Chpt. 8: Radioactivity

1. Chpt. 8: Radioactivity

2. Trefoil Symbol – symbol for ionising radiation

3. The Discovery of Radioactivity Henri Becquerel (1896): • Investigated the effect • of sunlight on crystals of • uranium salts • By chance he left sample • resting on a • photographic plate wrapped • in black paper in a dark • drawer and accidentally • discovered it gave off some • sort of radiation

4. Investigated the • radiation emitted - • concluded emission of • radiation was • spontaneous i.e. no heat, • light needed

5. Marie & Pierre Curie (1898): • worked with a Uranium ore • (pitchblende) and purified it. • after thousands of re- • crystallisations they isolated • two new elements from this • ore: • - polonium • - radium • 1903 – Nobel Prize for • physics • 1911 – Nobel prize for • chemistry

6. What is radioactivity? Radioactivity is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation. • Radiation consists of : • Alpha particles α • Beta particles ß • Gamma radiation γ

7. Note: For a nucleus to be stable it must contain a suitable ratio of protons to neutrons. If this ratio is not suitable the nucleus disintegrates throwing out fragments in an effort to achieve a stable ratio. This is radioactivity/radiation

8. Alpha Particles • What are alpha particles??? • 2 protons and 2 neutrons stuck together. • *He24 same as Helium nucleus • may be represented as He24

9. Properties of alpha particles: • Low penetrating power – stopped by few cm of • air or sheet of paper • Positively charged • Strongly Ionising – it knocks electrons of • matter it passes through producing positive • ions • Slowest moving radiation • Are deflected by magnetic and electric fields

10. Alpha Source Alpha Source Detector Detector Example A example of a radioactive element that emits alpha particles is americium-241 Use of americiun-241 – smoke detectors No Smoke present Smoke present Alpha particles do not reach the detector – alarm goes off Alpha particles reaching the detector - no alarm

11. Beta Particles • What are beta particles??? • Electrons • Formed when a neutron in an unstable nucleus • is changed into a proton and an electron. • The electron is ejected from the nucleus

12. Properties of Beta Particles: • More penetrating than alpha particles – can • penetrate up to about 5 mm of aluminium • Negatively charged • Less ionising than alpha particles • Deflected by magnetic and electric fields

13. Example An example of an element that emits beta particles is carbon-14 - carbon-14 is used to date archaeological artefacts (carbon dating)

14. Gamma Radiation • What is gamma radiation??? • High energy electromagnetic radiation – • radiant energy not a particle (similar to X-Rays) • An unstable nucleus emits gamma radiation in • order to lose surplus energy

15. Properties of Gamma Rays: • Not deflected in magnetic/electric fields • Has no charge or mass – can be emitted from a • nucleus that has emitted an α orβ particle • Poor ionising ability • It is fast moving similar to the speed of • light • High penetrating ability – only stopped by thick • slab of lead • Very harmful to human tissue – absorbed = • cancer

16. Example An example of an element that gives off gamma rays is cobalt – 60 - cobalt-60 used in cancer treatment (cancerous cells more susceptible to damage by ionising radiation than normal cells) - cobalt-60 used in food irradiation

17. Absorption of Radiation

18. Demonstration: To detect the presence of αparticles,βparticles and gamma radiation, and to investigate the penetrating power of each of these three types of radiation Note: To detect radiation we can use a Geiger - Müller tube connected to a ratemeter.

19. Distinction between Chemical Reactions and Nuclear Reactions • Chemical reactions involve transfer of electrons: • (a) Elements combine to form compounds • (b) Compounds break up to form elements • (c) Elements in compounds rearrange to form new compounds • Nuclear reactions involve the emission of • radioactive radiation from the nuclei of • unstable atoms: • - Nuclear reactions cause changes in nucleus • - Nuclear reactions cause elements to change into other elements

20. Nuclear Reactions Nuclear Equations – Alpha Emissions: When an alpha particle is emitted from a radioactive element the new element remaining is found by: - subtracting 4 from the given atomic mass no. - subtracting 2 from the given atomic no. (i.e. Move back two places on the periodic table to identify element)

21. Alpha Particles Emission: When an atom looses an alpha particle it changes into an atom of the element two places before it in the Periodic Table. The mass number of the parent element decreases by four

22. Example: If an atom of radium-226 looses an alpha particle what is the new element formed??? 22688Ra ? + 42He + energy Complete calculations: A: 226 – 4 = 222 - New atomic mass no. Z: 88 – 2 = 86 - New atomic no. 2) Use periodic table to identify new element: 22688Ra 22286Rn + 42He + energy

23. Complete the following equations: a) 21484Po ? + 42He + energy b) 238 92U ? + + energy 42He

24. Nuclear Equations – Beta Emissions: When a beta particle is emitted one of the neutrons is changed into an electron and a proton. (One neutron less and one more proton). The proton remains in the nucleus. There is: - no change in the atomic mass number (A) - the atomic number (Z) increases by 1

25. Beta Particles Emission: When an atom loses a beta particle, it changes into an atom of the element one place after it in the Periodic Table. The mass number of the atom stays the same.

26. Example: If an atom of carbon-14 looses a beta particle what is the new element formed??? 146C ? + 0-1e + energy Complete calculations A: no change – atomic mass number Z: 6 + 1 = 7 - new atomic number 2) Use periodic table to identify element 146C 147N + 0-1e + energy

27. Complete the following equations: 21482Pb ? + ? + energy 19879Au ? + ? + energy

28. Gamma Radiation: The loss of gamma radiation does not give rise to any new atoms – energy (hf) is simply lost from the nucleus Transmutation: The changing of one element into another is called transmutation

29. Half-Life • The half life of an element is the time taken for half of the nuclei in any given sample to decay. • Example: • Carbon 14 half life = 5,700 years • Cobalt 60 half life = 5 years • Polonium 234 half life = 0.15 milliseconds Significance of long half-life as an argument against nuclear power

30. Radioisotopes • Unstable isotopes that are radioactive • Most elements of atomic number above 83 do not have stable isotopes • Many useful isotopes are made artificially. In nuclear reactors e.g. Cobalt-60

31. Uses of Radioisotopes (Must know 3) Medical Uses: Cobalt 60 (gamma radiation) can be used to treat cancerous cells as cancerous cells are more susceptible to damage by ionising radiation than normal cells. Gamma radiation also used to sterilise medical instruments

32. 2. Archaeological: Carbon-14 (carbon dating) /beta emitter • Used to indicate age of carbon containing • objects(wood, fabric….) • Determination • Living organism contains C-12 and C-14 in • same proportions as in the air • On dying, the ratio of C-12/C-14 changes • with time due to radioactive decay of C-14 • The extent of the changes in the ratio allows • the age to be calculated

33. 3. Americium 241 (discussed previously): • Alpha particle emitter • These ionise the air molecules and conduct • electric current between two terminals • smoke cling to the ionised molecules and • slows them down • The current decreases and a transistor • switch activates the alarm

34. Background Radiation Radiation can be detected all around us. We call this background radiation. There are two types of sources: - natural (first 4) - artificial *Note: Radon is a radioactive gas that escapes from rocks and increases the risk of cancer