Halflife Engager n n To calculate halflife of

Half-life

Engager n

n To calculate half-life of different radioactive isotopes. n Calculate count rate after a given number of half lives. n Explain what the term Half-life means Progress To know what half-life is

Radioactive decay is 1. Random an event that is described by a probability i. e. we can predict that so many decays will happen in the next minute, but we cannot say exactly when they will happen 2. Spontaneous without premeditation or external stimulus i. e. we cannot make an unstable nucleus decay (e. g by increasing the temperature or pressure).

The half-life is the time taken for the number of radioactive nuclei in an isotope to half This can take hours, weeks, years, millions of years etc.

The time required for half of a sample of radioactive atoms to decay. Some isotopes have half-lives of fractions of a second but others have half lives of millions of years.

How to calculate Half-life

Calculating the half-life of Skittles Method • You have 50 skittles in your cup shake the cup and empty out • Remove the skittles with the ‘S’ showing, • Write down the number in table provided, • Repeat process until you removed all of your skittles, • Exchange your results with all groups. Draw a graph.

Half-life of skittles Number of shakes 0 1 2 3 4 5 6 7 8 9 10 Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Average skittle count

Graph for the Half-life of Skittles Count rate Number of shakes of the cup

activity is the number of decays per second the higher the activity, the more unstable atoms present

1. Plot a graph of activity against time. 2. Use your graph to find the half-life. Show clearly on your graph how you do this. 3. At what time will the activity be 122 Bq? activity is the number of decays per second the higher the activity, the more unstable atoms present Ext – find the 2 nd, 3 rd and 4 th half lives. Are they the same?

Connector - Worksheet on Half-life

Answers To Worksheet Mark in Red! 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 5 seconds. 20 seconds. A 5 minutes. 50 atoms 8 hours 24 hours 100 16 hours 60 years

Higher tier

L. O. Homework explain how radioactivity is measured ü explain how half-life can be used to estimate the age of objects ü

Rocks don’t come with labels saying how old they are! But scientists can work out their age using radioactivity.

Dating rocks

Task Turn this into a story board to explain how the ratio of uranium to lead can be used to estimate the age of the rock.

A rock formed and it contains no lead but does contain some radioactive uranium nuclei. n Over time, the uranium decays to form lead. n When half has decayed to lead, 1 billion years has passed (the half life). n If you measure the ratio of uranium to lead, you can work out the age of the rock from the number of half lives. n

Carbon dating n Living things contain a radioactive isotope called carbon-14. n It’s produced naturally by cosmic rays from space.

n Animals absorb carbon-14 when they eat or breathe. n Plants absorb carbon-14 when they exchanges gases during photosynthesis and respiration. n When something is alive, the activity of the carbon-14 inside it is the same as that in the air.

When a living thing dies, it no longer takes in any carbon-14. n So the amount present gets smaller and smaller over time. n Eventually there is none left. n 100% Age 0 50% 5730 yr 25% 12. 5% 11460 yr 17, 190 yr

100% Age 0 50% 5730 yr 25% 12. 5% 11460 yr 17, 190 yr As the carbon-14 in the object decays, the activity in the object decreases. Each time the activity halves, the sample has aged one half life.

the amount of Carbon-14 in the air has not changed for thousands of years n living things absorb Carbon-14 by gas exchange but this process stops as soon as they die n as the Carbon-14 in the object decays the activity of the object decreases n the ratio of activity from living matter to the activity of the sample is used to calculate the age n