Half-life -page 53

  • Created by: emj790
  • Created on: 01-03-17 20:29
What do radioactive sources contain?
Radioactive isotopes
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What do they give out?
Ionising radiation
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From where?
The nuclei of the atom
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Why can't you guess when the nucleus will decay?
Because it's a random process
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What can you predict?
How many will have decayed in a given time based on the half-life of the source
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What is activity?
The rate at which a source decays
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What is activity measured in?
Becquerels, Bq
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What is one becquerel?
1 decay per second
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How can activity be measured?
Using a Geiger-Muller tube
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How does it work?
It clicks each time it detects radiation
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What happens if you connect it to a counter?
It displays the number of clicks per second (the count-rate)
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How can you also detect radiation?
Using photographic film
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How is it used to detect radiation?
The more radiation the film's exposed to, the darker it becomes (just like when you expose it to light)
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What happens each time a radioactive nucleus decays?
One more radioactive nucleus disappears
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What does this mean about the activity?
As the unstable nuclei steadily disappear, the activity as a whole will decrease
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What is the problem with trying to measure this?
The activity never reaches zero
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So what do we do?
We use the idea of half-life to measure how quickly the activity drops off
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What is half-life?
It is the average time taken for the number of radioactive nuclei in an isotope to halve
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What is a short half-life?
When the activity falls quickly
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Why does this happen?
The nuclei are very unstable and rapidly decay
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Why are they dangerous?
They emit a high amount of radiation at the start but then it quickly becomes safe
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What is a long half-life?
When the activity falls more slowly
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Why does this happen?
Most of the nuclei don't decay for a long time
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So what does the source do?
It justs sits there, releasing small amounts of radiation for a long time
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Why can this be dangerous?
The nearby areas are exposed to radiation for (millions of) years
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The activity of a radioactive sample is measured as 640Bq. Two hours later it has fallen to 40Bq. Find its half-life
Find out how many half-lives it took to get to 40 by dividing it by 2 multiple times. Two hours is four half-lives we have found out, so the half-life is 2 hours / 4 = 30mins
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What would be on the axis of a graph for half-life?
x= time y= activity
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What do you need to take into account when plotting a graph for activity against time?
You need to take background radiation into account
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Name three types of ionising radiation
Alpha, Beta and Gamma rays
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A radioactive source has a half-life of 60hours and an activity of 480Bq. Find its activity after 240 hours (2 marks)
The number of half-lives in 240 hours is 240 / 60 = 4 half-lives. Divide 480 by 2, 4 times (because that is the number of half lives to get to 240) so at 240hours the activity is 30Bq
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Other cards in this set

Card 2


What do they give out?


Ionising radiation

Card 3


From where?


Preview of the front of card 3

Card 4


Why can't you guess when the nucleus will decay?


Preview of the front of card 4

Card 5


What can you predict?


Preview of the front of card 5
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