Alpha decay Alpha particles consist of two protons plus two neutrons. They are emitted by some of the isotopes of the heaviest elements. 1

Example: The decay of Uranium 238 92 Th 234 90 α 4 2 + Uranium 238 decays to Thorium 234 plus an alpha particle. Notes: 1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2) 2. The alpha particle can also be notated as: He 4 2 2

Question Show the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92). Pu 239 94 U 235 92 α 4 2 + 3

Beta decay Beta particles consist of high speed electrons. They are emitted by isotopes that have too many neutrons. One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle. 4

Example: The decay of Carbon 14 6 N 14 7 β- -1 + Carbon 14 decays to Nitrogen 14 plus a beta particle. Notes: 1. The beta particle, being negatively charged, has an effective atomic number of minus one. 2. The beta particle can also be notated as: e -1 5

Question Show the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg). Na 25 11 Mg 12 β- -1 + 6

Gamma decay Gamma decay is the emission of electromagnetic radiation from an unstable nucleus Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle. Example: Cobalt 60 Co 60 27 Co 60 27 γ + Cobalt 60 with excess ENERGY decays to Cobalt 60 with less ENERGY plus gamma radiation. 7

Do Now copy and complete Changing elements Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number. Decay type Atomic number Mass number alpha DOWN by 2 DOWN by 4 beta UP by 1 NO CHANGE gamma 8

Complete the decay equations below: Fe 59 26 Co 27 β- -1 + Ra 224 88 Rn 220 86 α 4 2 N 16 7 O 8 (a) (c) (b) 9

Write equations showing how Lead 202 could decay into Gold Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!) Element Sym Z Platinum Pt 78 Gold Au 79 Mercury Hg 80 Thallium Tl 81 Lead Pb 82 Bismuth Bi 83 Pb 202 82 Hg 198 80 α 4 2 + Hg 198 80 Pt 194 78 α 4 2 + Pt 194 78 Au 79 β- -1 + There are other correct solutions 10

Choose appropriate words to fill in the gaps below: When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______. Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged. Gamma rays consist of ______________ radiation that is emitted from a nucleus when it loses ________, often after undergoing alpha or beta decay. two four neutrons one mass electromagnetic energy WORD SELECTION: four one energy two neutrons mass electromagnetic 11

Today’s lesson Title Half-life Use the term half-life in simple calculations, including the use of information in tables or decay curves. Give and explain examples of practical applications of isotopes. Title Half-life

½ - life – copy please This is the time it takes for half the nuclei present in any given sample to decay Number of nuclei undecayed A graph of the count rate against time will be the same shape time half-life (t½)

Different ½ - lives Different isotopes have different half-lives The ½-life could be a few milliseconds or 5000 million years!half life applet Number of nuclei undecayed time half-life (t½)

Examples A sample of a radioactive isotope of half life 2 hours has a count rate of 30 000 counts per second. What will the count rate be after 8 hours?

Examples

Henri Becquerel discovered radioactivity in 1896 The activity of a radioactive source is equal to the number of decays per second. Activity is measured in bequerels (Bq) 1 becquerel = 1 decay per second Half life Henri Becquerel discovered radioactivity in 1896 17

Question 1 At 10am in the morning a radioactive sample contains 80g of a radioactive isotope. If the isotope has a half-life of 20 minutes calculate the mass of the isotope remaining at 11am. 10am to 11am = 60 minutes = 3 x 20 minutes = 3 half-lives mass of isotope = ½ x ½ x ½ x 80g mass at 11 am = 10g 18

Question 2 Calculate the half-life of the radioactive isotope in a source if its mass decreases from 24g to 6g over a period of 60 days. 24g x ½ = 12g 12g x ½ = 6g therefore TWO half-lives occur in 60 days half-life = 30 days 19

Example 2 – The decay of source Z Time Activity (Bq) 9 am 12 noon 3 pm 6 pm 9 pm midnight Source Z decays with a half-life of three hours. At 9 am the source has an activity of 16000 Bq The activity halves every three hours. 16000 8000 4000 2000 1000 500 When will the activity have fallen to 125 Bq? 6 am 20

Example 3 – The decay of isotope X Isotope X decays to Isotope Y with a half-life of 2 hours. At 2 pm there are 6400 nuclei of isotope X. Time Nuclei of X Nuclei of Y 2 pm 4 pm 6 pm 8 pm 10 pm midnight 6400 3200 3200 1600 4800 800 5600 400 6000 200 6200 When will the nuclei of isotope X fallen to 25? 6 am 21

Question 3 A radioactive source has a half-life of 3 hours. At 8 am it has an activity of 600 Bq. What will be its activity at 2 pm? at 8 am activity = 600 Bq 2 pm is 6 hours later this is 2 half-lives later therefore the activity will halve twice that is: 600  300  150 activity at 2 pm = 150 Bq 22

Question 4 – The decay of substance P Substance P decays to substance Q with a half-life of 15 minutes. At 9 am there are 1280 nuclei of substance P. Complete the table. Time Nuclei of X Nuclei of Y 9 am 9:15 9:30 9:45 10 am 10:15 1280 640 640 320 960 160 1120 80 1200 40 1240 How many nuclei of substance X will be left at 11 am? 5 23

Question 5 A sample contains 8 billion nuclei of hydrogen 3 atoms. Hydrogen 3 has a half-life of 12 years. How many nuclei should remain after a period 48 years? 48 years = 4 x 12 years = FOUR half-lives nuclei left = ½ x ½ x ½ x ½ x 8 billion nuclei left = 500 million 24

Experiment Dicium 25

Finding half-life from a graph The half-life in this example is about 30 seconds. A more accurate value can be obtained be repeating this method for a other initial nuclei numbers and then taking an average. half-life 26

Question 6 Estimate the half-life of the substance whose decay graph is shown opposite. half-life The half-life is approximately 20 seconds 27

Question 7 The mass of a radioactive substance over a 8 hour period is shown in the table below. Draw a graph of mass against time and use it to determine the half-life of the substance. Time (hours) 1 2 3 4 5 6 7 8 Mass (g) 650 493 373 283 214 163 123 93 71 The half-life should be about 2 hours: 28

Choose appropriate words or numbers to fill in the gaps below: The ________ of a radioactive substance is the average time taken for half of the _______of the substance to decay. It is also equal to the average time taken for the ________ of the substance to halve. The half-life of carbon 14 is about _______ years. If today a sample of carbon 14 has an activity of 3400 Bq then in 5600 years time this should have fallen to ______ Bq. 11200 years later the activity should have fallen to ____ Bq. The number of carbon 14 nuclei would have also decreased by ______ times. half-life nuclei activity 5600 1700 425 eight WORD & NUMBER SELECTION: 5600 nuclei eight half-life 425 1700 activity 29

Revision Simulations Half-Life - S-Cool section on half-life and uses of radioactivity including an on-screen half-life calculation and an animation showing thickness control. BBC AQA GCSE Bitesize Revision: Detecting radiation Natural sources of background radiation Artificial radiation Half life Alpha Decay - PhET - Watch alpha particles escape from a Polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half life.

Uses of radioactive isotopes

Smoke detection Uses

Thickness control

Thickness control

Used as Tracers

Used as Tracers

Killing microbes

Killing microbes

Checking welds

Used as Tracers

Carbon dating – write notes using the book page 265

Summary sheet

“Can you………?”

Test! Thursday 27th September 2012

Can you answer the questions on pages 261 and 265?