Half-Life Noadswood Science, 2012

Saturday, April 15, 2017 Half-Life To understand radioactive half-life

Radiation Complete the radioactive questions below: - List one source of natural and one source of artificial background radiation What are the three types of radiation? What are the three symbols of the three types of radiation? Which type of radiation has the strongest penetrating power? Which type of radiation have the weakest penetrating power? What machine can be used to detect radiation? What source of radiation is most harmful within the body? What source of radiation is the most harmful outside of the body?

Radiation List one source of natural and one source of artificial background radiation – natural = ground, air, building materials, food etc… and artificial = medical, nuclear power, weapons etc… What are the three types of radiation – alpha, beta and gamma What are the three symbols of the three types of radiation – α, β and γ Which type of radiation has the strongest penetrating power – gamma Which type of radiation have the weakest penetrating power – alpha What machine can be used to detect radiation – Geiger Muller tube What source of radiation is most harmful within the body – alpha What source of radiation is the most harmful outside of the body – beta and gamma

Half-life The nuclei of radioactive atoms are unstable – they break down and change into a completely different type of atom, known as radioactive decay For example, carbon-14 decays to nitrogen-14 when it emits beta radiation It is not possible to predict when an individual atom might decay, but it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay This is called the half-life of the radioactive isotope

Half-life The decay of radioisotopes can be used to measure the material’s age The half-life of an atom is the time taken for half of the radioisotopes in a sample to decay… At start there are 16 radioisotopes After 1 half-life half have decayed (8) After 2 half-lives another half have decayed (4 more = 12) After 3 half-lives another half have decayed (2 more = 14)

Half-life There are two definitions of half-life: - The time it takes for the number of nuclei of the isotope in a sample to halve The time it takes for the count rate from a sample containing the isotope to fall to half its starting level Different radioactive isotopes have different half-lives: - The half-life of carbon-14 is 5,715 years The half-life of francium-223 is just 20 minutes

Measurements The activity of a radioactive source is the number of radioactive atoms in the source that decay every second Activity is measured in becquerels (Bq) If a source has an activity of 100 Bq, this means that 100 atoms in the source decay every second Radioactive sources can have very small or very large half-lives – this is important to know, as a medical tracer would be dangerous if it’s half-life was too great (as it would be in the body for a long time) Sources such as thallium which is used in medical traces has a half-life of around 1 to 10 days (depending upon the isotope used)

Half-life It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time… The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days In the next two days, it drops from 40 to 20 - it halves In the two days after that, it drops from 20 to 10 - it halves again and so on…

Half-life Question Uranium decays into lead – the half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium – calculate the age of the sample (which was originally all uranium) 8 4 8 2 8 1 8 All of the sample was uranium Now only 4/8 of the uranium remains (the other 4/8 is lead) Now only 2/8 of uranium remains (the other 6/8 is lead) Now only 1/8 of uranium remains (the other 7/8 is lead) It must have taken 3 half-lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead) Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old (3 x 4,000,000,000)

Half-life question Potassium decays into argon – the half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium – how old is the rock? 4/4 = potassium 2/4 = potassium 1/4 = potassium (1 potassium for every 3 argon) The rock must be 2 half-lives old 2 x 1.3 billion years = 2.6 billion years old 1st half-life 2nd half-life

Half-life question The activity of a radio-isotope is 640cpm – two hours later it has fallen to 40cpm. What is it’s half-life? Initial count = 640cpm After one half-life = 320cpm After two half-lives = 160cpm After three half-lives = 80cpm After four half-lives = 40cpm There have been 4x half-lives in 2 hours = 1 half-life every 30 minutes

Half-life experiment Grab a handful of the half-life blocks and count how many you have (aim for around 20) Roll them out in a tray, picking out the ones which have landed with the black-side up – record how many are left in the tray and roll again… Continue to do this until you have no more blocks left, recording your results

Half-life experiment Record your results (how many blocks remain, as well as how many times you have rolled) Graph your results – hopefully you have found a realistic half-life pattern! Number of rolls Number of dice 20 1 2 Etc…

Half-life Complete the half-life worksheet…

Half-life Complete the half-life worksheet…