (your skin becomes contaminated).

Question 29

What is irradiation?

Answer 29

Lasts only for a short period of time.
The source emits radiation, which reaches the object.
E.g radiactive dust emitting beta radiation, which irradiates your skin.
Medical items are irradiated sometimes to kill bacteria on its surface to kill bacteria on its surface, but not to make the medical tools themselves radioactive.

Question 30

Compare and contrast the treatment of tumours using radiation applied externally or internally

Answer 30

External: A beam of gamma radiation rotates around the body and continuously focuses on the tumour whilst only passing momentarily across healthy cells, ensuring minimal damage. There is a greater risk of long term side effects, as the radiation passes through healthy tissues.
Internal: Radioactive material is held within a needle and is injected directly into a tumour.

Question 31

How do PET scans work?

Answer 31

A radioactive tracer which is tagged to the desired chemical is inserted into the body, where it travels in the body and records where the tracer emits radiation and produces a 3D visualization of the body.

Question 32

Explain some advantages and disadvantages of nuclear power for generating electricity?

Answer 32

Uranium fuel splits, releasing neutrons which are absorbed by further uranium nuclei, which split. No CO2 is produced. There is a safety risk of radiation leaking, or the chain reaction becoming uncontrollable, causing a meltdown.
Waste disposal is difficult – initially extremely hot, the waste needs to be placed deep in lakes, to cool down before being stored deep underground.

Question 33

What is nuclear fission? (6)

Answer 33

1. When a uranium nucleus absorbs a neutron, this triggers the nucleus to undergo fission.
2. When the nucleus splits it forms two smaller daughter nuclei. It also emits two or three neutrons plus gamma radiation.
3. The products of fission are radioactive.
4. Energy is also released and kinetic energy from the products is released from this fission reaction.
5. The neutrons can now be absorbed by more uranium nuclei and trigger fission again.
6. This is called a chain reaction.

Question 34

What are moderators?

Answer 34

Moderators control the speed at which the neutrons are travelling through the reactor.

Question 35

What are control rods?

Answer 35

Used to control number of neutrons in reactor.
They ca absorb neutrons to slow the rate of reaction.

Question 36

How is energy released in a nuclear reactor?

Answer 36

A controlled chained reaction is used to release energy in a nuclear reactor.

Question 37

What causes the explosion in a nuclear weapon?

Answer 37

The explosion in a nuclear weapon is caused by an uncontrolled fission chain reaction.

Question 38

What is nuclear fusion? (4)

Answer 38

1. At high temperatures and pressures, two light nuclei (e.g. hydrogen) are joined to form a heavier nucleus.
2. Some of the mass of nuclei can be converted into energy, which is released as radiation.
3. This takes place in stars e.g the sun.
4. The electrostatic repulsion of the protons in the two different nuclei means a lot of energy is required to bring the nuclei close enough to fuse.

Question 39

Why is it difficult to create a practical and economic fusion power station?

Answer 39

Fusion cannot happen at low temperatures and pressures and so makes it very difficult to make a practical and economic fusion power station.