# Half life

Mind Map by , created over 6 years ago

## Physics Mind Map on Half life, created by jessica-gollop on 04/13/2013.

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 Created by jessica-gollop over 6 years ago
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Half life
1 The unit for measuring radioactivity is the becquerel (Bq). 1 Bq means one nucleus is decaying per second.
2 The radioactivity of a sample always decreases over time (half life).
2.1 HALF LIFE IS THE AVERAGE TIME IT TAKES FOR THE NUMBER OF NUCLEI IN A RADIOACTIVE ISOTOPE TO HALF.
2.1.1 In other words its the time it takes for the count rate for a sample containing the isotope to fall by half its initial level.
2.2 When radioactive decay occurs an alpha, beta or gamma particle is released. This means that one more radioactive nucleus has disappeared.
2.3 As the unstable nuclei all steadily disappear, the activity (the number of nuclei that decay per second) will decrease.
2.3.1 So the older a sample becomes the less radiation it will emit.
2.4 How quickly the activity drops off varies a lot.
2.4.1 For some substances it takes just a few microseconds before nearly all the unstable nuclei have decayed, for others it can take millions of years.
2.5 A short half life means the activity falls quickly, because lots of nuclei decays quickly.
2.6 A long half life means the activity falls slower because most of the nuclei don't decay for along time.
3 Do half life question step by step
3.1 A typical question: The activity of a radioactive isotope is 640 cmp (counts per minute). Two hours later is has fallen to 80 cpm. Find the half life.
3.1.1