You are currently browsing the tag archive for the ‘radioactive decay’ tag.

If you are a teacher then please also visit my new site: intermathematics.com for over 2000+ pdf pages of resources for teaching IB maths!

**Modelling Radioactive decay**

We can model radioactive decay of atoms using the following equation:

**N(t) = N _{0} e^{-λt}**

Where:

**N _{0}**: is the initial quantity of the element

**λ**: is the radioactive decay constant

**t**: is time

**N(t)**: is the quantity of the element remaining after time t.

So, for Carbon-14 which has a half life of 5730 years (this means that after 5730 years exactly half of the initial amount of Carbon-14 atoms will have decayed) we can calculate the decay constant **λ. **

After 5730 years, N(5730) will be exactly half of N_{0}, therefore we can write the following:

**N(5730) = 0.5N _{0} = N_{0} e^{-λt}**

therefore:

**0.5 = e ^{-λt}**

and if we take the natural log of both sides and rearrange we get:

**λ = ln(1/2) / -5730**

**λ ≈0.000121**

We can now use this to solve problems involving Carbon-14 (which is used in Carbon-dating techniques to find out how old things are).

eg. You find an old parchment and after measuring the Carbon-14 content you find that it is just 30% of what a new piece of paper would contain. How old is this paper?

We have

**N(t) = N _{0} e^{-0.000121t}**

**N(t)/N _{0}** =

**e**

^{-0.000121t}**0.30** = **e ^{-0.000121t}**

**t = ln(0.30)/(-0.000121)**

**t = 9950 years old.**

**Probability density functions**

We can also do some interesting maths by rearranging:

**N(t) = N _{0} e^{-λt}**

**N(t)/N _{0}** =

**e**

^{-λt}and then plotting **N(t)/N _{0}** against time.

**N(t)/N _{0}** will have a range between 0 and 1 as when t = 0,

**N(0)**=

**N**which gives

_{0}**N(0)**/

**N(0)**= 1.

We can then manipulate this into the form of a probability density function – by finding the constant a which makes the area underneath the curve equal to 1.

solving this gives a = λ. Therefore the following integral:

will give the fraction of atoms which will have decayed between times t1 and t2.

We could use this integral to work out the half life of Carbon-14 as follows:

Which if we solve gives us t = 5728.5 which is what we’d expect (given our earlier rounding of the decay constant).

We can also now work out the expected (mean) time that an atom will exist before it decays. To do this we use the following equation for finding E(x) of a probability density function:

and if we substitute in our equation we get:

Now, we can integrate this by parts:

So the expected (mean) life of an atom is given by 1/λ. In the case of Carbon, with a decay constant λ ≈0.000121 we have an expected life of a Carbon-14 atom as:

E(t) = 1 /0.000121

E(t) = 8264 years.

Now that may sound a little strange – after all the half life is 5730 years, which means that half of all atoms will have decayed after 5730 years. So why is the mean life so much higher? Well it’s because of the long right tail in the graph – we will have some atoms with very large lifespans – and this will therefore skew the mean to the right.

**Essential Resources for IB Teachers**

If you are a **teacher** then please also visit my new site. This has been designed specifically for teachers of mathematics at international schools. The content now includes over **2000 pages of pdf content** for the entire SL and HL Analysis syllabus and also the SL Applications syllabus. Some of the content includes:

**Original pdf worksheets**(with full worked solutions) designed to cover all the syllabus topics. These make great homework sheets or in class worksheets – and are each designed to last between 40 minutes and 1 hour.**Original Paper 3 investigations**(with full worked solutions) to develop investigative techniques and support both the exploration and the Paper 3 examination.- Over 150 pages of
**Coursework Guides**to introduce students to the essentials behind getting an excellent mark on their exploration coursework. - A large number of
**enrichment activities**such as treasure hunts, quizzes, investigations, Desmos explorations, Python coding and more – to engage IB learners in the course.

There is also a lot more. I think this could save teachers 200+ hours of preparation time in delivering an IB maths course – so it should be well worth exploring!

**Essential Resources for both IB teachers and IB students**

1) Exploration Guides and Paper 3 Resources

I’ve put together a **168 page** Super Exploration Guide to talk students and teachers through all aspects of producing an excellent coursework submission. Students always make the same mistakes when doing their coursework – get the inside track from an IB moderator! I have also made **Paper 3 packs** for HL Analysis and also Applications students to help prepare for their Paper 3 exams. The Exploration Guides can be downloaded here and the Paper 3 Questions can be downloaded here.