04/01/12EdExcel Additional Science Putting Radiation to Use W Richards The Weald School
04/01/12 The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same NEUTRON – mass asneutral, same neutron (“1”)mass as proton (“1”)
04/01/12The structure of the atomParticle Relative Mass Relative Charge Proton 1 +1Neutron 1 0Electron 0 -1 MASS NUMBER = number of protons + number of neutrons SYMBOL PROTON NUMBER = number of protons (obviously)
Isotopes 04/01/12An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.A “radioisotope” is simply an isotope that is radioactive –e.g. carbon 14, which is used in carbon dating.
04/01/12 Introduction to RadioactivitySome substances are classed as “radioactive” – this meansthat they are unstable and continuously give out radiation: RadiationThe nucleus is more stable after emitting some radiation – thisis called “radioactice decay” and the activity is measured in“Becquerels (Bq)”.
Types of radiation 04/01/12 1) Alpha (α) – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of aUnstable New Alpha ______ atom) nucleus nucleus particle 2) Beta (β) – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, Beta high energy electron is called a _____ particle particle.Unstable New nucleus nucleus 3) Gamma – after α or β decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.Unstable New Gamma Words – frequency, proton, nucleus nucleus radiation energy, neutrons, helium, beta
04/01/12 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… = radioisotope = new atom formed After 1 half After 2 half After 3 half At start life half have lives another lives anotherthere are 16 decayed half have 2 haveradioisotopes (that’s 8) decayed (12 decayed (14 altogether) altogether)
04/01/12 A radioactive decay graphCount Time 1 half life
Dating materials using half-lives 04/01/12 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. Answer: The sample was originally completely uranium… 1 half life 1 half life 1 half life later… later… later… 8 4 2 1 8 8 8 8 …of the Now only 4/8 of Now only 2/8 of Now only 1/8 ofsample was the uranium uranium remains uranium remains uranium remains – the – the other 6/8 – the other 7/8 other 4/8 is lead is lead is lead So 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.
04/01/12 An exam question…Potassium decays into argon. The half life of potassium is1.3 billion years. A sample of rock from Mars is found tocontain three argon atoms for every atom of potassium.How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years
04/01/12 IonisationRadiation is dangerous because it “ionises” atoms – in otherwords, it turns them into ions by “knocking off” electrons:Alpha radiation is the most ionising (basically, because it’s thebiggest). Ionisation causes cells in living tissue to mutate,usually causing cancer.
Blocking Radiation 04/01/12Each type of radiation can be blocked by different materials: α β γ Sheet of Few mm of Few cm of paper aluminium lead
04/01/12 Uses of radioactivity 1Sterilising medical instruments Gamma rays can be used to kill and sterilise germs without the need for heating.
04/01/12 Uses of radioactivity 2 Beta detectorRollers Paper Beta emitter
04/01/12 Uses of radioactivity 3Smoke detectors Alpha emitter +ve electrode -ve electrode Alarm If smoke enters here a Ionised air particles current no longer flows
04/01/12 Uses of Radioactivity 4 - Treating CancerHigh energy gamma radiation can be used to kill cancerouscells. However, care must be taken in order to enure that thegamma radiation does not affect normal tissue as well.Radioactive iodine can be used to treat thyroid cancer. Iodineis needed by the thyroid so it naturally collects there.Radioactive iodine will then give out beta radiation and killcancerous cells.
04/01/12 Dangers of radioactivity Radiation will ionise atoms in livingAlpha cells – this can damage them and cause cancer or leukaemia.BetaGamma OUTSIDE the body β and γ are more dangerous as α radiation is blocked by the skin. INSIDE the body an α source causes the most damage because it is the most ionising.
04/01/12 Background Radiation 13% areman-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power
04/01/12 Protecting the Earth 1) The Atmosphere 2) The Magnetic Field The Earth has a number of The Earth’s magnetic fieldlayers that reflect radiation reflects charged particles back into space. from solar wind away from us.