Nuclear & Particle Physics

Initially, Thomson proposed the ‘plumb pudding’ model for atoms, saying the atom was made up of a blob of positive charge with negatively charged electrons sprinkled in and about this.

Rutherford’s Scattering experiment disproved this, by firing alpha particles at a gold foil and seeing where they emerged on a fluorescent screen. Most alpha particles passed straight through with very little deflection; however, some scattered a lot and others were reflected right back. 

This led to the conclusion that an atom was made up of largely empty space, with a very small, very dense, positively charged nucleus in the centre. This Nuclear Model is now widely adopted:

• The nucleus is surrounded by orbiting electrons in shells

• The nucleus is about one 10,000th of the size of the atom, but makes up most the mass

• The protons and neutrons are around 2000 times more massive than the electron

• Nuclear density is always ~1017 kg m-³; atomic density is far less

Isotopes are atoms with the same proton number but a different nucleon number – the different number of neutrons does not affect the chemical properties, however can make the atom unstable.

The radius of the nucleus is directly proportional to the cube root of the nucleon number:

R = r(0) x ³√A nuclear radius = r(0) x ³√nucleon number

r(0) can be taken as 1.4 E-15

The Four Forces

There are four forces that govern the laws of physics - the strong and weak nuclear, the electromagnetic, and gravitational.

The nucleons are held together by the strong nuclear force, but as the nucleus gets bigger and the electrostatic repulsion outweighs it on the outside, the nucleus becomes unstable.

Strong Nuclear Force

• Very short range: attractive to ~3 fm; repulsive to ~0.5 fm

• Works equally between all nucleons

• At very small separations, it must be repulsive, else the nucleus would collapse

Weak Nuclear Force

• The only thing that can change one quark into another

• Also very small range

• Acts between all particles

Electromagnetic Force

• Infinite range

• Acts between all charged particles

• Can be attractive or repulsive

Gravitational Force

• Infinite range

• Acts on all objects with mass

• Always attractive

Fundamental Particles

All particles come in particle/anti-particle pairs, where each part has the same mass, but opposite charge. 

Classification of Matter

All matter is categorised into hadrons and leptons:

Quarks

The hadrons are largely made up of up, down and strange quarks, and their respective anti-quarks. All of these have a charge, and the combinations of these quarks is what gives protons their +1 and neutrons their 0 charge.

Quark notation consists of just writing the letters next to each other:

• Protons are made up of uud

• Neutrons are made up if udd

Decay

We can express beta decay in terms of quarks:

Radioactivity

Radioactive decay is both spontaneous (cannot be induced) and random (cannot be predicted), and typically takes one of four forms: