# Half Life

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

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• Half Life Definition
• Examples of Half Life
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• Created by: Ellie
• Created on: 05-02-14 17:49
• Half Life
• 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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