STARTER If a company has 1 million phones and sells half of the stock every day, how long does it take to sell all the stock? 20 days A piece of paper is 0.5 mm, how many times would it need to be folded to reach the moon? (348, 400 km to the moon from earth) http://www.freestockphotos.biz/stockphoto/15149 41 times is more than enough How come?

Learning objectives Review what is meant by isotope and how this is radioactive. (D) Understand the ‘random’ nature of radioactive decay. (C) State and explain what is meant by ‘half-life’. (B) Draw key conclusions from graphs of radioactive decay. (A) Keywords: Proton - Isotope Neutron - Half-life Electron - Count Stable/Unstable NOTICES: …………….

THEY ARE ISOTOPES OF THE CARBON ATOM Recap of …. Isotopes Carbon 12 C 6 ????? Carbon 13 C 6 ????? Carbon 14 C 6 ????? ? ? ? What is being shown above? THEY ARE ISOTOPES OF THE CARBON ATOM Isotopes are atoms of the same element with different numbers of neutrons. The isotopes of an element have the same number of protons. The nuclei of some isotopes are unstable. They can split up or ‘decay’ and release radiation. Such isotopes are called radioactive isotopes or radioisotopes. When a radioactive isotope decays, it forms a different atom with a different number of protons.

What do these isotopes decay into? TASK – Isotopes of what? What do these isotopes decay into? Plutonium 240 Pu 94 Uranium 236 U 92 1) - Alpha Decay = Carbon 14 C 6 Nitrogen 14 N 7 - = 2) Beta Decay Ceasium 137 Cs 55 Barium 137 Ba 56 3) - = Beta Decay

RECAP: What are our 3 types of radiation? α β γ

CLASS ACTIVITY HOW MANY FLIPS UNTIL ALL COINS SHOW HEADS? You each have 3 coins. When told you must spin or flip your 3 coins. Tell the teacher when asked how many (if any landed) on heads and put these to one side. When told spin/flip your remaining coins telling the teacher which landed on heads and putting these to one side. Repeat until we stop. 1p coin - http://www.nakuruchildrensproject.org.uk/donate/4579869692 Do we expect that some coins will turn up heads each time we spin/flip the coins? Can we predict how many will be heads on each turn? Is it more likely there will be more heads when we have more coins? Also less heads when we have less coins?

CLASS ACTIVITY Time (no. spins/flips) No. of Heads 1 2 3 4 5 6 7 8 9 10 11 12 13 You each have 3 coins. When told you must spin or flip your 3 coins. Tell the teacher when asked how many (if any landed) on heads and put these to one side. When told spin/flip your remaining coins telling the teacher which landed on heads and putting these to one side. Repeat until we stop.

CLASS ACTIVITY What does this graph show? RIGHT CLICK ON GRAPH -> EDIT DATA -> INPUT THE DATA COLLECTED FROM YOUR CLASS. IT WILL UPDATE APPROPRIATELY RIGHT CLICK ON GRAPH -> EDIT DATA -> INPUT THE DATA COLLECTED FROM YOUR CLASS. IT WILL UPDATE APPROPRIATELY What does this graph show? What detailed statement can you make from the graph?

Random???? What does random mean? Unexpected, unusual, odd ….etc ‘Happening without method’ Random….we know it is going to happen just not exactly when. What things can you think of that are ‘random’ RADIOACTIVITY

RADIOACTIVITY Do we know when a nucleus is going to decay? Can we predict how many nuclei are going to decay at any time? Can we expect more nuclei to decay when there are more unstable nuclei in a sustance? Also less decays when there are less unstable nuclei? RIGHT CLICK ON GRAPH -> EDIT DATA -> INPUT THE DATA COLLECTED FROM YOUR CLASS. IT WILL UPDATE APPROPRIATELY

RADIOACTIVITY & HALF-LIFE You are going to keep track of radioactive decay of a bunch of nuclei You will need to do this in pairs TIME (SECONDS) No.OF DECAYS 1 2 3 4 5 6 8 11 15 Note how many nuclei decay with time – we will then plot this on graph 2 2 2 1 1 1 1 1 1

TASK – TIMING DECAY START Some more left RIGHT CLICK ON GRAPH -> EDIT DATA -> INPUT THE DATA COLLECTED FROM YOUR CLASS. IT WILL UPDATE APPROPRIATELY

DO WE NEED TO RUN THAT AGAIN?? ‘THOUGHT SO’ ‘ONE MORE TIME’

TASK – TIMING DECAY START Some more left RIGHT CLICK ON GRAPH -> EDIT DATA -> INPUT THE DATA COLLECTED FROM YOUR CLASS. IT WILL UPDATE APPROPRIATELY

TIMING DECAY - RESULTS

TIMING DECAY – USING DATA We want to plot how much activity is left in our radioactive substance, so there is one more thing to do our data. 80 - decays = 77 - decays = WE STARTED WITH 80 UNSTABLE RADIOACTIVE NUCLEI 75 - decays = We therefore need to start with 80 and minus the decays that occurred each second.

WHAT CONCLUSION CAN WE DRAW FROM OUT GRAPH? Radioactive decay WHAT CONCLUSION CAN WE DRAW FROM OUT GRAPH? A GRAPH LIKE THIS CAN TELL US THE ‘HALF-LIFE’ OF A RADIOACTIVE SUBSTANCE

What is meant by ‘half-life’? REMEMBER: The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation. What is meant by ‘half-life’? The ‘half-life’ of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve. OR……. the time it takes for the count rate from a sample containing the isotope to fall to half its starting level

Reading half-life graphs How do we read half-life from our graphs? 60 What is the half-life of the substance shown in the graph? 50 40 4 days Number of radioactive nuclei 30 What is the second half-life of the substance shown in the graph? 20 10 2 4 6 8 10 12 14 9 days Time (Days)

Reading half-life graphs Compare and contrast the two graphs

Reading half-life graphs Short half-life, more active sample 60 50 40 Number of radioactive nuclei 30 20 10 1 2 3 4 5 6 7 8 Days

Reading half-life graphs Long half-life, less active sample 60 50 40 Number of radioactive nuclei 30 20 10 5 10 15 20 25 30 35 40 Days

In a single elimination tournament… After each round, half of the teams are eliminated. This is similar to the half-life of a radioactive element. Day1 Day 3 Day2 Day 5 Day 4

After one “half life”, 50% of the material has decayed It did not disappear, but it did turn into a different element because of alpha, beta, or gamma decay. Radioactive materials decay from the “Parent” material into the “Daughter Product”. Daughter Product Original “Parent” Material

Each half-life, the amount of atoms gets cut in half.

One half-life.

Two half-lives.

Three half-lives.

Four half-lives.

Don’t worry about the last atom Don’t worry about the last atom. You start with so many trillions that you never really get there. (It will just decay and then they’re all gone.)

100% parent 0% daughter 0 half lives Since you don’t know how many atoms you started with, a ratio between parent to daughter will tell you how many half-lives have gone by. 100% parent 0% daughter 0 half lives

One half-life. 50% parent 50% daughter 1 half-life

Two half-lives. 25% parent 75% daughter 2 half-lives

Three half-lives. 12.5% parent 87.5% daughter 3 half-lives

Four half-lives. 6.25% parent 93.75% daughter 4 half-lives

Summary The half-life of a radioactive isotope is the average time it takes for he number of nuclei of the isotope in a sample to halve. The activity of a radioactive source is the number of nuclei that decays per second. The number of radioactive isotopes and the activity decrease by half every half-life.

Review Questions 1) A Radioactive substance consists of unstable nuclei that emit nuclear radiation to become stable. 2) Isotopes are atoms of the same element with different number of neutrons. When isotopes decay their nuclei split and release energy. 3) Half-life refers to the time taken for a nuclear samples activity to reduce by half.. 4) 32 days (4 half-lives) 5) 3.5 days (accept 3.4-3.6)