Radioactivity

- a radioactive substance contains unstable nuclei that become stable by emitting radiation

- there are 3 main types of radiation from radioactive substances 

    alpha

    beta

    gamma

-radioactive decay is a random event - you cant predict or influence when it will happen

- radioactive sources emit alpha, beta and gamma radiation

- Rutherford used alpha particles to probe inside atoms, he found that some of the alpha particles were scattered through large angles 

- the 'plum pudding' model could not explain why some alpha particles were scattered through large angles 

- an atom has a small positively charged central nnucleus where most of the atoms mass is located

- the nuclear model of the atom correctly explained why some of the alpha particles scattered through large angles

- isotopes of an element are atoms with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers

- alpha decay

    nucleus loses 2 protons and 2 neutrons

    2 protons and 2 neutrons are emitted as as alpha particle

- beta decay

    a neutron in the nucleus changes into a proton

    an electron is created in the nucleus and instantly emitted

- alpha radiation is stopped by paper and has a range of a few centimetres in air

    it consists of particles, each composed of 2 protons and 2 neutron, it has the greatest ionising power

- beta radiation is stopped by a thin sheet of metal and has a range of about 1 metre in air, 

    it consists of fast-moving electrons emitted from the nucleus, it is less ionising than alpha radiation and more ionising than gamma radiation

- gamma radiation is stopped by thick lead and has an unlimited range in air

    it consists of electromagnetic radiation

- alpah, beta and gamma radiation ionise substances they pass through, ionisation in a living cell can damage or kill the cell

- the half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve

- the count rate of a Geiger counter caused by a radioactive source decreases as the activity of the source decreases 

- the number of atoms of a radioactive isotope and the count rate both decrease by half every half-life

- the count rate after n half-lives = the initial count rate / 2(n)

- radioactive isotopes are used in medicine for medical imaging, treatment of cancer and as tracers to monitor organs

- how useful a radioactove isotope is depends on:

    its half-life

    the type of radiation it gives out

- for medical imaging with a radioactive isotope and for medical tracers, the half-life should be not too short and not too long

- a gamma eam or a radioactive implant can destroy cancer cells in a tumour

- nuclear fission is the splitting of an atoms nucleus into 2 smaller nuclei and the release of 2 or 3 neutrons and energy

- induced fission occurs when a neutron is absorbed by a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits, spontaneous fission occurs without a neutron being absorbed

- a chain reaction occurs in a nuclear recator when each fission event causes further fission events

- in a nuclear reactor, control rods absorb fission neutrons to ensure that, on average, only 1 neutron per fission goes on to produce further fission

- nuclear fusion is the process of forcing the nuclei of 2 atoms close enough together so that they form a single larger nucleus

- nuclear fusion can be brought about by making 2 light nuclei collide at very high speed

- energy is released when 2 light nuclei are fused together, nuclear fusion in the Sun's core releases energy

- a fusion reactor needs to be at a very high temperature before nuclear fusion can take place, the nuclei to be fused are difficult to contain

- radon gas is an alpha-emitting isotope that seeps into houses through the ground in some areas

- there are hundreds of fission reactors safely in use in the world, none of them is of the same type as the Chernobyl reactors that exploded

- nuclear waste contains many different radioactive isotopes that emit nuclear radiation for many years, the radiation is dangerous because it can cause cancer

- nuclear waste is stored in safe and secure conditions for many years after unused uranium and plutonium (to be used in the future) are removed from it