Calculating Half-Life

Half Life and Half Life Calculations

• Half Life- time it takes for half

of the radioactive sample to
decay.
– Ranges from a fraction of a

second to billions of years

Multiple Choice

Using the graph, determine the half life of the isotope.

• 50

• 35

• 4

• 40

• 138 days

​Polonium-210

Half life examples

​Strontium-90

• 28.5 years

Cobalt-60

• 5.27 years

Carbon-14

• 5730 years

Uranium-238

• 700 million years

1. Calculate the number of 1/2 lives past.

2. Make a chart

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​Calculating remaining isotope

1. what is the 1/2?

2. How much will remain after 50 years?

3. If you start with 20 g how much will remain after 3 1/2 lives?

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​Decay of a 10 g sample of Strontium-90

Multiple Choice

If you have 48 kg of Polonium-210 with a half life of 138 days, how much will be left after 276 days?

• 24 kg

• 48 kg

• 36 kg

• 12 kg

Multiple Choice

If you have 30kg of Strontium-90 with a half life of 28.5 years, how much would you have after 85.5 years?

• 15 kg

• 3.75 kg

• 7.5 kg

• 30 kg

Example 3

Cobalt-60, used in radiation therapy, has a half-life
of 5.26 y. A sample of cobalt-60 containing 5.00 x
1012 radioactive atoms sits in a lead case in the
medical stockroom for 10.09 years. How many
cobalt-60 atoms remain after this amount of time?

Multiple Choice

What is a half life?

• A slightly boring life

• The amount of time it takes for 1/2 of a radioactive isotope to decay

• A period of time when one is not productive

• A state of being in between two different stages