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5.3.3 Radioactivity

5.3.3 Radioactivity. (a) describe the spontaneous and random nature of radioactive decay of unstable nuclei. Radioactive decay. Stable. Unstable: Will emit radiation randomly once. Radioactive decay. Nuclear decay is spontaneous because:

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5.3.3 Radioactivity

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  1. 5.3.3 Radioactivity

  2. (a) describe the spontaneous and random nature of radioactive decay of unstable nuclei

  3. Radioactive decay Stable Unstable: Will emit radiation randomly once

  4. Radioactive decay Nuclear decay is spontaneous because: • the decay of a particular nucleus is not affected by the presence of other nuclei • the decay of nuclei cannot be affected by chemical reactions or external factors such as temperature and pressure and is random because: • it is impossible to predict when a particular nucleus in the sample is going to decay • each nucleus in a sample has the same chance of decaying per unit time

  5. (b) describe the nature, penetration and range of α- particles, β-particles and γ-rays

  6. Radiation penetration 2 Protons ALPHA 2 Neutrons High Energy Electron BETA High Frequency Wave GAMMA ALUMINIUM PAPER LEAD

  7. Radiation penetration Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: Alpha decay Alpha particle (helium nuclei) 2 neutrons and 2 protons are emitted from the nucleus. 238234 4 U  Th +  + energy 92 90 2 A decreases by 4, Z decreases by 2 (A-4, Z-2)

  8. Radiation penetration Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: • Betadecay High energy electron A neutron in the nucleus decays into a proton and a high energy electron which is emitted with an anti-neutrino. 1414 0 C  N +  + ν 6 7 -1 A stays the same, Z increases by 1 (A=, Z+1)

  9. Radiation penetration Type of decay: What is emitted? Description of decay: Effect on A and Z: • Gamma decay High energy electromagnetic radiation Nucleus loses energy and becomes more stable. Gamma radiation is the energy it loses. A stays the same, Z stays the same (A=, Z=)

  10. (c) define and use the quantities activity and decay constant

  11. Activity The activity A of a radioactive sample is the rate at which nuclei decay or disintegrate The activity of a source is defined as follows: Activity is measured in decays per second (or h-1 or day-1, etc) An activity of one decay per second is one becquerel (1 Bq) 1 Bq = 1 s-1

  12. Decay constant The decay constant λ is the probability that an individual nucleus will decay per unit time interval The decay constant of a source is defined as follows: For example, in a sample of one million nuclei, if 200 000 in one hour, then the decay constant is Decay constant λ = 0.20 h-1

  13. (d) select and apply the equation for activity A = λN

  14. Activity equation Activity of a sample depends on the decay constant λ The greater the decay constant, the greater the activity Activity also depends on the number of undecayed nuclei in the sample N A = λN

  15. Questions A = λN = 0.30 s-1 x 500 000 = 150 000 s-1 or 150 000 Bq Count rate = 20 m-1 therefore 0.33 s-1 Activity = 3.3 s-1 Decay Constant = 3.3 s-1 / 1.5 x 109 = 2.0 x 10-9 s-1 A sample of carbon-15 initially contains 500 000 undecayed nuclei. The decay constant for this isotope of carbon is 0.30 s-1. Determine the initial activity of the sample A small sample of radium gives a received count rate of 20 counts per minute in a detector. It is known that the counter detects only 10% of the decays from the sample. The sample contains 1.5 x 109undecayed nuclei. Determine the decay constant of this form of radium

  16. (e) select and apply the equations A = Aoe-λt and N = Noe-λtwhere A is the activity and N is the number of undecayed nuclei

  17. Decay equations 100 Undecayed Atoms [N] or Activity [A] (s-1) 50 0 0 14 28 Time [t] (s)

  18. Decay equations The decay in the graph can be expressed as an equation If N0 is the number of undecayed nuclei, then N that remain undecayed after time t is given by: N = Noe-λt

  19. Decay equations The activity A of a sample is proportional to the number of undecayed nuclei N. Hence the activity of the sample decreases exponentially: A = Aoe-λt

  20. Questions • Now attempt SAQ 13, 14 and 15 • Use Worked Example 5 & 6 for help

  21. (e) define and apply the term half-life

  22. Half-life 100 Undecayed Atoms 50 0 0 28 14 Time (s)

  23. Half-life The half-life t½ of a radioisotope is the mean time taken for half of the active nuclei in a sample to decay

  24. (g) select and use the equation λt1/2 = 0.693

  25. Decay constant and half-life The decay constant and half-life are connected by the formula: λt1/2 = 0.693

  26. Assessment • Chapter 14 SAQ’s 1 to 21 • End of Chapter 14 questions 1 - 5 • Radioactivity worksheet questions

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