Nuclear Radiation

Question 1

What is alpha radiation and what is its relative mass and charge?

Answer 1

A helium nucleus, mass of 4, charge of +2
4
He
2

Question 2

What is the penetration, range and ionisation of alpha radiation?

Answer 2

2cm of air, or a single sheet of paper (it loses its momentum quickly)
Strong ionisation

Question 3

What is beta radiation and what is its relative mass and charge?

Answer 3

High speed electron, mass of zero and a charge of -1

Question 4

What is the penetration, range and ionisation of beta radiation?

Answer 4

10-15cm of air, 2mm of aluminium 
Medium ionisation (its high speed means it travels further before it interacts with an atom)

Question 5

What is gamma radiation, and what is its relative mass and charge?

Answer 5

Electromagnetic wave, charge and mass of 0

Question 6

What is the penetration, range and ionisation of gamma radiation?

Answer 6

Metres or kilometres of air, inches of lead. 
Weak ionisation (no mass or charge, so it doesn’t interact with many substances)

Question 7

What is a use for alpha radiation?

Answer 7

In smoke alarms - alpha particles ionise air so a small current will flow, but smoke absorbs alpha particles so radiation detected/current flowing changes if it is smoky

Question 8

What is a use for beta radiation?

Answer 8

Thickness control in paper production - in paper mills, thickness is controlled by how much beta radiation passes through the paper. The counter controls the pressure of the rollers, therefore the paper’s thickness

Question 9

What is a use for gamma radiation?

Answer 9

Sterilisation - it kills bacteria and mould in food, and it can be used to sterilise medical equipment
It can also be used as a radioactive tracer and in radiotherapy

Question 10

What is meant by background radiation and what are the main sources?

Answer 10

Radiation that can be detected with no radioactive source present (and it must be subtracted from any radiation measurement of a source)
The largest contributor is radon gas which arises from various sources - medical, industrial and living things

Question 11

Explain the basic principles of a Geiger-Muller tube

Answer 11

It contains gas at low pressure. A pd is applied across a metal cylinder and central wire. Radiation ionises gas molecules, producing positive ions and electrons. These are then attracted to electrodes and a current is detected

Question 12

What is ionisation and why can it be dangerous to living things?

Answer 12

The creation of ions e.g. when an alpha particle knocks an electron out of its orbit so the atom gains a positive charge, therefore being ionised
It is dangerous as cells can be killed or DNA can be damaged which will cause the cell to mutate or be unable to replicate itself

Question 13

What is meant by spontaneous decay?

Answer 13

Nothing is able to influence the process - it is natural

Question 14

What is meant by random decay?

Answer 14

Radioactive decay is not continuous or predictable - a single nucleus may decay in 5 seconds of 5 years; we can only measure the average activity

Question 15

What is meant by the decay constant?

Answer 15

It gives the probability of decay, and its units are s^-1

Question 16

What is meant by activity?

Answer 16

The rate at which nuclei decay - radioactive decays per unit time. Its units are s^-1

Question 17

What is half-life?

Answer 17

The average time taken for the activity of a sample to fall to half of its original value or the average time for half the nuclei to decay

Question 18

Why will the count rate measured by a GM tube be much smaller than the actual activity of a sample?

Answer 18

• Radiation is emitted from the source in all directions but the GM tube can only pick up a small amount of this
• Radiation is absorbed by the air and within the material of the GM tube
• There are inefficiencies in the GM tube, where not all radiation will be detected (as it needs to ionise molecules of the gas inside it, which causes problems particularly with gamma radiation)

Question 19

What is fission?

Answer 19

The splitting of a large, unstable nucleus into 2 or more smaller fragments to become more stable

Question 20

Describe the process of nuclear fission of U-235

Answer 20

The U-235 nucleus absorbs a slow-moving, ‘thermal’ neutron, and splits into 2 nuclei and 2 or 3 neutrons. Energy appears in the KE of the products

Question 21

What is fusion?

Answer 21

Small nuclei joining together to form a larger nucleus and energy. Very high temperatures and pressures (usually caused by gravity when it happens in stars) in order to overcome electrostatic repulsion

Question 22

How does mass-energy conservation allow energy released in nuclear reactions to be calculated?

Answer 22

Some of the original mass is converted into energy - total mass must be conserved, so if we know the mss deficit in kg, we can find the energy equivalent in joules using E = mc^2

Question 23

What is binding energy and how does it relate to the stability of a nucleus?

Answer 23

Binding energy is the energy required to pull a nucleus apart into its separate nucleons. The greater the binding energy per nucleon, the more stable the nucleus

Question 24

Which nuclei will undergo fission and which nuclei will undergo fusion, and why do both these processes release energy?

Answer 24

In order to become more stable, light nuclei undergo fusion and heavy nuclei undergo fission.
Light nuclei make larger nuclei and energy is given out from the mass deficit when they fuse, and they gain a higher binding energy per nucleon by fusing
When heavy nuclei undergo fission to make lighter nuclei, they also have a mass deficit which also gives out energy.
Both methods aim to ‘reach’ an iron nucleus which is the most stable as it has the highest binding energy per nucleon

Question 25

Which elements are anomalies on the binding energy against mass number graph?

Answer 25

Helium, carbon and oxygen (both of which are multiples of the helium nucleus)

Question 26

Why do stars cease to produce energy when their atoms have a nucleon number of approximately 55?

Answer 26

55 is approximately the mass of an iron nucleus, and energy would need to be input to fuse any nuclei from iron and heavier, so no more energy can be produced once the star cannot fuse any nuclei (as binding energy per nucleon would be lower)

Question 27

Describe the basic construction of a nuclear fission reactor and explain the role of the fuel rods, control rods and the moderator

Answer 27

• Fuel rods are filled with enriched uranium and are inserted into the reactor
• Fission produces heat in the form of KE of particles
• Neutrons must travel at the correct speed to be absorbed by the U-235 nucleus
• Moderators are used to absorb some momentum of the neutrons (e.g. water)
• Control rods (e.g. boron) absorb neutrons to control the rate of reaction
• Fluid in a primary circuit heats water via a heat exchanger to produce steam
• This steam is then used to turn a turbine, turning a generator to produce electricity

Question 28

Why do neutrons produced in fission need to be slowed down?

Answer 28

If they move too fast then they pass through the uranium nucleus

Question 29

What materials may be used as the moderator in a nuclear reactor?

Answer 29

Water, graphite, heavy water

Question 30

What are the benefits of nuclear power?

Answer 30

• No CO2 emissions
• Long term supplies which are effectively renewable
• Reliable source of energy (unlike other renewable sources like wind, for example)

Question 31

What are the drawbacks of nuclear power?

Answer 31

• Production of nuclear waste - much of which has a very long half life with no cost effective solution to get rid of
• Security issues with terrorism etc
• Danger of radiation leaks
• Expensive in decommissioning costs etc