Physics P7

!904 J.J Thomson discovered particles called electrons that could be removed from atoms.Plum Pudding model.

1909- Rutherford's Lab tried firing a beam of alpha particles at thin gold foil. Alpha particles fired. Few comeback others passed through the = dense tiny center nucleus. Nuclear Model

Niels Bohr - electrons orbiting the nucleus.  

James Chadwick 1932 proved existence of the neutron.

Current Model:

Proton- +1 relative charge relative mass 1

Electron - -1 charge relative mass 1/2000 

Neutron - 0 charge relative mass 1 it can change in isotopes

- Helium Nuclei 2 neutrons + 2 protons. Do not penetrate far (only a few centimeters) absorbed by a sheet of paper. But they are strongly ionizing and most dangerous when in the body. 

- Alpha Decay Decreases the charge of and the mass of the Nucleus. The atomic number will -2 and the mass number will -2. The charge decreases. New element is formed

Beta Decay:

A beta particle is a fast moving particle released by the nucleus. Virtually no mass and Charge of -1.  They are moderately ionising penetrate moderately far into materials. Few meters in the air. Absorbed by a sheet of aluminum. For every beta particle emitted a neutron in the nucleus has turned into a proton. 

Neutron turns into a proton and releases beta particle.   Increases in protons. Increases atomic number.

GammaRays: Waves of Em radiation released by the nucleus. Penetrate far and travel long distances in air. Weakly ionsing. Absorbed by thick lead or meters of concrete.

Radioactivity is random. Half life can be used to make predictions.Activity is measured in Bq.

The radioactivity of a Source Decreases over Time.  Half-life is the time taken for the number of radioactive nuclei in an isotope to have. 

Short half life - activity falls quickly because the nuclei are unstable and rapidly decay.  Start dangerous because of the high amount of radiation emitted but they quickly become safe.

Long Half-life - Activity falls slowly because most of the nuclei don't decay for a long time. The source just sits there emitting small amounts of radiation for a long time. This can be dangerous to nearby areas.

Risk - Enter Living Cells and ionize the atoms and molecules within them. This can lead to tissue damage. Low dosage - Mutate cells lead to cancer.High Dosage - Kills cells completely causing radiation sickness vomiting.

Medical Tracers:

Radioactive isotopes can be injected or swallowed. The progress round the body can be followed by the detector. A computer converts the reading to a display showing where the strongest reading is coming from. Iodine 123- absorbed by thyroid gland but gives out radiation to indicate weather thyroid gland as it should.                                                                These are usually gamma. Short Half-life

 Radiotherapy:

High doses of ionizing radiation.

Gamma rays are directed carefully and at right dosage to kill cancer cells without damaging to many normal cells. Radiation emitting implants inside tumors (Beta)  

 Risk Vs Benefit:

Tracers used to diagnose life-threatening conditions while the risk of cancer is very small.         Whilst prolonged exposure to radiation poses future risks and causes many side effects, many people with cancer choose to have radiotherapy as it may get rid of their cancer.      Perceived risk.

Splitting of a Large Unstable nucleus: Nuclear fission is a type of nuclear radiation that is used to release energy from large and unstable atoms (Uranium or plutonium) by splitting them into smaller atoms.

Spontaneous unforced fission rarely happens. Usually the nucleus has to absorb a neutron before it will be split.                                                                                                             When the atom splits it forms two new lighter elements that are roughly the same size. (kinetic energy).2 or 3 neutrons are also released when an atom splits. If any of these neutrons are moving slow enough to absorbed by another nucleus they can cause more fission to occur. This is a Chain Reaction.

The energy not transferred to kinetic energy is carried away by gamma rays. The energy carried away in gamma rays and in kinetic energy stores of the remaining free neutrons can be used to heat water.

The amount of energy produced by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur. This is done using control rods which are inside the reactor. Absorb neutrons slow down the chain reactions and amount of energy released.

Uncontrolled chain reactions quickly lead to lots of energy produced. Explosion nuclear weapons work.

Joining Small Nuclei

In fusion 2 light nuclei collide at high speed and fuse to create a larger heavier nucleus. E.g hydrogen fuse to form helium.

The heavier nucleus has less mass than 2 original light nuclei combined. Some of the mass is released as radiation and energy.

Fusion realises more energy than fission.

Scientists haven't found a way of using fusion to generate energy for us to use. The temperatures and pressures are hard and expensive to do on Earth.

Background Radiation: Subtract from results ( systematic errors) it comes from 

Radioactivity of naturally occurring unstable isotopes which are all around us - in the air in food in building materials and in rocks under our feet. Cosmic Rays from the sun 

Human activity: fallout from nuclear explosions or medical. Tiny Proportion. 

Exposure to radiation is Irradiation: Doesn't make it radioactive. Lead lined boxes and remote controlled arms reduce Irradiation 

Contamination: Unwanted radioactive atoms get onto or into an object the object is contaminated. E.g touching source without gloves. Get inside your body = dangerous.

Irradiation - Beta nd gamma are more dangerous Inside body: Alpha most dangerous. Gamma mostly pass through