Half-life

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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.

Answer

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

(http://www.bbc.co.uk/staticarchive/993deedca159f4591ed7a6926288a59d88247e1c.gif)

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