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Chapter 5 Radiaoactivity(Notes)

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    Chapter 5 Radiaoactivity(Notes) Chapter 5 Radiaoactivity(Notes) Document Transcript

    • MAKTAB SULTAN ISMAIL PHYSICS FORM 5 CHAPTER 5 : RADIOACTIVITY 5.1 Understanding the nucleus of an atom 1. Draw the Rutherford model of an atom. electron - + nucleus Empty space The nucleus consists of proton ( positively charged ) and neutrons ( neutral). Electrons are orbiting around the nucleus. 2. Briefly describe the Geiger – Marsden experiment: The discovery of the nucleus. This experiment is the discovery of the nucleus. (i) A gold foil is bombarded by positively charged alpha particle. 1
    • Gold foil Microscope Alpha source α α Nucleus ( positive ) α deflected back α not deflected α deflected at small angle (ii) Observation made are ; a. most of alpha particle passed through without deflection. The alpha particles passes far from nucleus. They did not experience repulsion. b. A very small number of alpha particles were deflected by the gold foil .( small angle of deflection ) The alpha particles passes quite near the nucleus and experience a repulsive force. 2
    • c. A very small number of alpha particles were reflected back( about 180 0 deflection ) The alpha particles move very near or nearly collide head-on with the nucleus experiences large repulsive force. Experimental conclusion : (a) The atomic mass is placed at the centre of the atom and is positively charged. (b) The atom has a lot of empty space. (c) The electrons orbiting the nucleus make up most of the atomic volume. 3. Define the proton number, Z The number of protons in the nucleus . In a neutral atom, the number of proton = number of electrons. 4. Define the nucleon number, A ( mass number ) The total number of protons and neutrons. The mass of an atom is normally written as a.m.u. The unit normally used is atomic mass uint ( a.m.u ) or written as u only. For example : A = 235 U Z= 92 The atomic mass = 235.043925 a.m.u 1 a.m.u = 1.66 x 10 -27 kg 5. How to find the number of neutrons ? n = A-Z 6. What is nuclide? 3
    • A nuclide is a type of nucleus with a particular proton number and particular nucleon number. 7. Write down the nuclide notation. X is an element for which the nucleus of the atom contains Z protons and A nucleons. The nuclide notation is written as A X Z 8. Uranium -238, has 92 proton, write down the nuclide notation. How many neutron does it has ? 238 U 92 n = Z –A = 238- 92 = 146 9. What is meant by isotopes? 1. Atoms of an element which have the same proton number but different nucleon numbers. 2…Atoms with the same number of protons but different number of neutrons. 3. Atoms with the same chemical properties but different physical properties. 10. What is meant by radioisotopes.? Isotopes that are radioactive . or Atoms with the same proton number but different neutron numbers and are radioactive ( giving out radioactive radiations ). 5.2 Analysing Radioactive decay 1. What is radioactivity ? Draw its symbol . Radiaactivity is the spontaneous disintegration (decay ) of an unstable nucleus accompanied by the emission of radioactive radiations, α, β , and γ . Alpha particle 4
    • Beta particle Gamma ray Radioactive sign Note : radiation can be helpful when used in the correct way . It can be very harmful if not managed properly, Exposure to large doses of radiation is very dangerous. 2. What is back ground radiation ? What are the sources of background radiations ? Low level of radiations from the surrounding. The back ground radiations are detected even tough there is no radioactive source nearby. About 60 counts per minute. 5
    • Sources Percentage Radiactive gasses in the air 51% e.g radon gas , radioisotopes of various gasses . Rock and soil ; form the Earth 14 % ‘s crust. Cosmic rays from outer space: 10% Food and drinks , eg. 12 % Vegestables, salds, cow’s milk. Medical examination 12% Nuclear leaks , waste, and 1% fallout ( nuclear pollutions ) 3. What are the harmful effects of radiations ? Radiations cause ionisations in the molecules of the living cells. The ions can cause damage to the other atoms in the cells. It can cause somatic effects and genetic effects . Table shows the effects of radiation Somatic effect Genetic Effect Radiation burns Chromosome abnormalities Organ failure Mutation of genes Blood Disorder ( Leukemia ) Birth defects Cataracts Congenital defects Death Premature death Hair loss Cancer in later life. Nausea Leukemia Fatique Genetic disorders Impotency - 4. What are the safety precautions in the handling of radioactive substances ? a. Read and follow the advice and instructions marked on radioactive sources. b. Gloves must be worn at any time while handling radioactive materials. c. Use forceps or tongs when handling. 6
    • d. Wear laboratory coats and eyes protections goggles when handling. e. Eating, drinking and smoking are prohibited while handling. f. Wash hands and forearms thoroughly after handling radioactive materials. g. Place radioactive liquids in unbreakable containers. h. Place radioactive solids in lead containers. i. Use dosimeter for radioactive workers. 5. Briefly explain Radiation as an ionisation process. When radiation passes through a medium, it is energetic enough to knock electrons out of the atoms to produce charged particles called ions. - Negative ion + Radiation Radiation Neutral atom Positive ion 6. Radioactive detectors : a. Photographic film- It darkens when radiation is detected. The degree of darkening will depend on the intensity of radiation, Higher dose of radiation gives rise to darker film. b. Cloud chamber Perspek window Piece of felt Cold & saturated vapor radioactive source 7
    • solid metal plate carbon dioxide sponge Tracks of the three main types of radioactive emission Alpha particle beta particle gamma rays How does it operates ? a. The radiation produces ions in the air that is cold and saturated with alcohol vapour . b. The water alcohol vapour condenses on the ions to make the tracks of the radiations which is white in colour. c. The cloud chamber works on the ionisation properties of radiations. 8
    • c. Geiger –Muller Tube and ratemeter. Argon gas Mica window aluminium cylinder ( cathode) Radiations tungsten wire ( anode) EHT ( 450V) amplifier Rate meter When the radiations enter from the thin mica window, the ratemeter will give a reading. The reading is normally recorded as counts per minute. e.g 9856 count per minute. Hoe does it works ? 1. The radiation ionises the argon gas to positive and negative ions. 9
    • 2. The electrons are attracted to the central anode and the positive ions move to the negative electrode. 3. A pulse of current is produced in the anode and cunted by a scaler or ratemeter. d. Spark counter: Suitable for alpha particles detection. Wire gauze radioactive source 2 mm Small uninsulated wire EHT perspek box Working principle : a. When a radioactive source is brought near the wire gauze, the radiation ionises the air below it. b. The motion of the ions to the gauze and the wire causes sparks to be produced. c. The sparks can be seen and heard.. 7. What are the three kinds of radioactive emissions ? The three kinds of radioactive emissions are alpha particles ( α- particle s), beta particles( β-particles) and gamma rays ( γ-rays ). 8. What is Alpha particle ? 10
    • It is a helium nucleus . Positively charged and very heavy. It’s nuclide notation is 4 2+ He 2+ 2 9. What is a beta particle ? It is high energy electron which has small mass and negatively charged. It’s nuclide notation is 0 - e -1 10.What is a Gamma ray ? It is an electromagnetic wave of very high frequency and short wavelength, with no charge . Briefly describe the ionising power and the penetrating power of radiations. Compare the ionising power and the penetrating power of α, β and γ . Ionising power – the rate at which ions ( charged particles are produced ) 11
    • Penetrating power - the distance travelled by the particle before it finally stops. Alpha particle Beta-particle Gamma ray Ionising power High Medium Low e.g e.g e.g 100 000 Ion pairs 1000 ion pairs per 10 ion pairs per per cm of air cm cm. Penetrating power Low Medium High e.g e.g e.g abt 7 cm in air few mm thick of few cm thick of aluminium lead. 11.Describe the general properties of alpha, beta particles and gamma rays . What happens to the radioactive emissions in electric and magnetic fields ? Types Alpha particle Beta particle Gamma rays properties 1. natural property. Heium nucleus Electron which is Electromagnetic which is positively negatively charged. radiation which has charged. no charged and no 0 mass. 4 2+ e He -1 2 2.velocity about 5% velocity Can travel up to Same as velocity of of light 90% of velocity of light. light. 3. Range in air About 5-7 cm About 10m Very far 12
    • 4.penetrating Low Medium High power Paper aluminium concrete or lead α β γ Comparison of penetrating power of radiactive admission. 5. Ionisation power High , bigger mass Medium, smaller Low, has no mass mass 6. Detection Can be detected Photographic plate, Photographic plate using cloud Cloud chamber, and cloud chamber. chamber, a GM tube. charged electroscope, GM tube with thin mica. 7. Deflection in Deflected as Deflected as Not deflected. magnetic field. shown below. shown. Large Small deflection deflection due to due to large mass it’s small mass. of alpha. Apply Fleming-Left Apply Fleming-Left Hand Rule to Hand Rule to determine the determine the deflection. deflection. 13
    • south alpha Gamma rays. magnetic field into the paper North Beta particle Radioactive source 8. Deflection by Deflected toward Deflected toward Not deflected electric field the negative plate. the positive plate. because it has no It shows that It shows that beta charge. alpha is positively is negatively charged. charged Gamma ray positive plate negative plate beta particle alpha particle Radioactive source 12.Radioactive decay ,What are the changes to A and Z values , 14
    • a. alpha decay : The emission of an alpha particle, the daughter nuclide A value will be reduced by 4 and it’s Z value will be reduced by 2. A A-4 4 X Y + He Z Z-2 2 Example s : 238 234 4 U Th + He 92 90 2 230 234 4 Th Ra + He 90 88 2 b. beta decay : In a beta decay , the daughter nuclide A value is unchanged but the Z value will be increased by 1. A neutron disintegrate to proton and a electron. A A 0 X Y + He Z Z+1 -1 Examples ; 234 234 0 Th Pa + e 15
    • 90 91 -1 234 234 0 Pa U + e 91 92 -1 c. Gamma decay : There is no change in A and Z values. A A X X + γ radiation. Z Z Example : 226 222 4 Ra Rn + He + γ radiation 88 86 2 2. Explain a Radioactive decay series and give an example. The daughter nuclide of a radioactive decay may still be unstable. It will eventually decay into another nuclide. This process continues as a radioactive decay series until a stable nuclide is reached. For example, Bismuth-214 to Lead -206 ( stable ) 214 214 210 210 210 206 Bi Po Pb Bi Po Pb 83 a 84 b 82 c 83 d 84 e 82 It involes 5 decay processes : a. emission of beta and gamma 16
    • b. emission of alpha c. emission of beta and gamma d. emission of gamma e. emission of alpha and gamma. 3. What is half-life ? write its notation . The half- life ( T or T ½ ) ) of a radioactive nuclide is the time taken for the number of undecayed nuclei to reduce to half its original number. Example : Original number, 1000 500 250 T T Or The half- life ( T ) of a radioactive nuclide is the time taken for the mass to decay and reduce to half its original mass Example : 600 g 300 g 150 g 75 g T T T Or The half- life ( T ) of a radioactive nuclide is the time taken for the activity to decay and reduce to half its original activity. T Example : 1200 counts per min 600 counts per min T 600 counts per min 300 counts per min T 300 counts per min 150 counts per min. The half-life ( T ) for different nuclei are different. Some have short half-life and some have long half-life. Short half-live means the radioactivity could finished off in a short duration. Long half-life means radioactivity will remain for some time. Half –life of some radioactive nuclides Radioactive nuclide Half-life ( T ) Sodium-24 15 hours Boron -12 20.2 milliseconds Nitrogen-13 9.96 minutes 17
    • Radon-222 3.82 days Cobalt-60 5.27 years Uranium -235 703 million years Carbon -14 5600 years. 4. Different nuclides have different half-life. What is the range ? Half-life ranges from milliseconds to million of years. 5. Draw a graph of count rates against time . show how half-lives can be determined from the graph . There are two types of graphs . The mass against –time graph and the activity against –time graph. Mass /g Graph of mass against time m m/2 m/4 m/8 18
    • 0 T1 T2 T3 time , t T½ = T1 atau T½ = T2 –T 1 T½ T½ T½ T½ m m/2 m/4 m/8 m/16 Activity / counts per min. Graph activity against time N0 N0/2 N0/4 N0/8 0 T1 T2 T3 time ,t T½ = T1 atau T½ = T2 –T 1 19
    • N0 N0/2 N0/4 N0/8 N0/16 6. Radioactive tracers are used to determine the leakage of water pipe. Briefly explain the choice of the tracer in terms of its properties such as penetrating power and half-life. Choice Reasons High or medium penetrating power To penetrate the ground and the e.g beta source is used. water pipe, Half-life should be short about a because short half-life is safer to few days or hours use. 7. Briefly explain how you could determine the age of a fossil using C-14 dating . The C-14 content in a living organism is constant . When it dies the quantity of C-14 decreases with time due to the decay of C-14. 20
    • Example : 1. The piece of the fossil sample is tested for C-14 activity . Let say it’s activity is 100 counts per minute. 2. The activity is compared to a fresh sample , let say the activity is 800 counts per minute. 3. Knowing the half-life of C-14 is 5 600 years, Then Using the schematic method , 800 400 200 100 5 600 years 5 600 years 5 600 years Age of fossil is = 3 T ½ = 3 x 5600 = 16 800 years The age of the fossil could be determined. 5.3 Understanding uses of radioactivity, briefly describe its use in 1. medicine a. Iodine -131 is used to check the function of the thyroid gland. The patient is given an injection of iodine -131 to act as a tracer . The activity of the iodine -131 is detected and monitored at the throat of the patient. b. Cobalt-60 gives out gamma rays. It can be used to kill bacteria during sterilization process. It can also be used to kill cancerous cells in tumors . c. Sodium -24 is injected into patient’s body and is used to detect blood clot or thrombosis. 2. agriculture a. To study the effectiveness of fertilizer . This is done using phosphorous -32 which is mixed with the fertilizer. b. To control pests . Gamma rays can kill bacteria. Gamma rays is used to cause infertility of agriculture pests and insects. c. Food control: Gamma ray from radioisotope can kill bacteria , virus, fungus etc in agriculture produce and this will ensure that the produce last longer and have longer storage. 21
    • 3. industries a. To determine water leakage in underground pipe using sodium-24 b. to study the wear and tear of a car engine using iron-59. c. beta particles are used to determine the thickness of a material. d. To check the content of food in a container or packaging using the absorption of beta particles. e. To monitor the process of mixing in tobacco industry. f. Using alpha particle from polonium-210 to reduce the risk of fire in textile factory. 5.4 Understanding nuclear energy 1. Define the atomic mass unit ( a.m.u ) or written as u. Atomic mass unit ( a.m.u ) is 1/12 the mass of the carbon-12 atom. One –carbon-12 atom is defined to be the mass of exactly 12 u. 1 a.m.u = 1.66 x 10 – 27 kg. 2. What is nuclear fission .? Explain briefly the fission of uranium -235 nucleus. Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei with the emission of radiations and enormous amount of energy. Example : 235 1 141 92 1 U + n Ba + Kr + 3 n 92 0 56 36 0 n U U n U n n U 22
    • n U n n U U n U n U n U n n U U n U Chain reaction process The uranium sample must have a certain minimum mass to sustain the chain reaction. This is known as the critical mass. The neutrons are slowed down using graphite rod so that they can be more easily captured by the uranium nuclei. The uranium-235 nucleus captures a neutron to become a uranium -236 which is highly unstable. It quickly splits into two fission fragments plus three free neutrons. 3. what is a chain reaction ? A chain reaction is a self-sustaining reaction in which the products of a reaction can initiate another similar reaction. 4. What is nuclear fusion ? Give an example . 23
    • Nuclear fusion is the combining of two lighter nuclei to form a heavier nucleus with the generation of enormous amount of energy. Normally occurs at high temperature. Example 1 : 1 2 3 H + H He + γ rays + energy. 1 1 2 Example 2 : 2 3 4 1 H + H He + n + energy 1 1 2 0 Example 3 : On the sun, 2 2 3 1 H + H He + n + energy 1 1 2 0 5. What is lost mass or mass defect in a nuclear reaction ? In both nuclear fission and fusion, the products of the reaction have a smaller mass than the mass before reaction. There is a lost mass ( mass defect ) during the reaction. The mass defect , m = Total mass before reaction – total mass after reaction. 6. Write the equation of mass and energy and define its symbols. E =mc 2 . E = energy generated in Joule 24
    • m = lost mass ( mass defect ) in kilogram C = velocity of light in ms -1. 7. Give an example using the Einstein Equation. E = mc 2. Example : Radi isotop radium-226 decay to radon-222 with the emission of alpha particle as below. Given the atomic mass of each atom as follows: 226 222 4 Ra Rn + He 88 86 2 Ra = 226.054 u Rn = 222.016 u He = 4.003 u Calculate : (a) mass defect during the nuclear reaction (b) energy generated ( 1 u = 1.66 x 10 –27 kg, speed of light ,c = 3 x 10 8 ms –1 ) E = mc 2 Total mass Rn + He = (222.016 + 4.003 )u = 226.019 u Total mass of Radium = 226.054 Lost mass( mass defect ) = (226.054 – 226.019 )u m = 0.035 u E = 0.035 x 1.66 x 10 –27 x ( 3 x 10 8 ) 2 J E =0.5229 x 10 –27 + 16J E = 0.5229 x 10 –11 J 25
    • E = 5.229 x 10 -12 J 8. Generation of electricity , describe the nuclear reactor . Dalam reaktor nuklear, tindakbalas berantai Uranium-235 berlaku pada kadar yang terkawal dan tenaga yang besar dihasilkan. Tenaga haba dari pembelahan nukleus Haba diserap oleh air dan menjadi stim pada tekanan tinggi tenaga kinetik pada turbin tenaga elektrik dalam dinamo besar. Rod pengawal, Rod grafit, rod uranium Cecair panas Stim panas ke turbin Air sejuk 26
    • Cecair (air) Pam Dinding konkrit Perisai sinaran plumbum 5.5 Realising the importance of proper management of radioactive substances 1. List the harmful effects of radiation . Radiations cause ionisations in the molecules of the living cells. The ions can cause damage to the other atoms in the cells. It can cause somatic effects and genetic effects . Table shows the effects of radiation Somatic effect Genetic Effect Radiation burns Chromosome abnormalities Organ failure Mutation of genes Blood Disorder ( Leukemia ) Birth defects Cataracts Congenital defects Death Premature death Hair loss Cancer in later life. Nausea Leukemia Fatique Genetic disorders Impotency - 2. Write down the safety precautions in the handling of radioactive substances. a. Read and follow the advice and instructions marked on radioactive sources. b. Gloves must be worn at any time while handling radioactive materials. c. Use forceps or tongs when handling. d. Wear laboratory coats and eyes protections goggles when handling. 27
    • e. Eating, drinking and smoking are prohibited while handling. f. Wash hands and forearms thoroughly after handling radioactive materials. g. Place radioactive liquids in unbreakable containers. h. Place radioactive solids in lead containers. i. Use dosimeter for radioactive workers. 3. Explain briefly the radioactive waste management involving low-level waste, intermediate –level waste and high level waste . Low level Medium Level High level It comes from hospitals, It contains about 4 % It contains about 95 industries and radioactivity. Normally % radioactivity. laboratories. It contains from nuclear reactor. about 1 % short-lived It is sealed inside radioactivity. Buried at isolated site . stainless steel Disposed off deep canisters and It is buried in shallow underground. disposed off deep landfill sites. underground. Advantages and Disadvantages of Nuclear power stations : Advantages Disadvantages Nuclear power station need less The cost to build a nuclear power fuel compare to fossil fuels to station is higher than to build an oil- produce same amount of energy. powered station The emission of carbon dioxide is Nuclear power stations produce minimal, It does not add to the waste in the form of used fuel rod. greenhouse effect, It does not produce gasses such as The hot water discharged from sulphur. nuclear power stations causes thermal pollution to the environment. The reactor in nuclear power People who work in the nuclear power stations can be used to produce station and those who live nearby useful radioisotopes, may be exposed to excessive 28
    • radiation. Efo/2007. 29