Physics - EDEXCEL IGCSE - Radioactivity and Particles

Section 7 - Radioactivity and Particles:

Atomic number - the number of protons in the nucleus

Mass number - the number of protons and neutrons in the nucleus

Alpha, beta and gamma rays are all radiation that damages cells (ionises) and is emitted from unstable (full) nuclei.

Alpha - this type of radiation contains 2 neutrons and 2 protons which make it essentially the helium nucleus. This is the weakness type of radiation and cannot even penetrate paper.

Beta - this type of radiation contains only electrons created when a neutron turns into a proton and electron. This is stronger than the alpha radiation but cannot penetrate anything above aluminium.

Gamma - this type of radiation is part of the electromagnetic spectrum. It is very strong but cannot penetrate lead.

Alpha = -2 atomic number, -4 mass number

Beta = +1 atomic number, -1 neutron number, -0 mass number

Gamma = N/A

Uranium (Atomic number of 92, mass number of 235) —> Thorium (Atomic number of 90, mass number of 231) + alpha (atomic number of 2, mass number of 4)

Carbon (Atomic number of 6, mass number of 14) —> Nitrogen (atomic number of 7, mass number of 14) + beta (atomic number of 0, mass number of 0)

Radiation:

Ionising radiation imprints on camera film.

The Geiger-Muller detector beeps in the presence of radiation.

Background Radiation Examples:

• Radon gas in the ground.
• Cosmic rays
• Food and drink
• Buildings
• Boron in the soil
• Medical instruments.

The radiation from a radioactive source will decrease over time. The half-life is the time taken for the radiation to decrease by a half. It is different for different sources of radiation.

Uses of radiation:

1. Tracers - a radioactive source is put into a system such as a piping network. It will then build up at the blockage and can then be detected to tell people where the fault is.
2. Medical tracers - these are similar to the normal tracer except that these are put int a body. The radioactive source builds up, for example, at a blocked blood vessel and doctors will then know where to operate on.
3. Radiotherapy - radiation is used to target cancer cells.
4. Carbon dating - the amount of radiation is measured and carbon’s half-life is used to date the object.

Dangers of radiation:

Radiation can damage the structure of the cells DNA, when these damaged cells replicate, cancerous tissue may form. Therefore, some radiation is said to be carcinogenic. It damaged cells and tissues by changing their atomic structure, thereby causing them to stop functioning properly.

Radioactive waste can poison waters, destroy ecosystems and cause widespread harm to nature.

Experiment involving the alpha particles:

Rutherford designed an experiment which his two assistants Geiger and Marsden carried out.

Geiger and Marsden beamed alpha particles at a gold foil. They expected the particles to go straight through the gold foil. However, they found, rather surprisingly, that a few went through but emerged at a bent angle, some even got deflected. Using this surprising result, Rutherford formed out present day view of atoms. This new model explained why the positive alpha particles were sometimes repelled and why the faster they went the faster they were repelled.

Nuclear fission:

The nucleus of a uranium-235 atom can be split through fission whereby a neutron is fired at the uranium. This splits the uranium nuclei into 2, leaving 2 ‘daughter nuclei’ and some neutrons. These neutrons are then used to cause a chain reaction through hitting other uranium nuclei.

In nuclear fission, control rods are used to absorb neutrons to prevent there being too many of them. If there were too many neutrons then the reaction would get out of hand.

A moderator is also used. This is usually water and is used to slow the neutrons down enough to be able to hit the nuclei at the right speed.