Nuclear physics

Rutherford scattering

A stream of alpha particles is bombarded at a thin foil of gold leaf. Most alpha particles travel straight through the nucleus, with some being deflected a small amount, and some being fully reflected off the foil. This disproved the plum pudding model of the atom.

What three things did the Rutherford scattering experiment prove about the atom?

That the atom is mostly empty space - as most of the particles passed straight through

That the centre of the atom is positively charged - as some of the positively charged alpha particles were deflected by a large angle

That the centre of the atom is very dense by very small - as a very small number of the particles were deflected by >90^o

Range of alpha radiation (in air)

2-10 cm

Ionisation power of alpha radiation

High

Is alpha radiation deflected by fields?

Yes

What is alpha radiation absorbed by?

Paper

Range of beta radiation (in air)

~1m

Ionisation power of beta radiation

Weak

Is beta radiation deflected by fields?

Yes

What is beta radiation absorbed by?

Aluminium foil

Range of gamma radiation (in air)

Infinite (follows the inverse-square law)

Ionisation power of gamma radiation

Extremely weak

Is gamma radiation deflected by fields?

No

What is gamma radiation absorbed by?

Several meters of concrete/Several inches of lead

How to identify the type of radiation

Measure background count using a GM tube. Place different materials (paper, aluminium and lead) in front of the source and GM tube and observe the results.

Medical uses of gamma radiation

Detector

To sterilise surgical equipment

In radiation therapy

How to protect against radiation whist handling sources?

Using long handled tongs to move the source

Storing the source in a lead-lined container

Keeping the source as far away as possible from yourself and others

Never pointing the source towards others

Sources of background radiation

Radon gas

Artificial sources (nuclear tests)

Cosmic rays

Rocks containing naturally occurring radioactive isotopes

What profile does radioactive decay follow?

Exponential

Activity of a sample

The number of nuclei that decay per second

Why might a nucleus become unstable?

If the strong nuclear and electromagnetic forces are out of balance

What are the four reasons for a nucleus being unstable?

It has too many neutrons (Beta-minus emission)

It has too many protons (Beta-plus emission)

It has too many nucleons (Alpha emission)

It has too much energy (Gamma emission)

Why does the number of nucleons per proton increase as the nucleus get larger?

In order to keep the nucleus stable, as more protons increase the strength of the electromagnetic force, so more neutrons are needed to keep the protons further apart

How to calculate nuclear radius using the distance of closest approach method

Fire an alpha particle of a known kinetic energy at the nucleus. At the point where its kinetic energy is zero, the electric potential energy will be the same as the initial kinetic energy. At this point, the equation E_elec = Q₁Q₂/4πϵ₀r. Then by rearranging for r, the radius can be estimated.

How to calculate nuclear radius using electron diffraction

Electrons are accelerated so that their De Broglie wavelength is around 10^-15 m. They are then directed at a film of the material of which the nuclear radius is desired. This will form a diffraction pattern of a series of concentric circles. Using a graph of diffraction angle and intensity, the first minimum theta can be used in the equation sin(θ) = 0.61λ/r

How to calculate nuclear radius using nuclear trends

Using the equation R = R₀A^1/3 and substituting values into the equation ρ = m/v and v = 4/3πr³ the nuclear radii can be equated. (A = nucleon number)

Mass defect

The difference in energy/energy lost when comparing the mass of the nucleons separately and the mass of the nucleus as a whole

What happens to the mass defect?

The energy from this mass is released when the nucleons fuse

Binding energy

The energy required to separate a nucleon into its constituent parts

Nuclear fission

The splitting of a large nucleus into two daughter nuclei

Nuclear fusion

Where two smaller nuclei join together to form one larger nucleus

Why do fusion and fission release energy?

The products have a much higher binding energy per nucleon

For binding energy per nucleon: A < Fe⁵⁶ < B

A = Fusion

B = Fission

How is fission induced?

Firing a (thermal) neutron into the fissile nucleus

Critical mass

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

Super-critical mass

Any amount of fissile material over criticality, causing the chain reaction to speed up

Moderator (Nuclear reactor)

A material such as water or graphite, which slows down the neutrons in fission reactions to thermal speeds

Control rods

Absorb neutrons in the reactor in order to control chain reactions

Coolant (Nuclear reactors)

Absorbs the heat released during fission, which creates steam to turn a generator

Enriched uranium

Uranium containing >~ 5% U-235

Nuclear meltdown

When a sample of material undergoing fission overheats and starts to melt the control rods, casing or moderator