P6 Radioactive Materials 15/01/2019 15/01/2019 P6 Radioactive Materials OCR 21st Century W Richards The Weald School

P6.1 Why are some materials radioactive? 15/01/2019 P6.1 Why are some materials radioactive?

The structure of the atom 15/01/2019 ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)

Introduction to Radioactivity 15/01/2019 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.

Background Radiation 13% are man-made Radon gas Food Cosmic rays 15/01/2019 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power

Structure of the atom 15/01/2019 15/01/2019 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”

The Rutherford Scattering Experiment 15/01/2019 15/01/2019 Alpha particles (positive charge, part of helium atom) Thin gold foil Most particles passed through, 1/8000 were deflected by more than 900 Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

What keeps a nucleus together? 15/01/2019 - + The “strong force”

Nuclear Fusion in stars 15/01/2019 15/01/2019 Nuclear fusion happens in stars when hydrogen nuclei are brought close enough together: Proton Neutron Einstein (1879-1955) We can calculate how much energy this reaction releases using my famous E=mc2 equation.

Isotopes 15/01/2019 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.

Types of radiation 15/01/2019 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

Sheet of paper (or 6cm of air will do) Blocking Radiation 15/01/2019 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

Changes in Mass and Proton Number 15/01/2019 Alpha decay: Am 241 95 Np α + 237 4 93 2 Beta decay: Sr 90 38 Y β + 90 39 -1

A radioactive decay graph 15/01/2019 15/01/2019 Activity (Bq) “1 Becquerel” means “1 radioactive count per second” Time

Half life 15/01/2019 15/01/2019 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

A radioactive decay graph 15/01/2019 15/01/2019 Count 1 half life 1 half life 1 half life Time

P6.2 Using Radioactive Materials 15/01/2019 P6.2 Using Radioactive Materials

Ionisation 15/01/2019 Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons: Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer, and causes molecules to break into bits called ions that then take part in other reactions.

Uses of Radioactivity 1 Sterilising medical instruments 15/01/2019 15/01/2019 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.

Uses of Radioactivity 2 - Tracers 15/01/2019 15/01/2019 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?

Uses of Radioactivity 3 - Treating Cancer 15/01/2019 15/01/2019 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?

Radiation dose is measured in units called “sieverts” (Sv). Exposure to Radiation 15/01/2019 People like me work with radiation a lot so we need to wear a “dosimeter” to record our exposure to radiation: Radiation dose is measured in units called “sieverts” (Sv).

Background Radiation by Location 15/01/2019 In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event: Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc

How we are exposed to Radiation 15/01/2019 We can be exposed to radiation by “irradiation” or by “contamination”: Irradiation “Irradiation” is when radiation “hits” us from the outside, like background radiation. Contamination “Contamination” is when we take radioactive sources in, like the case of Alexander Litvinenko Gamma

New nuclei (e.g. barium and krypton) Nuclear fission 15/01/2019 More neutrons Neutron Uranium nucleus Unstable nucleus New nuclei (e.g. barium and krypton)

Chain reactions 15/01/2019 Each fission reaction releases far more energy than burning the same mass of coal would do!

Fission in Nuclear power stations 15/01/2019 These fission reactions occur in the fuel rods and they become very hot. Water (a coolant) cools the rods (which then turns to steam) and the control rods are moved in and out to control the amount of fission reactions taking place.

Disposing of radioactive waste 15/01/2019 The key to dealing with radioactive waste is to IMMOBILISE it. There are a number of ways of doing this depending on how __________ the waste is: High level waste is immobilised by mixing with ____ making ingredients, melting and pouring the glass into steel containers. Intermediate waste is set in cement in _____ drums. The containers are then kept in stores, often _________. Words – glass, steel, underground, radioactive