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  • Created by: jp3louis
  • Created on: 20-02-16 16:10

What is half-life?

The radioactivity of a sample decreases over time.

Radioactive substances are unstable

The nuclei of radioactive atoms break up to form different atoms

This is called radioactive decay

Half-life is how long it takes for a radioactive substance to loose half of their radiation due to radioactive decay.

A short half-life means the radioactive substance halves very quickly (maybe a few hours)

A long half-life means the radioactive substance halves slowly (a few months+)

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How to do half-life questions step by step.

An example question:

The activity of a radioisotope is 640 cpm (counts per minute). Two hours later, it has fallen to 80 cpm. Find the half life of this sample.


So we start of with 640 cpm

A half life of this would be 320 as we DIVIDE it by 2 (HALF-life)

We then half it again to equal 160. This is the 2nd half-life.

We then half it again to equal 80. This is the 3rd half-life.

Two hours is the same as 120 minutes (I find it easier to work in minutes)

Because we worked out it took 3 half-lives to get to 80cpm, we divide the two hours (120 minutes) by 3. The answer to this question is 40 minutes!

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Finding the half-life of a sample using a graph

A half life is found from a graph like this:

1: On the counts per minute, find the half way mark. For this graph it would be at 40 as 80 divided by 2 is 40 counts per minutes.

2. Draw a straight line from 40 until you reach the red line. Then draw a line down until you reach the x axis. Whatever number it lands on the x axis is the 1st half-life


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