8. Nuclear physics

Question 1

What is the plum pudding model?

Answer 1

A model where atoms are spheres of positive charge, with small areas of negative charge evenly distributed throughout.

Question 2

How did Rutherford scattering work?

Answer 2

An alpha source (e.g. lead) was aimed at a one atom thick layer of foil in a vacuum chamber of walls which are scintillators (fluorescent coating).

Question 3

What were the Rutherford scattering findings?

Answer 3

Most alpha particles passed straight through the foil without being deflected, meaning the atom is mostly empty space.

Few alpha particles were deflected at large angles, meaning the centre of the atom is positively charged.

Very few alpha particles were deflected back by more than 90, meaning that the centre of the atom is very dense and very small.

Question 4

What are the properties of different radiation?

Answer 4

Alpha: travels 2-10 cm in air, is highly ionising, is absorbed by paper.

Beta: travels 1 m in air, is moderately ionising, is absorbed by 3 mm of aluminium.

Gamma: infinite range (following the inverse square law), very weakly ionising, is absorbed by several meters of concrete or inches of lead

Question 5

How can radiation be used in industry?

Answer 5

With a detector one side and a source the other, radiation can be used to monitor the thickness of certain materials when they are being produced.

Question 6

How can gamma radiation be used in medicine?

Answer 6

As a detector​: 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.

To sterilise surgical equipment​ : gamma radiation will kill any bacteria present on the equipment.

In radiation therapy​: 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.

The risks are reduced where possible: e.g. reduced exposure times, use of shielding.

Question 7

Why does gamma radiation follow the inverse square law and how can this e proved?

Answer 7

Because it spreads out in all directions equally.

Intensity = k/distance^2.
This can be found experimentally by measuring the radioactivity of a source at different distances from a geiger muller tube, adjusting for the background radiation and then plotting count against 1/distance^2

Question 8

What are the risks of radiation and how can they be minimised?

Answer 8

Any radiation can ionise body tissue, potentially causing damage to cells or cell mutations such as cancer.

The risks are minimised by: 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 9

What are the sources of background radiation?

Answer 9

Radon gas (released from rocks)
Artificial sources (from nuclear weapons testing etc.)
Cosmic rays entering the Earth’s atmosphere from space.

Question 10

Why are some nuclei unstable?

Answer 10

Nuclei are ​held together by the strong nuclear force, however protons experience the repulsive electromagnetic force.

A nucleus could have:
(1) Too many neutrons (beta-minus decay)
(2) Too many protons (beta-plus decay or electron capture)
(3) Too many nucleons (alpha decay)
(4) Too much energy (gamma emission)

Question 11

What is the pattern of stability?

Answer 11

It is not uniform because after about 20 protons the electromagnetic force of repulsion becomes larger so more neutrons are needed to increase the distance between the protons.

Question 12

How can the nuclear radius of an atom be estimated using the distance of closest approach of a charged particle?

Answer 12

A charged particle approaching a nucleus will have an initial kinetic energy which can be measured, as it moves towards the ​positively charged nucleus​ it will experience an ​electrostatic force of repulsion and slow down as its kinetic energy is converted to electric potential energy​. ​The point at which the particle stops and has no kinetic energy is its distance of closest approach.

Question 13

How can the nuclear radius of an atom be estimated using electron diffraction?

Answer 13

Electrons are accelerated and fired through a very thin film so they go between the nuclei and form a diffraction pattern (a set of concentric circles), from which a graph of intensity against diffraction angle can be plot so that the diffraction angle of the first minimum can be found.
Then use the equation sinθ = 0.6λ/R.

Question 14

How would you interpret a nuclear radius, R, - nucleon number, A, graph?

Answer 14

By taking the log of both sides of R = kA^n
n is 1/3 and k is approximately 1.4 fm and is renamed R naught.

Question 15

How would you find the density of the nucleus?

Answer 15

ρ = m/V = A x m(nucleon) / (4/3)πR^3 where R = R(naught)A^1/3 => => => m(nucleon) / (4/3)πR^3 = a constant

Question 16

What is the binding energy?

Answer 16

The energy required to separate the nucleus into its constituents. (or the energy released when a nucleus forms from its constituents i.e. the mass defect)

Question 17

What is the mass defect?

Answer 17

The difference between the mass of a nucleus and the mass of its individual constituents.

Question 18

What is one atomic mass unit, 1 u, defined as?

Answer 18

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

Question 19

What is nuclear fission?

Answer 19

The splitting of a large nucleus into two daughter nuclei. Energy is released during fission because the smaller daughter nuclei have a higher binding energy per nucleon.

Question 20

What is fusion?

Answer 20

Two smaller nuclei joining together to form one larger nucleus. Energy is released during fusion because the larger nucleus has a much higher binding energy per nucleon.
Fusion releases much more energy that fission, but can only occur at very high temperatures because a massive amount of energy is needed to overcome the electrostatic force of repulsion between nuclei. .

Question 21

What is the binding energy per nucleon?

Answer 21

The binding energy of a nucleus divided by the number of nucleons.

Question 22

How can fission be induced?

Answer 22

By firing a thermal neutron into a heavy nucleus, causing it to become extremely unstable.
Thermal neutrons have a low energy, as opposed to neutrons with a higher energy which would rebound away from the nucleus after a collision, which would not cause a fission reaction.
The products if fission are the two daughter nuclei and at least one neutron which can then cause more fission reactions forming a chain reaction.

Question 23

What is the critical mass?

Answer 23

The minimum mass of fuel required to maintain a steady chain reaction.
(i.e. using less means the chain would eventually stop)

Question 24

What is the function of a moderator?

Answer 24

To slow down the neutrons released in fission reaction to thermal speeds through elastic collisions between the nuclei of the moderator atoms and fission neutrons.
The closer the moderator atoms are in size to a neutron, the larger the proportion of momentum is transferred.
Water is often used as it contains hydrogen, is inexpensive, and not very reactive.
Graphite is also used upon occasion.

Question 25

What is the function of the con tool rods?

Answer 25

To absorb the neutrons in the reactor in order to control chain reactions and therefore control the rate of fission reactions occurring and the amount of energy produced.
This is done by changing the height of the control rods.
They are made of materials which absorb neutrons without undergoing fission, like boron or cadmium.

Question 26

What is the function of coolant?

Answer 26

To absorb 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.
Since water has a high specific heat capacity so can transfer large amounts of thermal energy it is sometimes used as a coolant.
Molten salt or Helium can also be used/

Question 27

How is fuel?

Answer 27

Enriched uranium, mined uranium (about 99% U-238) is enriched (to be about 5% U-235).
The U-238 absorbs fission neutrons and so helps control the rate of fission reactions.
The fuel rods are inserted into the reactor remotely yo limit workers’ exposure to radiation.

Question 28

What is a safety precaution around nuclear reactors?

Answer 28

A very thick concrete shielding which blocks radiation from escaping from the reactor and affecting the workers in the power station.
The shielding may become reactive after longterm use.

Question 29

What is an emergency shut down?

Answer 29

When, in an emergency, the control rods are fully dropped into the reactor core to stope fission reactors from occurring as soon as possible by absorbing all the free neutrons in the core.

Question 30

How is nuclear waste managed?

Answer 30

Short lived radioactive waste, like tools and gloves can be disposed of close to the surface.

The two daughter nuclei are usually extremely unstable and have a very high activity, because of this spent fuel rods are very dangerous and can stay radioactive for thousands of years so is processed carefully:
-the waste is removed remotely so exposure is limited.
-they are then placed in cooling ponds which are very close to the reactors for up to a year as they may still be producing heat due to radioactive emissions.
-any plutonium or usable uranium is removed from the spent fuel rods so they can be recycles.
-the waste is the vitrified (encased in glass) and placed in thick steel casks and stored in deep caverns in geologically stable locations.
Locations used to store radioactive waste are chosen so that it makes a minimal impact on the environment and people living in the area are consulted before hand.

Question 31

What are the risks and benefits of nuclear power?

Answer 31

They produce no polluting gases, are reliable production of power and need far less fuel.
They produce radioactive waste, nuclear meltdowns could have catastrophic consequences, and are very expensive too build.

The risks are minimised as far as possible is that the benefits outweigh them.

Understanding nuclear physics behind nuclear power allows society to make informed decisions about energy production.

Question 32

Explain in terms of binding energy why energy can be released during nuclear fusion?

Answer 32

Total Mass of nuclei is more than the mass of the fusion product.
Binding energy increases when a nucleus is formed by fusion