Nuclear Physics & Radioactivity VCE PHYSICS Unit 1 Topic 1.

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Nuclear Physics & RadioactivityVCE PHYSICSUnit 1Topic 1Explain why some atomic nuclei are stable and others are not.Describe the radioactive decay of unstable nuclei in terms of half life.Model radioactive decay as random decay with a particular half life, including mathematical modelling in terms of whole half lives.Apply a simple particle model of the atomic nucleus to the origin of , and radiation, including changes to the number of nucleons.Describe the detection and penetrating properties of , and radiation.Describe the effects of , and , radiation on humans including short- and long-term effects from low and high doses, external and internal sources, including absorbed dose (Gray), dose equivalence (Sieverts), and effective dose (Sieverts)Describe the effects of ionising radiation on living things and the environment.Explain nuclear transformations using decay equations involving , and radiation.Analyse decay series diagrams in terms of type of decay and stability of isotopes.Describe natural and artificial isotopes in terms of source and stability.Describe neutron absorption as one means of production of artificial radioisotopes.Identify sources of bias and error in written and other media related to nuclear physics and radioactivity.Describe the risks for living things and/or the environment associated with the use of nuclear reactions and radioactivityUnit Outline1.0 Atomic Structure.Atoms are made up of a nucleus which contains PROTONS and NEUTRONS, surrounded by ELECTRONS, circulating in groups or shells.THE HELIUM ATOMPROTONS have a mass of 1 A.M.U. ( 1 Atomic Mass Unit = 1.67 x 10-27 kg) Each carries a Positive charge of 1.6 x 10-19 Coulomb. NEUTRONS have a mass of 1 A.M.U. and carry NO CHARGE.ELECTRONS have a mass of 1/1840th of an A.M.U. (9.1 x 10-31 kg) Each carries a Negative charge of 1.6 x 10-19 Coulomb.Normal atoms are electrically neutral, thus the number of Protons = the number of Electrons. The number of neutrons varies (from 0 in Hydrogen atoms to a number much greater than the number of protons, eg Uranium atoms have 92 protons and 146 neutrons) 1.1 Atoms and IsotopesA shorthand method of representing the structure of an atom is:AXZ where, X = the elements chemical symbol A = the MASS NUMBER = total number of Protons + Neutrons in the nucleus, Z = The ATOMIC NUMBER = the number of protons in the nucleus and therefore the number of electrons grouped around the nucleus.For example an atom of Uranium can be represented as:238U92Thus, this atom contains 92 protons, 92 electrons and (238 - 92 = 146) neutrons ISOTOPES are different forms of the same element. They differ because they contain varying numbers of NEUTRONS in their nucleus. Uranium has 4 main isotopes:233U92 92 protons, 92 electrons, 141 neutrons.234U92 92 protons, 92 electrons, 142 neutrons.235U92 92 protons, 92 electrons, 143 neutrons.238U92 92 protons, 92 electrons, 146 neutrons.Atoms and Isotopes1. Fill in the blank spaces in the table2. Fill in the blank spaces in the table.1939201919243959514895232909014290210841288484214212Po84214Po84ElementMass Number Atomic Number Number of ProtonsNumber of NeutronsNumber of ElectronsPotassium ( 39K19 )Americium ( 243Am95 ) Thorium ( 232Th90 )NameNumber of protonsNumber of neutronsMass NumberAtomic NumberSymbolPolonium84126210Po84Polonium84212Polonium130841.2 Atomic and Nuclear Energy UnitsIn the large scale world energy is measured in Joules.In the small scale world of individual atomic or nuclear reactions, the Joule is too large a unit, so a smaller unit, the electron volt (eV) is used to quote energy values.By definition 1 electron volt (1 eV) is the energy obtained by 1 electron when passing through a voltage of 1 volt. Attaching metal plates to the terminals of a battery will provide a region where electrons can pass through a voltageAfter crossing between the two charged plates, the electrons energy will have increased by 1 eVIf the voltage between the plates is 1000 V the electrons energy will increase by 1 keVIf the voltage between the plates is 10 million volts, the electrons energy will increase by 10 MeV.An electron carries a charge of 1.6 x 10-19 Coulombs. When passing through a voltage of 1.0 V, its energy will increase by 1.6 x 10-19 J. So 1 eV = 1.6 x 10-19 JAtomic Energy3. Calculate the energy (in joule) an electron would gain in passing through a potential difference of 6.2 eV.4. In order to raise an electron from one energy level to another within an atom it must absorb all the energy of an incoming photon of energy 1.25 x 10-18 J. How much more energetic will the electron be after the collision ? (Quote your answer in eV)1 eV = 1.6 x 10-19 Joule. So 6.2 eV = (6.2)(1.6 x 10-19) J = 9.92 x 10-19 J 1.6 x 10-19 J = 1 eV. So 1.25 x 10-18 J = (1.25 x 10-18/1.6 x 10-19 ) eV= 7.8 eV 1.3 Uranium - Mining & EnrichmentUranium ore is mined and processed at the mine site into a greeny-yellow coloured solid material called YELLOWCAKE. Chemically, yellowcake is Uranium Oxide - U3O8This material is packed into 200 L drums and exported to overseas uranium processing plants.The U3O8 is made up of 2 isotopes; 99.3% 238U and 0.7% 235U.It is the 235U which is the desired product. It is this uranium isotope which is FISSIONABLE (able to be split apart) by slow or thermal neutrons (with energies < 5eV)In order to sustain a Nuclear Chain Reaction (see Slide 1.4) in a nuclear reactor or nuclear weapon, the proportion of 235U needs to be increased. This is achieved by the ENRICHMENT process.Nuclear reactor fuel needs to be enriched to about 3% to 4% 235UNuclear weapons fuel needs to be enriched to 90% 235U.Approximately 17 kg of 235U is needed to produce an effective weapon.However, only 4 kg of 90% pure PLUTONIUM (239Pu) is needed to produce an equally effective weapon.Uranium7. The enrichment processA: Increases the proportion of 234U in the sampleB: Increases the proportion of 235U in the sampleC: Increases the proportion of 238U in the sampleD: Increases the proportion of all the isotopes in the sample5. What is the chemical composition of yellowcake ? U3O86. Naturally occurring Uranium ore containsA: 4 Isotopes of UraniumB: 3 Isotopes of UraniumC: 2 Isotopes of UraniumD: Only a single isotopic form of Uranium8. Nuclear reactor fuel needs the proportion of 235U in the fuel sample to be at least A: 3% to 5% of the total B: 10% to 12% of the totalC: 25% to 30% of the totalD: 50% to 75% of the sample1.4 Fissile MaterialsAny nucleus capable of undergoing fission is called a FISSILE MATERIAL.The main fissile materials known are: 233U92, 235U92 and 239Pu94 are more likely to undergo fission by capture of slow (< 5 eV) neutrons. 238U92 and 232Th90 need fast neutrons (> 1 MeV) to undergo fission.235U92,239Pu94,238U92and 232Th90Fission is defined as the splitting of atomic nuclei233U92,1.5 Nuclear FissionWhen slow neutron collides head on with a 235U atom, the nucleus undergoes fission . It splits into 2 fission products with atomic numbers approximately half that of the original 235U, PLUS (on average) 2.5 Neutrons PLUS (on average) 160 - 200 MeV of energy.Both Uranium isotopes are capable of being fissioned by neutrons:235U is fissioned by neutrons of all energies with a high probability of fission by low energy (< 5 eV), thermal neutrons. 238U is fissioned by fast neutrons (>1 MeV). It captures neutrons of lesser energy without suffering fission.The products shown here are typical but not unique, many combinations of product nuclei are possible, with Atomic Nos ranging from 34 to 74.Shown on the left is a typical fission process initiated by a neutron with the first target nucleus splitting to release further neutrons.Fission10. What are the products of the nuclear fission of 235U ?9. Define nuclear fission.Fission is defined as the splitting of atomic nuclei 235U splits into 2 fission products with atomic numbers approximately half that of the original 235U, PLUS (on average) 2.5 Neutrons PLUS (on average) 160 - 200 MeV of energy. 1.6 Mass into EnergyThe typical 235U fission as mentioned on a previous slide is: Adding up the mass of the reactants (measured in a.m.u.s), we get: 1.0087 + 235.0439 = 236.0526 a.m.u.Adding the masses of the products we get: 140.9139 + 91.8973 + 3.0261 = 235.8373 a.m.u.The mass of the products is 0.2153 a.m.u. LESS than the mass of the reactants.This lost mass has been converted to energy, the amount of which can be calculated from the Einsteins famous equation E = mc21 a.m.u. = 1.66 x 10-27 kg. 0.2153 a.m.u. = 3.57 x 10-28 kg. E = (3.57 x 10-28)(3.0 x 108)2= 3.2 x 10-11 JConverting this energy in Joules to energy in eV we get:3.2 x 10-11/1.6 x 10-19 = 2.0 X 108 eV = 200 MeVThus EACH fission of a 235U nucleus releases about 200 MeV of energy, initially as Kinetic Energy of the fragments which is then converted to Heat Energy by collisions with other nuclei.This heat energy is used to create steam to spin a turbine which drives a generator producing electricity.Mass into Energy11. The energy released in the nuclear fission process arises from the conversion of what to energy ?12. What equation is used to convert mass to energy ? Who formulated this equation ? 13. Show that 0.5 amu is the equivalent of 478 MeV of energy Note: (1 amu = 1.67 x 10-27 kg)In nuclear fission mass is converted into energyE = mc2 , Equation formulated by Albert Einstein 0.5 a.m.u. = (0.5)(1.67 x 10-27) kg = 8.5 x 10-28 kg. E = mc2 = (8.5 x 10-28)(3 x 108)2 = 7.65 x 10-11 JNow 7.65 x 10-11 J = (7.65 x 10-11)/(1.6 x 10-19) eV = 4.78 x 108 eV= 478 MeV Chapter 2Topics covered:Neutron Moderation.Chain Reactions.Critical Mass.Neutron Flux.Neutron Absorption by 238U2.0 Neutron ModerationThe neutrons produced by a 235U fission are high energy fast neutrons. This is achieved using a MODERATOR.To increase the likelihood that these neutrons go on to cause further fissions of 235U nuclei, they must be slowed down.The most commonly used moderators are Graphite (C), Heavy Water (D2O), and Light Water (H2O).Moderators are all low Atomic Weight materials which will suffer a large recoil when hit by a neutron. This large recoil takes a large amount of Kinetic Energy from the neutron slowing it sufficiently for it to become a slow neutron.This slow neutron MAY then go on to collide with another 235U nucleus, setting up a so called chain reaction. Moderation14. What is the moderation process used for in nuclear reactors ? 15. Name 3 materials that can be used to moderate fast neutrons.Moderation is used to slow neutrons down to thermal energies so they are capable of initiating further fissions of 235U Graphite, Light Water, Heavy Water 2.1 Chain ReactionsIn order to produce a nuclear chain reaction, the neutrons liberated from the first fission must go on to produce further fissions.Slow neutron escapes,no further fissionSlow neutroncapture, further fissionIn a nuclear reactor, with enriched fuel, the chain reaction is controlled, so only one of the liberated neutrons goes on to produce one further fission, as shown above.In a nuclear weapon, with highly enriched fuel, the chain reaction is uncontrolled, so every liberated neutron goes on to produce further fissions. LOTS OF ENERGY IS RELEASED VERY QUICKLY.In naturally occurring Uranium (with 99.3% 238U and 0.7% 235U), a chain reaction is not possible. Too many neutrons will be lost through the first two mechanisms above. Further fissions are not guaranteed because the neutrons initially released may behave in a number of different ways. For example:-Chain Reactions16. How are the chain reactions in a nuclear reactor and a nuclear weapon different ?In reactors the chain reaction is strictly controlled while in weapons it is totally uncontrolled2.2 Critical Mass For a chain reaction (of 235U fissions) to occur, there needs to be enough 235U nuclei present in the sample so that the released neutrons from the first fission find a target 235U nucleus and those subsequently released also find targets.In other words, there exists a lower limit of 235U distribution in a sample, below which a chain reaction cannot be sustained.This lower limit is called the CRITICAL MASS. It is the mass of material below which a chain reaction cannot be supported.A sample of material below the Critical Mass is said to be Sub CriticalWhenever fissile material is transported around the world it is always moved in sub critical amounts.Critical Mass for 235U (as weapons fuel) is approximately 8 kg. 2.3 Neutron FluxIn all the various fission reactions which 235U undergoes, there are, on AVERAGE, 2.5 neutrons per fission produced.The number of neutrons actually available to initiate further fissions is called the NEUTRON FLUX .SUB CRITICAL Will not support a Chain Reaction.CRITICAL Will just sustain a Chain Reaction (as in a Nuclear Reactor). SUPER CRITICAL Will lead to an uncontrolled Chain Reaction (as in a Nuclear Weapon).By variation of the SIZE, SHAPE and PURITY of the 235U sample and by controlling the number of neutrons available throughGeometry, Neutron Speed andNeutron Absorption, it is possible to organize the neutron flux to create one or more of the following conditions:Neutron Flux17. Define Critical Mass18. What is neutron flux ?19. What factors affect neutron flux ?Critical Mass is It is the mass of radioactive material below which a chain reaction cannot be supported.The number of neutrons actually available to initiate further fissions is called the neutron flux . Neutron flux can be controlled by variation of the SIZE, SHAPE and PURITY of the 235U sample and by controlling the number of neutrons available through:GeometryNeutron Speed andNeutron Absorption. 2.4 Neutron Absorption by 238USome fissile materials can absorb neutrons and NOT undergo fission. Instead, the material will undergo decay producing a nucleus with a higher atomic number which itself is fissile. For example 238U can absorb a neutron to produce 239Pu, via the process:The Plutonium can then undergo a fission reaction (initiated by a slow neutron) in much the same way as 235U does, yielding, on average, 3 more neutrons and 210 Mev of energy: A substance like 238U which can be converted into a fissionable material is called a FERTILE material.Chapter 3Topics coveredThermal ReactorsBreeder Reactors3.0 Thermal Nuclear Reactors Normal thermal nuclear reactors use the heat generated by the fission reaction to produce steam to drive a generator to produce electricity. Any thermal reactor requires the following components:Fuel in the form of fuel rods which contain 235U.A Moderator used to slow down fast neutrons.Control rods which capture neutrons allowing for reactor control.Coolant to carry heat away from the reactor core.Radiation Shield to protect operators from lethal radiation. 3.1 Typical Reactors3.2 Breeder Reactors75% 235UBlanket ofNatural or depleted UraniumCoreLiquid SodiumCoolantHeat exchangerControl RodsWaterSteamBreeder reactors require different fuel to thermal reactors.They do not have a moderator, the core is surrounded by a blanket of natural or depleted uranium, which will capture fast neutrons from the core, producing 239Pu.They are cooled using liquid sodium.Reactors20. What are the 5 main requirements for a thermal nuclear reactor ?21. How are breeder reactors different from thermal reactors ?Fuel in the form of fuel rods which contain 235U.A Moderator used to slow down fast neutrons.Control Rods which capture neutrons allowing for reactor control.Coolant to carry heat away from the reactor core.A Radiation Shield to protect operators from lethal radiation. Thermal reactors consume their fuel whereas breeder reactors generate more fuel than they originally had.Chapter 4Topics covered:Nuclear weaponsFission BombsFusion Bombs Neutron Bombs4.0 Nuclear Weapons Fission Bombs (1)When fired the 2 Uranium masses are brought together to form a super critical mass in which an uncontrolled chain reaction occurs. The explosion will occur within 10-6 sec of the masses being brought together.The first of the nuclear weapons to be developed Little Boy was dropped on Hiroshima on August 6th 1945.4.1 Nuclear Weapons Fission Bombs (2)The second nuclear weapon used, called Fat Man was dropped on Nagasaki on August 9th 1945.It was a Plutonium Implosion fission device.Shells of 238U and Beryllium surrounded the core to reduce neutron loss.A large number of conventional TNT charges, exploded simultaneously, compressed the 239Pu into a small supercritical mass, which produced the uncontrolled chain reaction leading to the explosion. It consisted of a large number of sub critical masses of 239Pu4.2 Nuclear Weapons Fusion Bombs Often called Hydrogen Bombs or Thermonuclear Weapons, these weapons rely on the Fusion (as in our sun), where heavy isotopes of Hydrogen (Deuterium and Tritium) fuse together to form Helium releasing massive amounts of energy. ADVANTAGES 1. Produce less radioactive fallout than fission bombs 2. Raw materials are readily available.DISADVANTAGES 1. Hard to start the fusion reaction. A conventional Fission starter bomb is used to produce the required temperature to get the fusion started.4.3 Nuclear Weapons Neutron BombsWhen exploded, usually in the air above the target, a small blast (relative to other nuclear bombs), releases large amounts of fast neutrons and rays.Blast damage is restricted to a radius of about 0.3 km, but they deliver a lethal radiation dose to people over a radius of approx 1.2 km.They are regarded as clean bombs because they produce little long lived fallout, leaving the blast area safe to enter after a few days.This bomb is designed to inflict minimum property damage while, at the same time causing maximum loss of life.They operate in much the same way as fusion bombs without the outer casing of 238U. Nuclear Weapons22. What is the difference between the first nuclear weapons (little boy and fat man) and thermonuclear and neutron weapons ?23. Why do military planners prefer Neutron Bombs ?The origial weapons were fission weapons whereas the thermonuclear weapons rely on fusion for releasing energyNeutron bombs are designed to inflict minimum property damage while, at the same time causing maximum loss of life. Chapter 5Topics covered:Radiation Radiation Radiation RadiationDecay ProcessesDetection of Radiation5.0 RadiationRADIATION is a general term used to describe the exposure of earthly beings to the ELECTROMAGNETIC SPECTRUM.Radiation can be broken up into two general types:(a) NON - IONISING RADIATION with frequencies below about 1016 Hz.(b) IONISING RADIATION with frequencies above 1016 Hz. The difference between these two types of radiation is that below 1016 Hz the radiation is not energetic enough to strip 1 or more outer shell electrons from atoms it contacts, whereas above 1016 Hz the radiation is energetic enough to strip electrons, forming highly reactive IONS. Radiation24. What are the two general forms of radiation ? What frequency divides one type from the other ?25. What types of radiation the fall into the Ionizing category ?Ionising and Non ionising radiation, dividing frequency 1016 HzPart of the UV spectrum, X Rays, Cosmic Rays 5.1 Alpha () RadiationN.B. The forms of radiation mentioned subsequently arise from processes which occur WITHIN the NUCLEUS of the atom and DO NOT involve the electrons which circulate around the nucleus. radiation consists of a package of 2 protons and 2 neutrons ejected from the nucleus of an atom.The package is, in fact, a Helium Nucleus (He2+)The package is ejected at approximately 0.1c, 10% of the speed of light, a relatively slow speed. The particle has a range of a few centimetres in air and can be easily stopped by a piece of paper or a layer of skin.Since the particle carries a charge (2+), its path through space can be affected by electric and magnetic fields Sources of particles are harmless outside the body but very dangerous if ingested. They are a form of ionising radiation which cause internal body damage by ionising large numbers of atoms and/or compounds around the point of lodgement. This ionisation disrupts the normal operation of the cells made up of these atoms or compounds.When an unstable atom emits an particle, its atomic number falls by 2 and mass number falls by 4. Thus 238U92 will decay to 234Th90 238U92 234Th90 + 4He2Alpha Radiation26. What change to mass number and atomic number occur when an alpha particle is emitted from a radioactive nucleus ?27. List 3 properties of alpha radiation 28. 210Po84 undergoes alpha decay to produce an isotope of lead (Pb). Write the equation for this decay. particle emission Mass No goes down 4, Atomic No goes down 2 Any 3 of: ejected at 10% of speed of light; has a range of a few cm in air; can be stopped by a piece of paper or a layer of skin; sources harmless outside the body but dangerous if ingested; will cause ionisation at site of lodgement210Po84 206Pb82 + 5.2 Beta () Radiation radiation consists of a stream of charged particles, which can carry either a single negative or positive charge.- particles are ELECTRONS, + particles are POSITRONS.In the case of - radiation, a NEUTRON within the nucleus of an unstable atom converts to a PROTON (which remains in the nucleus) and an ELECTRON (which is ejected from the nucleus), plus an antineutrinonp + -The change of a neutron to a proton means the Atomic Number will go up by 1, while the Mass Number remains unchanged.If Thorium undergoes - decay, it forms an isotope of Protactinium:234Th90234Pa91+ -In the case of + radiation, a PROTON within the nucleus of an unstable atom converts to a NEUTRON (which remains in the nucleus) and a POSITRON (which is ejected from the nucleus), plus a neutrinop n + + The change of a proton to a neutron means Atomic Number will go down by 1, while the Mass Number remains unchanged. If Thorium undergoes + decay, it forms an isotope of Actinium:234Th90234Ac89+ ++ -+ -+ ++ +(Neutrinos and antineutrinos are neutral (non charged) particles with a very small mass that travel near the speed of light. They are produced in decay but knowledge of their properties in not part of the course)5.3 The Nature of RadiationBeta () radiation, whether a stream of positrons or electrons, is ejected from the nucleus at approximately 0.9c (90% of the speed of light). Being both much smaller and more energetic , they have much greater penetrating power than particles. They can be stopped by several sheets of paper or a thin sheet of Aluminium.Being charged particles, their path through space can be affected by electric and magnetic fields.This ionisation disrupts the normal operation of the atoms and compound in the cells made up of these atoms and compounds.They have a longer range (approx 1.0 m) in air than particles.Sources of particles are relatively harmless outside the body, but extremely dangerous if ingested.They are a form of ionising radiation (less able to ionise than s due to lesser mass) and cause internal body damage by ionising the atoms and compounds close to the point of lodgement.Beta Radiation29. What type of material can be used as a safety shield to protect a person from beta radiation ?30. With what speed are beta particles emitted from the nucleus ?31. Which of the nucleons undergoes change in the production of - radiation ? Write an equation for this process32. Each of the following radioactive elements undergo beta minus decay. Write the equations for each decay. (Element No 7 is Nitrogen (N), No 39 is Yttrium (Y), No 16 is Sulphur (S)(a) 14C6, (b) 90Sr38, (c) 32P15Several sheets of paper or a thin piece of aluminium 90% of the speed of light A neutron converts to a proton plus an electron plus an antineutrino n p+ + e- + v-14C6 14N7 + , 90Sr38 90Y39 + , (c) 32P15 32S16 + Positrons33. What are positrons and how are they formed ? What affect does the formation of a positron have on the Mass Number and Atomic Number ?Positrons are positively charged electrons. They are formed when a proton converts to a neutron a positron (ejected) plus a neutrino. Mass number is unchanged, Atomic number goes down by 1. 5.4 Gamma () RadiationGamma Radiation is NOT made up of a stream of particles, but is simply a form of electromagnetic radiation like X rays, Microwaves or U.V. radiation.In contrast to and radiation, radiation has NO MASS. radiation is ejected from the nucleus at c, the speed of light.Having no charge, radiation is NOT affected by electric or magnetic fields.Because of its speed, radiation is extremely penetrating.It has less ionising ability than radiation but still extremely dangerous because of its penetrating ability.The Star (*) represents a nucleus with excess energyIt is difficult to stop, easily passing through a few cm of lead. radiation arises from atomic nuclei that have excess energy. This excess energy is given up by the nucleus by emitting radiation. emission does not change the composition of the nucleus so no new products are formed. emission is shown thus:5.5 Penetrating Power Each form of radiation has varying penetrating power as shown 5.6 Effects of Magnetic Fields and particles carry an electric charge and so their paths through space can be affected by the presence of a magnetic field. Having no charge, radiation is NOT affected by a magnetic field.Gamma Radiation34. How is gamma radiation different from alpha and beta radiation ?35. List 3 properties of gamma radiation radiation is pure energy whereas and are matter Any 3 of: Has no mass; ejected at the speed of light; extreme penetrating power; stopped by several cm of lead; often accompanies and emission; emission causes no change to nucleus, no new products formedGeneral Radiation36. List the known forms of radiation in order from least to most penetrating. 37. Would the path followed by a stream of neutrons be affected by the presence of a magnetic field? Explain your answer., , and neutrons Neutrons carry no electric charge so their path would not be affected by a magnetic field5.7 Decay ProcessesRadioactivity is defined as the spontaneous and uncontrollable decay of a nucleus resulting in the emission of particles or rays.Nuclear decay processes occur because the nucleus is unstable. In an attempt to reach a more stable configuration, it may eject matter ( or particles) or energy ( rays).When an atom undergoes a radioactive decay process, it may give out a single , or to achieve stability or it may undergo a series of decays giving out any or all particles or rays to finally reach a stable end product.234Th90 234Pa91234U92230Th90226Ra88222Rn86218Po84214Pb82214Bi83214Po84210Pb82210Bi83208Tl81208Pb82Times given below arrow = half life, wheres = sec; m = min; h = hours; d = day; y = year(See Slide 6.2)There a number of well documented Radioactive Decay Series, a standard set of pathways followed by various radioactive nuclei which decay through a number of steps to a final non-radioactive stable end product.The three most common of these are called: The Uranium Series, The Thorium Series and the Actinium Series.The Thorium series is shown.Radioactivity38. Define radioactivity39. What is a "radioactive decay series "? How many series exist ?Radioactivity is defined as the spontaneous and uncontrollable decay of a nucleus resulting in the emission of matter or energy. A radioactive decay series is a set of pathways (ejection processes) that radioactive nuclei follow in the process of searching for stability. There a 3 known series, Uranium, Thorium and Actinium series5.8 Detection of Radiation (1)Radiation cannot be seen or felt, so methods of detecting the presence of radiation are needed. A number of detectors are commonly used.1. THE GEIGER COUNTER - This consists of a detection tube and associated electronics. Ionising radiation enters the tube through a mica window causing the neon gas in the detector to become ionised.The gas ions enable a current to flow between charged plates in the detector and the electronics amplify this to produce a signal or click.The geiger counter is capable of detecting ALL types of radiation.5.9 Detection of Radiation (2)2. FILM BADGES - These are worn on the lapel and contain an ordinary piece of photographic film. The film blackens in the presence of radiation and the extent of blackening is a measure of the extent of exposure. 3. T.L.D. - THERMOLUMINESCENT DOSIMETER - Again worn on the lapel, this device contains the fluorides of either Lithium or Calcium which have the property of storing a small amount of radiation energy when exposed to radiation. If the Dosimeter is subsequently heated it releases the stored energy as visible light.The amount of light released is a measure of the dose of radiation.Radiation Detection40. Name 3 devices that can be used to detect the presence of nuclear radiation.(a) Geiger Counter, (b) Thermoluminescent Dosimeter, (c) Film Badges Chapter 6Topics covered:Stable & Unstable Isotopes.Natural & Artificial Radioactive Isotopes.Half Life.6.0 Stable & Unstable IsotopesAtomic nuclei with approximately equal numbers of protons and neutrons, are generally stable with no tendency to emit either matter or energy. This is true for elements with atomic numbers up to approximately Z = 40. (Hydrogen to Zirconium).The inherent stability of these nuclei is because the electric repulsive force between the protons is balanced by the strong nuclear force which binds the nucleus together. These statements are only generalisations, as there are isotopes with Z < 40 which are radioactive, (eg 24Mg12) and there are nuclei with Z > 40 which are stable and not radioactive, (eg 208Pb82).Above Atomic No 82 most nuclei are unstable and naturally emit radiation either as mass ( or particles) or energy ( rays)6.1 Natural & Artificial Radioactive IsotopesNaturally occurring radioactive nuclei are those which spontaneously emit mass or energy. They are generally the heavier nuclei, with Z > 82.It is possible to produce unstable nuclei from stable ones by bombardment with neutrons (n).Cobalt 59 when irradiated with neutrons forms Cobalt 60 Cobalt 60 then undergoes - decayCobalt 60 is a - source and is used in both cancer treatment and food irradiation.Nuclear Stability41. What ratio of nucleons within the nucleus leads to stability - that is no tendency to emit mass or energy ?42. Above what mass number are nuclei generally unstable and likely to be radioactive ?Atomic nuclei with equal numbers of protons and neutrons are generally stableAbove Atomic Number 826.2 Half LifeThe rate at which spontaneous radioactive decay occurs is dependent on the Half Life (t1/2) of the radioactive species.Half Life (t1/2) is the time it takes for half of the radioactive nuclei in a sample to decay, or the time taken for the disintegration rate to drop by one half.Half lives range from less than 10-20 sec to more than 1020 sec (approx 1012 years) .N0 = Original Number of Radioactive Nuclei in the Sample Thus, after 1 half life,1/2 of the original nuclei are left.After 2 half lives, 1/4 of the original sample is left.After 3 half lives, 1/8 of the original sample is left, etc.This trend indicates an inverse relationship between the number of nuclei and time.When the number of radioactive nuclei have fallen to half the original number, one half life has passed.Half Life44. Iodine 131 (131I53) is a radioactive isotope used in medical diagnosis. It has a half life of 8 days. If a sample containing 100 g of Iodine 131 is delivered to a hospital, how many g of Iodine 131 will be left in the sample after 48 days ? 43. Define half life.Half Life is the time it takes for half the radioactive species present in the original sample to decay. 48 days = 6 half lives. Original sample had 100 g after 1 t1/2 50 g left, after 2 t1/2 25 g left, after 3 t1/2 12.5 g left, after 4 t1/2 6.25 g left, after 5 t1/2 3.125 g left, after 6 t1/2 1.56g left Chapter 7Topics covered:Radiation Dose.Quality Factor.Dose Equivalent.Effect of Radiation on Humans.7.0 Radiation DoseIn all the applications of radiation in medicine, industry and research, it is important to know the exact dose of radiation which has been absorbed.The amount of radiation absorbed by the target is called the Radiation Dose or The Energy Absorbed Per Kilogram of object at the target site.The unit used to measure Radiation Dose is called the GREY (Gy) (where 1 Gy = 1 7.1 The Quality FactorThe effects of the various types of radiation on living cells depends upon the ionising ability of the radiation.Since the ionising ability of radiation is much greater than either or radiation, there needs to be an adjustment to the radiation dose to accommodate this difference.This is accomplished by the Quality Factor (Q.F.), which is a form of weighting assigned to the radiation dose. For and radiation the Q.F. = 1 For radiation the Q.F. = 20For Neutrons or Protons separately the Q.F. = 10Radiation Exposure45. Name two forms of external radiation exposure.46. What is the unit used for Radiation Dose ?47. What is the quality factor used for ?48. How is the dose equivalent calculated ? What unit measures dose equivalent ?Any 2 of: gamma rays particlarly during air travel; radioactive soils; background radiation (some of which is leftover from the big bang); radon gas leaching from disturbed soils. The S.I. unit for radiation dose is the GREY where 1 Gy = 1 Jkg-1The Quality Factor is used to adjust the radiation dose to take account of the varying ionising abilities of the different forms of radiation. Dose equivalent = Absorbed Dose x Quality FactorDose Equivalent is measured in Sieverts (Sv)7.2 Dose EquivalentAltering the radiation dose by the quality factor leads to a radiation measure called the Dose Equivalent measured in Sieverts (Sv). Thus:Dose equivalent (Sv) = Absorbed Dose (Gy) x Quality FactorThe Sievert is a large unit and dose equivalents are often quoted in millisieverts (mSv)In the past may different measures of radiation dose have been used .Eg. The REM (1 REM = 0.01 Sv)The RAD is a smaller unit of Absorbed Dose (1 RAD = 0.01 Gy)Another superseded unit is the ROENTGEN (1RN = 1Gy)Background Radiation49. What is the major contributor to background radiation exposure ?50. What is the average dose equivalent from all forms of background radiation per year ?48% of background radiation is Radon Gas 1 to 3 mSv per year7.3 Radiation Effects on HumansLOW LEVEL (BACKGROUND) EXPOSUREAll of us are exposed to low levels of radiation because we live on Earth. This exposure comes from both naturally occuring and man made sources, such as chest and dental X rays and Cosmic Rays reaching the Earths surface.In total these sources give us an exposure of 1 to 3 mSv per year.HIGH LEVEL EXPOSURE1. Doses of 100 Sv or greater will cause death within hours to days by causing damage to the central nervous system.2. Doses of 10 to 50 Sv will cause death within 1 to 2 weeks by causing damage to the gastrointestinal tract.3. Doses of 3 to 5 Sv will cause death in 50% of those exposed within 1 to 2 months due to bone marrow damage.International conventions set desirable limits for exposure (above those due to background radiation). The whole body dose for the general public is set at 1 mSv/year.For workers in the nuclear industry the limit is set at 50 mSv/year. 7.4 Effects of Neutron Radiation on HumansApart from some medical neutron generators, the greatest source of neutron radiation in our environment comes from man made fission reactions. Neutrons have NO electric charge and thus cannot produce ionizations directly. Neutrons do, however, have a high probability of interacting with Hydrogen nuclei (protons).This process then goes on to produce unwanted ionizations. Our bodies have an abundance of Hydrogen either as water or complex organic compounds.A 10 mSv dose of fast neutron radiation is 5 times more likely to cause cataracts than a similar dose of radiation.A 6.5 Sv dose of neutron radiation is lethal. Death will occur within a few days.Deadly Radiation51. What dose equivalent exposure level leads to death within hours to days ?52. How many times greater is the international standard for radiation exposure per year for nuclear industry workers compared to the general public ?53. Which form of radiation is the deadliest ?Doses of 100 Sv or greaterThe limit is 50 times greaterNeutron radiation is the deadliest form of radiationOllie Leitl 2003


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