# Physics P2 6.5 Half-life.

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What is the half-life of a radioactive isotope?
The average time it takes for the number of nuclei of the isotope in a sample to halve.
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What is the activity of a radioactive source?
The number of nuclei that decay per second.
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What happens to the number of atoms of a radioactive isotope and the activity?
They both decrease by half every half-life.
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How can we measure the radioactivity of a sample of a radioactive material?
By measuring the count rate from it.
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What happens to the radioactivity of a sample over time and what does this depend on?
It decreases over time, how quickly the count rate falls to nearly zero depends on the isotope, some take a few minutes, others take millions of years.
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What do we use the idea of half-life for?
To ensure how quickly the radioactivity decreases, it is the time taken for the count rate from the original isotope to fall to half its initial value or the time it takes for the number of unstable nuclei in a sample to halve.
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What happens to the count rate of a radioactive sample over time?
It decreases.
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What has happened to the original count rate of a radioactive sample after two half-lives have passed?
It has decreased to one quarter of its original value.
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**LOOK UP GRAPH OF RADIOACTIVE DECAY - A GRAPH OF COUNT RATE AGAINST TIME**
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## Other cards in this set

### Card 2

#### Front

What is the activity of a radioactive source?

#### Back

The number of nuclei that decay per second.

### Card 3

#### Front

What happens to the number of atoms of a radioactive isotope and the activity?

### Card 4

#### Front

How can we measure the radioactivity of a sample of a radioactive material?

### Card 5

#### Front

What happens to the radioactivity of a sample over time and what does this depend on?