P1b:Radioactivity SJT
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This document discusses the structure of atoms, radioactivity, and uses evidence about radiation and the universe to support the Big Bang theory. It describes: 1) The basic structure of atoms including protons, neutrons, electrons, isotopes, and radioisotopes. 2) Different types of radiation including alpha, beta, and gamma rays and how they are produced and can be blocked. 3) How radioisotopes are used in medical tracers and carbon dating to measure material ages based on half-life decay. 4) Evidence from light spectroscopy showing redshift of spectra from distant galaxies, which suggests everything is moving away from a single point supporting the Big Bang theory of an expanding universe.
P1b:Radioactivity SJT
- 1. Radiation and the Universe b.Radioactivity Mr S Thompson
- 2. The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)
- 3. The structure of the atom Particle Relative Mass Relative Charge Proton 1 +1 Neutron 1 0 Electron 0 -1 MASS NUMBER = number of protons + number of neutrons SYMBOL PROTON NUMBER = number of protons (obviously)
- 4. Isotopes An isotope is an atom with a different number of neutrons: Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. Notice that the mass number is different. How many neutrons does each isotope have? A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.
- 5. Introduction to Radioactivity Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactice decay”.
- 6. Types of radiation 1) Alpha ( ) – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a Helium atom) 2) Beta ( ) – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle. 3) Gamma – after or decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed. Unstable nucleus Unstable nucleus Unstable nucleus New nucleus New nucleus New nucleus Alpha particle Beta particle Gamma radiation
- 7. Ionisation 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.
- 8. Blocking Radiation Each type of radiation can be blocked by different materials: Sheet of paper Few mm of aluminium Few cm of lead
- 9. Deflection by Magnetic Fields Alpha and beta particles have a charge: + + - 2 protons, 2 neutrons, therefore charge = +2 1 electron, therefore charge = -1 Because of this charge, they will be deflected by electric and magnetic fields: + -
- 10. Background Radiation Radon gas Food Cosmic rays Gamma rays Medical Nuclear power 13% are man-made
- 11. Uses of radioactivity 1) Medical uses – gamma rays can be used to destroy cancerous cells or to sterilise medical instruments 2) Tracers – a tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: Gamma source Tracers can also be used to develop better plant fertilisers and in medicine to detect tumours: The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.
- 12. Uses of radioactivity 2 Rollers Beta emitter Beta detector Paper
- 13. Dangers of radioactivity OUTSIDE the body and are more dangerous as radiation is blocked by the skin. INSIDE the body an source causes the most damage because it is the most ionising. Alpha Beta Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia. Gamma
- 14. Half life 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… At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) After 3 half lives another 2 have decayed (14 altogether) After 2 half lives another half have decayed (12 altogether) = radioisotope = new atom formed
- 15. A radioactive decay graph Time Count 1 half life
- 16. Dating materials using half-lives Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. 8 8 Answer: The sample was originally completely uranium… …of the sample was uranium 4 8 2 8 1 8 Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old. 1 half life later… 1 half life later… 1 half life later…
- 17. An exam question… Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years
- 18. Evidence about the origins of the universe…
- 20. If you pass the light through a gas something different is seen… helium Some wavelengths of light are absorbed by the gas – an “absorption spectrum”.
- 21. After helium If the light source is moving away the absorption spectra look a little different… helium Before
- 22. The absorption lines have all been “shifted” towards the longer wavelength end (red end)… After Before A similar effect happens with sound – this is called “The Doppler Effect” Hear Doppler Effect This is called red shift. The faster the light source moves the further its light will be “shifted”
- 23. Light from different stars and from the edge of the universe also shows this “red-shift”. This suggests that everything in the universe is moving away from a single point. This is the BIG BANG theory
- 24. Red shift summary Light from other galaxies has a longer wavelength than expected. This shows that these galaxies are moving away from us very quickly. This effect is seen to a greater extent in galaxies that are further away from us. This indicates that the further away the galaxy is, the faster it is moving. This evidence seems to suggest that everything in the universe is moving away from a single point, and that this process started around 15 billion years ago. This is the big bang Theory.
- 25. Observing the Universe Consider different types of telescope: Ground-based telescopes Space-based telescopes What are the advantages and disadvantages of each?