GCSE (1-9) Physics: Radioactivity Nuclear Decay and Half Life

Objectives:

FLASHBACK FLASHBACK: A radioactive source is giving off all three forms of ionising radiation. Compare the relative danger of each form on a human being with reference to their relative ionisation ability and penetrance( what they can get through) [6 marks]

Determining Activity- Method 1 Naturally, we are unable to physically see a radioactive substance decay. As such, it is important that we use a different method to determine how much radiation (decay) is being released. The most common methods are Geiger-Muller (GM) counters or photographic film. Photographic Film Photographic film (silver chloride film) turns black when exposed to ionising radiation. The more radiation there is, the blacker it becomes. A small piece is put into a little structure that has an outer box with windows of paper, aluminium and lead Why does the detector have these windows?

Determining Activity- Method 2 Geiger-Muller Counters GM counters have a more complicated structure. They basically work by allowing ionising radiation to enter which ionises the air inside. This results in charges that can be picked up as a current. This then allows the scientist to detect Activity measured in counts per time or Becquerels (Bq)

Determining Activity- Comparison Compare and contrast the usefulness of each of these methods of determining radioactivity. (6 marks)

Activity- Experimental Plan Prepare a table of results that you can fill in once the class demonstration is completed. We will be testing the experimental question: “ What is the relationship between lead thickness and the radioactivity reading of a radioactive source” 2. Write a short hypothesis which predicts with scientific explanation, what pattern of results you expect to get 3. Draw a simple labelled diagram of the set-up The thickness will be in mm and the activity is in decays/10s

Why is important for multiple trials to occur? Data Record Why is important for multiple trials to occur? Thickness of Lead (mm) Activity in 10s (count) Trial 1 Trial 2 Trial 3 Trial 4 AVERAGE Decay is always random and so cannot be predicted. You never know which nucleus will decay next!

Controlling the Experiment Before we take any sample reading we must always take a reading of the background radiation. Background radiation is the radiation that is constantly around us from various sources. As it’s different in every location you have to take a reading in your sample reading location first then minus this value from your sample!! Use these charts to summarise the sources of background radiation. Then suggest why it’s different in different locations

Class Demonstration- Half Thickness Gather around the set-up. Take note of the method steps that your teacher is undertaking, you’ll need to be able to remember these! Note: The radioactive sources are dangerous and as such have to be respected. If anyone messes around or is not listening during the experiment, it must be stopped. It could end up producing lovely results so focus is key! Vernier calliper to measure thickness of the lead at each point.

Data Analysis Once you have calculated the average count for each thickness, you need to plot a graph for activity against thickness. Don’t forget that for each of your averages you need to minus the background reading After you have plotted your data, make sure to draw a neat line/curve of best fit. This should be a single line that attempts to show the pattern best! Activity (count) Once complete, write a conclusion and refer to whether your hypothesis was correct! Thickness/mm

Mathematical Relationship The relationship between the rate of decay (or activity) of a radioactive isotope and the thickness of material is known as an exponential curve. An exponential shows that one value (e.g. activity) changes by the same ratio in each set change in the other value (e.g. thickness) -400BC antiquarks Thickness isn’t the only property this is true for….

Half-Life Half-life is the time taken for the activity (or number of undecayed nuclei) of a sample to reduce to half Within a sample of a radioactive isotope, there are a number of undecayed nuclei. As a radioactive isotope is unstable it will randomly decay its nuclei, releasing radiation (activity). Although you never know which particular nuclei will decay, the sample as a whole follows a general decay curve. -400BC Initial Activity Half of initial activity Go across to line and down to time antiquarks NEW Initial Activity It’s still 2 days! (2 to 4!) Half life for this element is 2 days!

Mathematical Relationship -400BC So every half life, the activity or the number of undecayed nuclei remaining halves! We can use this information to determine the age of a radioactive substance! antiquarks

Practice Plot the following and determine the half life

Half Life Practice DIFFICULTY 1. A radioactive substance has a half-life of 30 minutes.  What fraction of the atoms will have decayed after 1 hour? -400BC 2. A researcher measures 200 counts per minute coming from a radioactive source at midday. At 3 o'clock, she finds that this has dropped to 25 counts per minute. What is the half-life of the radioactive source? DIFFICULTY antiquarks 3. The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

6 Mark Question -400BC William has been provided with a rock sample that is giving off some radiation. He has been tasked with determining the half-life of this sample. Suggest how William could achieve this task with reference to experimental precautions he must take to ensure an accurate value. antiquarks

6 Mark Question William has been provided with a rock sample that is giving off some radiation. He has been tasked with determining the half-life of this sample. Suggest how William could achieve this task with reference to experimental precautions he must take to ensure an accurate value. -400BC Defines half-life as the time taken for activity to reach half it’s original value Set up with a GM counter to measure activity Take a background reading and minus this value from the sample reading Take multiple readings of activity and take an average Decay is random Plot a graph of activity against time Draw a line from half of original value and determine the time Do this multiple times and take an average antiquarks