OCR 21st Century Science Unit P6 a Revision

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Presentation transcript:

OCR 21st Century Science Unit P6 a Revision Radioactive materials

The structure of the atom

Introduction to Radioactivity Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation at random intervals: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactive decay”. This process is NOT affected by temperature or other physical conditions. 30/12/2018

Structure of the atom 30/12/2018 A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it… Ernest Rutherford, British scientist: I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment” 30/12/2018

The Rutherford Scattering Experiment 30/12/2018 …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….

Radioactivity Radioactivity

Isotopes An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? O 8 16 O 8 17 O 8 18 Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating. 30/12/2018

Isotopes

Types of radiation summary 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. Beta particle New nucleus Unstable nucleus 3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Words – frequency, proton, energy, neutrons, helium, beta Unstable nucleus New nucleus Gamma radiation 30/12/2018

Alpha Radiation Alpha radiation

Beta Radiation Beta radiation

Beta radiation summary

Gamma radiation Gamma radiation

Gamma radiation summary

Sheet of paper (or 6cm of air will do) Blocking Radiation Each type of radiation can be blocked by different materials:    Sheet of paper (or 6cm of air will do) Few mm of aluminium Few cm of lead 30/12/2018

Penetration of radiation Blocking radiation summary Penetration of radiation

Ionisation Ionisation

Ionisation summary Ionisation

Half life 30/12/2018 The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… = radioisotope = new atom formed After 2 half lives another half have decayed (12 altogether) After 3 half lives another 2 have decayed (14 altogether) After 1 half life half have decayed (that’s 8) At start there are 16 radioisotopes 30/12/2018

Half life graph …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….

Uses of Radioactivity 1 Sterilising medical instruments 30/12/2018 Uses of Radioactivity 1 Sterilising medical instruments Gamma rays can be used to kill and sterilise germs without the need for heating. The same technique can be used to kill microbes in food so that it lasts longer. 30/12/2018

Uses of Radioactivity 2 - Tracers 30/12/2018 Uses of Radioactivity 2 - Tracers A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: Gamma source The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected. Tracers can also be used in medicine to detect tumours: For medicinal tracers, you would probably use a beta source with a short half life – why? 30/12/2018

Uses of Radioactivity 3 - Treating Cancer 30/12/2018 Uses of Radioactivity 3 - Treating Cancer High energy gamma radiation can be used to kill cancerous cells. However, care must be taken in order to enure that the gamma radiation does not affect normal tissue as well. Radioactive iodine can be used to treat thyroid cancer. Iodine is needed by the thyroid so it naturally collects there. Radioactive iodine will then give out beta radiation and kill cancerous cells. What sort of half life would you want the radioactive iodine to have? 30/12/2018

For fission to occur the uranium-235 or plutonium-239 nucleus must first absorb a neutron The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released. +  neutron Uranium 235 Strontium 90 Xenon 144 fission neutrons uranium 236 A slow moving neutron is absorbed by the nucleus The nucleus splits into two almost equal parts This releases large amounts of energy Two or three new neutrons are released

The neutrons may go on to start a chain reaction Nuclear fission results in a chain reaction because each time a nucleus splits it releases more neutrons, which can go on and cause more fission reactions to occur... and so on. This is why a chain reaction releases a lot of energy so rapidly. If a chain reaction is uncontrolled, heat builds up very quickly. A chain reaction must be controlled to maintain a steady output of heat. + 

Fission summary

Nuclear fusion is the joining of two atomic nuclei to form a larger one. +  tritium deuterium helium neutron fusion

Nuclear fusion is the process by which energy is released in stars.

3 Types of nuclear waste. High level waste (HLW) – from ‘spent’ fuel rods. This doesn't last long. Intermediate level waste (ILW) – the amount of this is growing. Low level waste – protective clothing and medical equipment.

Where does intermediate level waste come from? High level waste decays quickly at first. Its activity falls and it becomes intermediate level waste. It can remain radioactive for thousands of years.