Nuclear and radioactivity

Question 1

What conclusion can we draw from the observation that most alpha particles passed straight through the foil with no deflection in the Rutherford scattering experiment?

Answer 1

The atom is mostly empty space.

Question 2

What conclusion can we draw from the observation that a small amount of particles were deflected by a large angle in the Rutherford scattering experiment?

Answer 2

The centre of the atom is positively charged.

Question 3

What conclusion can we draw from the observation that very few particles were deflected back by more than 90 degrees in the Rutherford scattering experiment?

Answer 3

The centre of the atom is very dense, the nucleus is small.

Question 4

Alpha radiation information

Answer 4

Range: 2 - 10 cm in air
Highly ionising
Deflected by electric and magnetic fields
Absorbed by paper

Question 5

Beta radiation information

Answer 5

Range of around 1 m in air
Weakly ionising
Deflected by electric and magnetic fields
Absorbed by aluminium foil

Question 6

Gamma radiation information

Answer 6

Infinite range - follows inverse square law
Very weakly ionising
Not deflected by electric and magnetic fields
Absorbed by several metres of concrete or several inches of lead

Question 7

What are the uses of gamma radiation as a detector?

Answer 7

A radioactive source with a short half-life (to reduce exposure), which emits gamma radiation, can be injected into a patient and the gamma radiation can be detected using gamma cameras in order to help diagnose patients.

Question 8

What are the uses of gamma radiation to sterilise surgical equipment?

Answer 8

Gamma radiation will kill any bacteria present on the equipment.

Question 9

What are the uses of gamma radiation in radiation therapy?

Answer 9

Gamma radiation can be used to 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 10

What safety precautions should be used when handling radioactive sources?

Answer 10

• using long handled tongs to move the source
• storing the source in a lead-lined container when not in use
• keeping the source as far away as possible from yourself and others
• never pointing the source towards others

Question 11

What are the sources of background radiation?

Answer 11

• radon gas, which is released from rocks
• artificial sources, caused by nuclear weapons testing and nuclear meltdowns
• cosmic rays, enter the Earth’s atmosphere from space

Question 12

What is the decay constant?

Answer 12

The probability of a nucleus decaying per unit time.

Question 13

What is the half-life?

Answer 13

The time taken for the number of nuclei to halve.

Question 14

How can half-life be used to date objects?

Answer 14

Nuclei with a long half-life, like carbon-14, can be used to date organic objects. This is done by measuring the current amount of carbon-14 and comparing it to the initial amount, the percentage of which is approximately equal in all living things.

Question 15

How can half-life be used for medical diagnosis?

Answer 15

Nuclei with relatively short half-lives are used as radioactive tracers. For example, technetium-99m is ideal for use in medical diagnosis as it is a pure gamma emitter (low ionisation), has a half-life of six hours (which is short enough to limit exposure but long enough for tests to be carried out), and it can be prepared easily on site.

Question 16

What are the 4 reasons why a nucleus might decay, and what type of radiation happens as a result?

Answer 16

1 - too many neutrons: beta-minus
2 - too many protons: beta-plus
3 - too many nucleons: alpha
4 - too much energy: gamma

Question 17

Why does the number of protons and neutrons in a stable nucleus not increase uniformly beyond around 20 of each?

Answer 17

Beyond this amount, the electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together, so more neutrons are needed to increase the distance between protons in order to decrease the magnitude of the electromagnetic force to keep the nucleus stable.

Question 18

How can you estimate the nuclear radius of an atom?

Answer 18

By calculating the distance of closest approach - the point at which the approaching alpha particle has no kinetic energy, only potential energy.

Question 19

How can electron diffraction be used to calculate the nuclear radius?

Answer 19

The electrons are accelerated to very high speeds so that their De Broglie wavelength is around 10^-15, and are directed at 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. You can then plot a graph of intensity against diffraction angle, from which you can find the diffraction angle of the first minimum.

Question 20

What is the formula for nuclear radius from electron diffraction?

Answer 20

sin x = 0.62 lambda / R (where x is the angle of diffraction of the first minimum, lambda is the De Broglie wavelength of the electrons, and R is the radius of the nucleus that the electrons were scattered by.

Question 21

What is the mass defect?

Answer 21

The mass difference between the mass of the individual nucleons and the mass of the nucleus.

Question 22

What is the binding energy?

Answer 22

The energy required to separate the nucleus into its protons and neutrons.

Question 23

What is u (the atomic mass unit)?

Answer 23

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

Question 24

Why is energy released during fission?

Answer 24

The smaller daughter nuclei have a higher binding energy per nucleon.

Question 25

Why is energy released during fusion?

Answer 25

The larger nucleus has a much higher binding energy per nucleon.

Question 26

Why can fusion only occur at extremely high temperatures?

Answer 26

A massive amount of energy is required to overcome the electrostatic force of repulsion between nuclei.

Question 27

What is the critical mass?

Answer 27

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

Question 28

What does a moderator do?

Answer 28

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 in each collision.

Question 29

Why are water and graphite often used as moderators?

Answer 29

They are inexpensive and not very reactive.

Question 30

What are control rods?

Answer 30

They absorb neutrons in the reactor in order to control chain reactions.

Question 31

What is coolant?

Answer 31

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.

Question 32

Why doesn't gamma radiation used to sterilise surgical equipment make it radioactive?

Answer 32

To become radioactive, the nucleus has to be affected, which ionising radiation does not do.

Question 33

State 3 benefits of using nuclear power.

Answer 33

1 - little greenhouse gas emissions 2 - smaller amount of fuel used to produce same amount of energy 3 - energy can be produced continuously

Question 34

Explain why the kinetic energy of neutrons must be reduced in a thermal nuclear reactor.

Answer 34

To increase the probability of fission when the neutron collides with the fissile material.

Question 35

What are the uses of beta minus radiation?

Answer 35

Controlling the thickness of materials when they are being manufactured. Beta minus radiation is passed through the material, and if the detector picks up a decrease in radiation the material has gotten thicker so the rollers move closer together (and vice versa).

Question 36

What are cloud chambers?

Answer 36

Chambers filled with highly saturated water vapour. They are used to visualise radiation trajectories, as when ionising radiation passes through the vapour, it leaves a trail that we can see.

Question 37

What are the uses of gamma radiation?

Answer 37

- medical diagnosis (not very ionising so does less damage to the body, a radioactive source can be injected or swallowed and then detected with a PET scanner) - treating cancer as high doses can damage cancerous cells, however can also damage healthy cells - sterilising surgical equipment

Question 38

What trails does beta minus radiation create in a cloud chamber?

Answer 38

Thin, wispy, wiggly trails which show how the electrons are deflected by the atoms.

Question 39

What trails do alpha particles create in a cloud chamber?

Answer 39

Thick straight trails as they are high energy and not deflected by atoms.

Question 40

What is a use of alpha radiation?

Answer 40

Smoke alarms. A current flows when they move unimpeded, but when blocked by smoke the circuit is broken and the alarm is triggered.

Question 41

What are the two experimental methods for finding the nuclear radius?

Answer 41

Distance of closest approach and electron diffraction.

Question 42

When is a nucleus in a metastable state?

Answer 42

After alpha or beta decay, the nucleus often has excess energy - it is excited. This energy is lost by emitting gamma rays.

Question 43

What is activity (A)?

Answer 43

The number of nuclei that decay each second - proportional to the size of the sample.

Question 44

What are the uses of isotopes with long half lives (eg carbon-14)?

Answer 44

Dating objects by measuring how much of the undecayed isotope is left and hence how old it is.

Question 45

What are the uses of isotopes with short half lives (eg Technetium-99)?

Answer 45

Medicine, as they will not continue to harm the body for long periods of time.

Question 46

What is the average binding energy per nucleon?

Answer 46

The total binding energy divided by the number of nucleons. Higher average binding energy per nucleon means that more energy is needed to remove nucleons from the nucleus - more stable.

Question 47

What are nuclear reactors kept in and why?

Answer 47

Thick concrete casing to stop radiation from escaping - protects the workers in the power station.

Question 48

How is waste from nuclear reactors dealt with?

Answer 48

Waste products are highly radioactive and initially very hot, so are placed in cooling ponds until the temperature falls to a safe level. The waste is then stored in sealed containers and buried deep underground.