Half Life

AQA Seperate Science Physics : 2B Electricity and the Atom.

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  • Half Life Definition
  • Decrease in Radioactivity
  • Examples of Half Life
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  • Created by: Ellie
  • Created on: 05-02-14 17:49
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  • Half Life
    • Radioactivity decreases over time
      • Each time decay happens an alpha, beta or gamma is given out.
        • This means one more radioactive nucleus has disspaeared.
        • As the unstable Nuclei sll disappear the activity will decrease. So the older the sample the less radiation will be emitted
          • Activity : the number of nuclei that decay per second
      • We cant measure activity because the activity never reaches zero
    • THE AVERAGE TIME IT TAKES FOR THE NUMBER OF NUCLEI IN A RADIOACTIVE ISOTOPE SAMPLE TO HALVE.
      • A SHORT HALF LIFE: means the activity falls quickly because lots of the nuclei decay quickly
      • A LONG HALF LIFE: means the activity falls slowly because most of the nuclei wont decay for a long time
    • EXAMPLE:
      • The activity of a radioisotope is 640cpm. Two hours later it has dropped to 80cpm. Find the half life of the sample
        • Initial Count: 640
          • After one half Life: 320
            • After two half lives: 160
              • After three half lives: 80
                • 2 hours represents 3 half lives. 2 hours = 120 minutes.
                  • 120 / 3 = 40 minutes
                    • The activity of a radioisotope is 640cpm. Two hours later it has dropped to 80cpm. Find the half life of the sample
                      • Initial Count: 640
                        • After one half Life: 320
                          • After two half lives: 160
                            • After three half lives: 80
                              • 2 hours represents 3 half lives. 2 hours = 120 minutes.
                                • 120 / 3 = 40 minutes
      • Reading The Half Life of Graphs
        • The half life is found from the graph by finding the interval time on the bottom axis corresponding to a halving of the activity on the vertical axis

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