Nuclear Physics (Unit 5)

The Experiement:

A beam of alpha particles were directed at a thin piece of gold foil.

The Results:

• Most aplha particles travelled straight through the foil with very little deflection.
• Some of the alpha particles were deflected by an angle < 90
• Very few alpha particles were defelected by an angle > 90

Conclusions:

• The nucleus is small - as very few of the alpha particles are deflected
• The nucleus is positively charged - as the aplha particles were repelled by the nucleus

We can estimate the closest that a scattered particle could get to a nucleus in the gold foil as we know its kinetic energy initially and we know that the energy required to overcome the electrostatic repulsion and approach the nucleus.

Where Q1 is 2e - the charge on an alpha particle and Q2 is Ze - the charge on the nucleus when Z = the proton number

Electron Diffraction is the most accurate way to estimate the size and scattering of nuclei in a material, because electrons are leptons unlike alpha particles and thus do not interect via the strong nuclear force like alpha particles do.

It works because electrons show wave particle duality. A beam of high speed electrons (taking relativistic effects into account) has a wave length of approximately:

If this beam is directed at a material situated in front of a screen, then a pattern appears on the screen.

As with light diffraction patterns the first minimum occurs where:

Using these measurements the size and spacing fo the materials nuclei can be concluded.

the radius of the nucleus is proportional to the cube root of the number of nucleons, so by adding the constant      we can form the equation:

We can prove from this that the density of all nuclei is approxiamtely the same as:

and

Nucler Matter is increadibly dense and all has about the same density.

Alpha Radiation:

• Has the symbol      
• It is a Helium Nucleus, two protons and two neutrons and thus has a charge of +2 and a mass of 4u.
• It moves quite slowly and is highly ionising but has a very short range (because it ionises everything in its path and soon loses all its energy)
• It's ionising power is due to its mass and charge, it can easily pull electrons out of orbit around their atoms
• However it loses energy with every ionisation and can only ionise around 10,000 particles.

Beta Radiation:

• Has the symbol
• Comes in two forms, Beta-plus and Beta-minus

Beta-plus:

• Is a positron (+1 charge) and negligable mass
• Anihilates as soon as it comes into contact with an electron so virtually zero range and can only ionise one atom as it destroys an electron.

Beta-minus:

• Is an electron (-1 charge) and negliable mass
• Due to its high speed(higher than alpha) it can knock electrons out of orbit but due to its much smaller mass its less ionising, can ionise about 100 atoms
• It can penetrate paper or a hand but a few cms of aluminium will stop it.

Gamma:

• Has the symbol
• Is the emission of a high frequency electromagnetic wave
• Even more weakly ionising than Beta radiation
• Has a much greater range, only stopped by thick lead or concrete.

Intensity:

Gamma rays spread out as you get further from the source, it follows an inverse square law,

We are surrounded by background radiation caused by:

The air- Radon gas is released into the air by rocks and is radioactive.

The ground and buildings - All rocks contain radioactive isotopes and so emit radiation

Cosmic Rays - Cosmic rays are high energy protons from space which produce nuclear radiation when they collide with particles in the upper atmosphere.

Living things - Carbon 14 in all plants and animals decays radioactively very slowly.

Man made radiation - Radiation that escaoes from medical or industrail sources (this is a very small contributer to background radiation)