Decay

Half Lives

​What is it?

the time taken for half of the nuclei to decay

Some radioisotopes have long half lives, like U-238, and some have short ones, like I-131. Long half lives can be used to date ancient artefacts. Carbon-13 is useful in this instance. Short half-lives are useful in medicine.

Let's count!

​If you have 100g of a radioactive isotope, how much would you have after two half-lives-

a) if the half life is 3 years?

b) if the half life is 3 hours?

Multiple Choice

A radioisotope has a half-life of 10 years. In the year 2000, there was 50g of this sample in a container. How much would there be in the year 2020

• 25g

• 12.5g

• 100g

Multiple Choice

The half life of plutonium is 24300 years. If nuclear bomb released 8kg of this isotope, how many years would pass before the amount is reduced to 1kg?

• 72900

• 50

• 8100

• 5

Exponential Decay

​trendlines in science

Trendlines are used to make clear the trend in the data

You can have linear and non-linear trends

Non-Linear trends can be exponential, logarithmic, quadratic or trigonometric

Linear trends- well, they are just straight line!

Here is a non linear exponential decaying trend

​Here is one that seems to be periodic

Radioactive Decay

Radioactive decay is an exponentially decaying function

It is a rapidly decreasing function

But a very useful function because we can determine the half life of these radioactive isotopes

Which experiment had a longer half life?

​

Multiple Choice

Determine the amount of the radioisotope left 4 years

• approx 60g

• approx 6g

• approx 155g

• 10g

Multiple Choice

Determine the half life or radioisotope 'x'

• approx 5 years

• approx 1 year

• approx 1/2 year

• approximately 50 years

Can we use half life to predict amounts?

Suppose we start with 120 atoms of a radioactive sample, how many will remain after three half lives?

What percentage is that?

The half life of Carbon-14 is 5730 years. You have a sample that has only 25% of the original amount. How many years has passed?