3.8 Nuclear Physics

Question 1

Describe the Rutherford scattering experiment.

Answer 1

A beam of alpha particles are fired at a thin gold leaf. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected by any angle.

Question 2

What are the conclusions of the Rutherford scattering experiment?

Answer 2

Most of the particles passed straight through- the nucleus is small.
Some alpha particles are slightly deflected- the nucleus is positively charged.
Some alpha particles are significantly deflected- the nucleus is massive.

Question 3

Give a brief outline of the history of the atom.

Answer 3

The Greeks believed all matter was made up of identical atoms.
John Dalton thought that each element was made up of a different type of atom. And the atom couldn’t be broken up.
J.J.Thompson discovered electrons could be removed from from atoms so Dalton’s theory was disproved. He suggested the plum pudding model.
Rutherford suggested atoms didn’t have uniformly distributed charge and density. He discovered the nucleus.

Question 4

What are the properties of alpha radiation?

Answer 4

The most strongly ionising
Have a range of up to 4cm in air.
Can be absorbed by paper.

Question 5

What are the properties of beta radiation.

Answer 5

Weakly ionising
Has a range of up to 20cm-3m in air
Absorbed by around 3mm of aluminium

Question 6

What are the properties of gamma radiation?

Answer 6

Very weakly ionising
Range of over 1km
Can be absorbed by many cm of lead or several m of concrete

Question 7

What re the uses of alpha radiation?

Answer 7

Smoke alarms- they are very ionising, so they allow current to flow but don’t travel very far.

Question 8

What are the dangers of alpha radiation?

Answer 8

They can’t penetrate your skin but are dangerous when ingested, they quickly ionise tissue in one area causing a lot of damage.

Question 9

What are the uses of beta radiation?

Answer 9

Controlling the thickness of a material- the penetration depends on the thickness of the material.

Question 10

What are the uses of gamma radiation?

Answer 10

Radioactive tracers in medicine- gamma is weakly ionising so it does less damage to the body tissue. A source with a short half life is used.
Treating cancerous tumours- damages all cells so, the beam is rotated.

Question 11

What are the dangers of gamma radiation?

Answer 11

Can cause cancer
Can cause infertility
Avoid long-term exposure

Question 12

What is the definition of activity?

Answer 12

The number of decays per unit time.

Question 13

What is the inverse square law for radiation?

Answer 13

Intensity = k / x^2

Question 14

What is the experimental verification of inverse-square law?

Answer 14

Set up the apparatus
Turn ont he Geiger count and take a reading of the background count. Do this 3 times and take an average.
Place the counter a distance of d away from the tube.
Record count rate at that distance 3 times and take an average.
Move the counter to a distance of 2d and repeat.
Take readings for multiple distances.
Calculate corrected count rate.
Plot a graph of corrected count rate against distance.
As distance doubles, count rate should fall to 1/4 of original value, supporting inverse square law.

Question 15

Why does gamma radiation follow inverse square law?

Answer 15

At any distance r from the source, the radiation will be spread over the area of a sphere, 4(pi)(r^2).
When r doubles, the area over which radiation is spread quadruples and intensity decreases by a factor of 4.

Question 16

What is the application of inverse square law in handling radioactive sources?

Answer 16

Always hold source away from your body as a source becomes significantly more dangerous the closer it gets.
Use tongs when handling to maximise distance.
Keep as far away as possible.

Question 17

What are the sources of background radiation?

Answer 17

The air- radioactive radon gas
The ground and buildings
Cosmic radiation
Living things
Man-made radiation

Question 18

What is corrected count rate?

Answer 18

The count rate when you subtract background radiation from it.

Question 19

How do you determine activity from the count rate of a detector?

Answer 19

Find the corrected count rate
Find the cross sectional area of the detector
Find the area of the sphere at the distance of the detector
Multiply count rate by (Area of sphere) / (cross-sectional area)
Convert answer to Bq

Question 20

What is the nature of radioactive decay?

Answer 20

Random
Spontaneous

Question 21

What is the definition of half-life?

Answer 21

The average time it takes for the number of unstable nuclei to halve.

Question 22

What are the uses of measuring half-life?

Answer 22

Radioactive dating of objects
Medical diagnosis- radioactive tracers must have a long enough half-life to collect data but, radioactivity falls to a safe level quickly.

Question 23

Why are long half-lives dangerous?

Answer 23

Some substances stay radioactive for a long time, like nuclear waste. So they need to be stored safely to prevent damage to environment and people.

Question 24

What are the 4 reasons that nuclei decay?

Answer 24

Too many neutrons
Not enough neutrons
Too many nucleons
(Too much energy)

Question 25

When does beta- emission occur?

Answer 25

In neutron rich nuclei

Question 26

When do nuclei emit gamma radiation?

Answer 26

When they have to much energy. After alpha or beta decay, nuclei are often in the excited state and release a photon when they de excite. During electron capture.

Question 27

In nuclear reactions, what are the conservation rules?

Answer 27

Energy, momentum, charge and nucleon number must be conserved.

Question 28

When nuclei size increases, why does the line of stability tend away from the N = Z line?

Answer 28

The electrostatic repulsive force between the protons has a long range and it increases on each proton when the proton number increases. The strong force acts between adjacent nucleons and has a short range. So, as Z increases, more neutrons are required to increase the separation of protons, to reduce electrostatic repulsion.

Question 29

What are the 2 methods of determining nuclear size?

Answer 29

Closest approach of a scattered particle Electron diffraction

Question 30

How do you determine nuclear radius using the closest approach method?

Answer 30

When alpha particle approach the nucleus, their kinetic energy is converted to electric potential energy. They momentarily stop when Ek = Ep So, Ek = (Qq) /(4*pi*epsilon*r) This method provides an upper limit to the radius of a nucleus.

Question 31

How is electron diffraction used to estimate nuclear radius?

Answer 31

If e- are travelling fast enough, their wavelength can be similar to the diameter of a nucleus. So, the arrangement of atoms behaves like a diffraction grating. Radius = (0.61*wavelength) / (sin(theta)) When theta is the angle of the first minimum Wavelength = hc/E

Question 32

What are the advantages of using electron diffraction to determine nuclear size over the distance of closest approach?

Answer 32

Electrons can get closer to the nucleus ad there’s no electrostatic repulsion. Electrons are easier to accelerate as they have a higher charge density. Electrons are easier to produce. There’s no strong nuclear interaction. Using alpha particles only provides an upper limit to the radius.

Question 33

Why is nuclear density constant?

Answer 33

The strong force has a short range and is only present between adjacent nucleons. So, the nucleons are equal distances from each other. So nuclear density is independent of nucleon number.

Question 34

Why is carbon dating only valid when a sample is between 200 - 60,000 years old.

Answer 34

Before 200 years have passed, not enough carbon has decayed for activity to fall by a significant amount. After 60,000 years, too many half-lives have passed so activity is too close to background.

Question 35

What is mass deficit?

Answer 35

Total mass of nucleons - mass of a nucleus

Question 36

Why is there a mass deficit?

Answer 36

The nucleons have less energy when they’re in the nucleus.

Question 37

What is binding energy?

Answer 37

The energy required to separate all of the nucleons in a nucleus.

Question 38

How do you calculate binding energy?

Answer 38

Binding energy = mass deficit * c^2

Question 39

How is fission induced?

Answer 39

When thermal neutrons are absorbed Creating a chain reaction if you have critical mass

Question 40

What happens to the mass of a nuclei into fission?

Answer 40

The total mass of the particles formed is less than the mass of the original nucleus. The remaining energy is released in the form of a photon or in the Ek or the products.

Question 41

What is the function of the moderator?

Answer 41

It slows neutrons to thermal speeds as fission neutrons are too fast and can’t be absorbed by the nuclei. When the neutrons collide with the moderator, they transfer Ek to the moderator. Momentum is conserved and the collision is elastic.

Question 42

What materials are typically used for the moderator?

Answer 42

Water Graphite

Question 43

What is the purpose of the control rods?

Answer 43

They control the chain reaction by absorbing neutrons so the rate of fission is controlled. In emergency shutdown, the control rods are automatically released to stop the reaction.

Question 44

What is the purpose of the coolant in a nuclear reactor?

Answer 44

It’s sent around the reactor to remove heat. It is often the same water in the moderator. The heat is used to make steam and power electricity-generating turbines.

Question 45

What is the critical mass?

Answer 45

The smallest mass where there can be a self-sustaining chain reaction.

Question 46

Why is critical mass used in a nuclear reactor?

Answer 46

So that there is a steady chain reaction as it has the perfect surface area to volume ratio so the perfect amount of neutrons escape.

Question 47

What is reactor shielding?

Answer 47

Thick concrete shielding to absorb gamma from the reactor.

Question 48

What is the source of HLW and how is it dealt with?

Answer 48

Used fuel rods Stored underwater until it decays into ILW

Question 49

What is the source of ILW and how is it dealt with?

Answer 49

Decayed HLW and materials contaminated by contact with reactor fuel. It is chopped up and mixed with concrete or glass and is stored deep underground.

Question 50

What is the source of LLW and how is it dealt with?

Answer 50

Contaminated equipment, materials and protective clothing. It is sealed in drums, surrounded by concrete and placed in landfill.

Question 51

What is the source of HLW how is it dealt with and why? [6 marks]

Answer 51

The (highly radioactive/ most dangerous) waste are the fission fragments from the fission of uranium-235 or from (spent) fuel rods The waste is initially placed in cooling ponds Plutonium/uranium is separated to be recycled High level waste is vitrified/made solid into (pyrex) glass Then placed in (stainless) steel/lead/concrete cylinders To be stored deep underground The waste is (initially) is very hot/generates heat so has to be placed in water/cooling ponds (to remove the heat) The waste is initially highly radioactive and needs to be screened in water to absorb radiation The waste is initially highly radioactive and need to be remotely handled to avoid human contact The waste will be radioactive for thousands of years to long term storage in geologically stable areas must be found.