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Nuclear Physics & Radioactivity

Nuclear Physics & Radioactivity. VCE PHYSICS Unit 1 Topic 1. Unit Outline. Explain why some atomic nuclei are stable and others are not. Describe the radioactive decay of unstable nuclei in terms of half life.

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Nuclear Physics & Radioactivity

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  1. Nuclear Physics & Radioactivity VCE PHYSICS Unit 1 Topic 1

  2. Unit Outline • Explain 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 α, β and γ radiation, including changes to the number of nucleons. • Describe the detection and penetrating properties of α, β and γ radiation. • Describe the effects of α, β and γ, 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 α, β and γ 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 radioactivity

  3. 1.0 Atomic Structure. P N P N e- e- NUCLEUS THE HELIUM ATOM Atoms are made up of a nucleus which contains PROTONS and NEUTRONS, surrounded by ELECTRONS, circulating in groups or “shells”. 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. PROTONS 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. 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) NEUTRONS have a mass of 1 A.M.U. and carry NO CHARGE.

  4. 1.1 Atoms and Isotopes A shorthand method of representing the structure of an atom is: AXZ For example an atom of Uranium can be represented as: 238U92 Thus, this atom contains 92 protons, 92 electrons and (238 - 92 = 146) neutrons where, X = the element’s 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. 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.

  5. Atoms and Isotopes 1. Fill in the blank spaces in the table 19 19 19 20 39 95 148 243 95 95 232 90 90 142 90 2. Fill in the blank spaces in the table. 210 84 212Po84 128 84 214Po84 214 84

  6. - + e e e e e 1 V 0 V 1.0 Volt Battery 1.2 Atomic and Nuclear Energy Units In 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 voltage After crossing between the two charged plates, the electron’s energy will have increased by 1 eV If the voltage between the plates is 1000 V the electron’s energy will increase by 1 keV If the voltage between the plates is 10 million volts, the electron’s 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 J

  7. Atomic Energy 3. Calculate the energy (in joule) an electron would gain in passing through a potential difference of 6.2 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 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) 1.6 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

  8. 1.3 Uranium - Mining & Enrichment Uranium ore is mined and processed at the mine site into a greeny-yellow coloured solid material called YELLOWCAKE. Chemically, yellowcake is Uranium Oxide - U3O8 This material is packed into 200 L drums and exported to overseas uranium processing plants. 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% 235U Nuclear 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. 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)

  9. Uranium 5. What is the chemical composition of yellowcake ? U3O8 6. Naturally occurring Uranium ore contains A: 4 Isotopes of Uranium B: 3 Isotopes of Uranium C: 2 Isotopes of Uranium D: Only a single isotopic form of Uranium 7. The enrichment process A: Increases the proportion of 234U in the sample B: Increases the proportion of 235U in the sample C: Increases the proportion of 238U in the sample D: Increases the proportion of all the isotopes in the sample 8. 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 total C: 25% to 30% of the total D: 50% to 75% of the sample

  10. 1.4 Fissile Materials Fission is defined as the “splitting of atomic nuclei” Any nucleus capable of undergoing fission is called a FISSILE MATERIAL. The main fissile materials known are: and 232Th90 238U92 235U92, 239Pu94, 233U92, 233U92, 235U92and 239Pu94are more likely to undergo fission by capture of “slow” (< 5 eV) neutrons. 238U92 and 232Th90 need “fast” neutrons (> 1 MeV) to undergo fission.

  11. 1n0 + 235U92141Ba56 + 92Kr36 + 3 1n0 + Energy 1.5 Nuclear Fission 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. When 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. The products shown here are typical but not unique, many combinations ofproduct nuclei are possible, with Atomic No’s 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.

  12. Fission 9. Define nuclear fission. Fission is defined as the “splitting” of atomic nuclei 10. What are the products of the nuclear fission of 235U ? 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.

  13. 1.6 Mass into Energy 1n0+ 235U92141Ba56 + 92Kr36 + 3 1n0 + Energy The 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 Einstein’s famous equation E = mc2 1 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 J Converting 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 MeV Thus 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.

  14. Mass into Energy 11. The energy released in the nuclear fission process arises from the conversion of what to energy ? In nuclear fission mass is converted into energy 12. What equation is used to convert mass to energy ? Who formulated this equation ? E = mc2 , Equation formulated by Albert Einstein 13. Show that 0.5 amu is the equivalent of 478 MeV of energy Note: (1 amu = 1.67 x 10-27 kg) 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 J Now 7.65 x 10-11 J = (7.65 x 10-11)/(1.6 x 10-19) eV = 4.78 x 108 eV = 478 MeV

  15. Chapter 2 Topics covered: • Neutron Moderation. • Chain Reactions. • Critical Mass. • Neutron Flux. • Neutron Absorption by 238U

  16. Slow Neutrons After Fission of 235U n 92Kr n n 141Ba Fast Neutrons Moderator eg. Graphite D2O 2.0 Neutron Moderation The neutrons produced by a 235U fission are high energy “fast” neutrons. To increase the likelihood that these neutrons go on to cause further fissions of 235U nuclei, they must be slowed down. This is achieved using a MODERATOR. 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”.

  17. Moderation 14. What is the moderation process used for in nuclear reactors ? Moderation is used to slow neutrons down to thermal energies so they are capable of initiating further fissions of 235U 15. Name 3 materials that can be used to “moderate” fast neutrons. Graphite, Light Water, Heavy Water

  18. 2.1 Chain Reactions n 92Kr n 235U Slow neutron capture no further fission n First fission 141Ba 238U n n 235U n Ba Moderator Kr n In order to produce a nuclear “chain reaction”, the neutrons liberated from the first fission must go on to produce further fissions. Further fissions are not guaranteed because the neutrons initially released may behave in a number of different ways. For example:- Slow neutron escapes, no further fission Slow neutron capture, further fission In 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.

  19. Chain Reactions 16. 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 uncontrolled

  20. 2.2 Critical Mass 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 Critical Whenever fissile material is transported around the world it is always moved in sub critical amounts. 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. Critical Mass for 235U (as weapons fuel) is approximately 8 kg.

  21. 2.3 Neutron Flux In 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 . • By variation of the SIZE, SHAPE and PURITY of the 235U sample and by controlling the number of neutrons available through • Geometry, • Neutron Speed and • Neutron Absorption, • it is possible to organize the neutron flux to create one or more of the following conditions: 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).

  22. Neutron Flux 17. Define Critical Mass Critical Mass is It is the mass of radioactive material below which a chain reaction cannot be supported. 18. What is neutron flux ? The number of neutrons actually available to initiate further fissions is called the neutron flux . 19. What factors affect 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: Geometry Neutron Speed and Neutron Absorption.

  23. 2.4 Neutron Absorption by 238U 239U92 +  238U92 + n 239N p93239Pu94 +  +  239Pu + n 133Cs + 103 Ru + 3 n + 210 MeV Some 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: A substance like 238U which can be converted into a fissionable material is called a FERTILE material. 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:

  24. Chapter 3 Topics covered • Thermal Reactors • Breeder Reactors

  25. 3.0 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.

  26. 3.1 Typical Reactors A PWR (Pressurised Water Reactor) Reactor A BWR (Boiling Water Reactor) Reactor

  27. 3.2 Breeder Reactors Breeder 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. Control Rods Heat exchanger Steam Blanket of Natural or depleted Uranium 75% 235U Water Core Liquid Sodium Coolant

  28. Reactors 20. What are the 5 main requirements for a thermal nuclear reactor ? • 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. 21. How are breeder reactors different from thermal reactors ? Thermal reactors consume their fuel whereas breeder reactors generate more fuel than they originally had.

  29. Chapter 4 Topics covered: • Nuclear weapons • Fission Bombs • Fusion Bombs • Neutron Bombs

  30. 4.0 Nuclear Weapons – Fission Bombs (1) Hard Metal Casing Hard Metal Casing Hard Metal Casing Hard Metal Casing Hard Metal Casing Uranium or Fission Bomb Sub Critical Masses of 235U Fuel Conventional Explosive (TNT) Conventional Explosive (TNT) Conventional Explosive (TNT) Conventional Explosive (TNT) Conventional Explosive (TNT) Uranium or Fission Bomb Sub Critical Masses of 235U Fuel Uranium or Fission Bomb Uranium or Fission Bomb Uranium or Fission Bomb Sub Critical Masses of Sub Critical Masses of Sub Critical Masses of 235U Fuel 235U Fuel Neutron Source Neutron Source Neutron Source Neutron Source Neutron Source 235U Fuel The first of the nuclear weapons to be developed – “Little Boy” was dropped on Hiroshima on August 6th 1945. 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.

  31. Fat Man 4.1 Nuclear Weapons Fission Bombs (2) 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. The second nuclear weapon used, called “Fat Man” was dropped on Nagasaki on August 9th 1945. It was a Plutonium Implosion fission device. It consisted of a large number of sub critical masses of 239Pu Shells of 238U and Beryllium surrounded the core to reduce neutron loss.

  32. atomicarchive.com 4.2 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.

  33. Conventional Explosive (TNT) Lithium Deuteride 238 U 239 Pu 4.3 Nuclear Weapons Neutron Bombs When 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.

  34. Nuclear Weapons 22. What is the difference between the first nuclear weapons (little boy and fat man) and thermonuclear and neutron weapons ? The origial weapons were fission weapons whereas the thermonuclear weapons rely on fusion for releasing energy 23. Why do military planners prefer Neutron Bombs ? Neutron bombs are designed to inflict minimum property damage while, at the same time causing maximum loss of life.

  35. Chapter 5 Topics covered: • Radiation • α Radiation • β Radiation • γ Radiation • Decay Processes • Detection of Radiation

  36. 5.0 Radiation Frequency (Hz) 1024 1022 1020 1018 1016 1012 108 106 104 1014 1010 Infra Red High Energy Radio Waves Low Energy Microwaves Gamma rays Non - Ionising Radiation Ionising Radiation X Rays Cosmic Rays UV Visible Light RADIATION is a general term used to describe the exposure of earthly beings to the ELECTROMAGNETIC SPECTRUM. 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. 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.

  37. Radiation 24. What are the two general forms of radiation ? What frequency divides one type from the other ? Ionising and Non ionising radiation, dividing frequency 1016 Hz 25. What types of radiation the fall into the Ionizing category ? Part of the UV spectrum, X Rays, Cosmic Rays

  38. 5.1 Alpha () Radiation P N P N HELIUM NUCLEUS N.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. 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.  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. 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 + 4He2 Since the  particle carries a charge (2+), its path through space can be affected by electric and magnetic fields

  39. Alpha Radiation 26. What change to mass number and atomic number occur when an alpha particle is emitted from a radioactive nucleus ? α particle emission – Mass No goes down 4, Atomic No goes down 2 27. List 3 properties of alpha radiation 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; α sources harmless outside the body but dangerous if ingested; will cause ionisation at site of lodgement 28. 210Po84 undergoes alpha decay to produce an isotope of lead (Pb). Write the equation for this decay. 210Po84206Pb82 + α

  40. 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 antineutrino 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 neutrino p + + + ν+ 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: n - p + + ν- 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: 234Th90 + + 234Ac89 + ν+ (Neutrinos and antineutrinos are neutral (non charged) particles with a very small mass that travel near the speed of light. They are produced in β decay but knowledge of their properties in not part of the course) 234Th90 + - 234Pa91 + ν-

  41. 5.3 The Nature of  Radiation Beta () 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.

  42. Beta Radiation 29. What type of material can be used as a safety shield to protect a person from beta radiation ? Several sheets of paper or a thin piece of aluminium 30. With what speed are beta particles emitted from the nucleus ? 90% of the speed of light 31. Which of the nucleons undergoes change in the production of β- radiation ? Write an equation for this process A neutron converts to a proton plus an electron plus an antineutrino n  p+ + e- + v- 32. 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) 32P15 • 14C614N7 + β, • 90Sr3890Y39 + β, • (c) 32P1532S16 + β

  43. Positrons 33. 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.

  44. 5.4 Gamma () Radiation 24Mg*1224Mg12 +  Gamma 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. It 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: The Star (*) represents a nucleus with excess energy

  45. www.nukeworker.com/study/radiation_faqs/rf07-... 5.5 Penetrating Power Each form of radiation has varying penetrating power as shown

  46. 5.6 Effects of Magnetic Fields http://outreach.atnf.csiro.au/education/senior/cosmicengine/images/sun/magnetic_field_radiation.gif Both  and β 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.

  47. Gamma Radiation 34. How is gamma radiation different from alpha and beta radiation ? γ radiation is pure energy whereas α and β are matter 35. List 3 properties of gamma radiation Any 3 of: Has no mass; ejected at the speed of light; extreme penetrating power; stopped by several cm of lead; often accompanies α and β emission; emission causes no change to nucleus, no new products formed

  48. General Radiation 36. List the known forms of radiation in order from least to most penetrating. α, β, γ and neutrons 37. Would the path followed by a stream of neutrons be affected by the presence of a magnetic field? Explain your answer. Neutrons carry no electric charge so their path would not be affected by a magnetic field

  49. 5.7 Decay Processes - - - - -  -     8 x 104 y 1.6 x 103 y 1.6 x 10-4 s 2.5 x 105 y 24.1d 26.8 m 20.4 y 6.75 h 19.7 m 4.2 m    3.8 d 5.0 m 3.1 m Radioactivity 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. 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. Times given below arrow = half life, where s = sec; m = min; h = hours; d = day; y = year (See Slide 6.2) 234Th90 234Pa91 234U92 226Ra88 230Th90 222Rn86 218Po84 214Pb82 214Bi83 210Bi83 208Tl81 208Pb82 214Po84 210Pb82

  50. Radioactivity 38. Define radioactivity Radioactivity is defined as the spontaneous and uncontrollable decay of a nucleus resulting in the emission of matter or energy. 39. What is a "radioactive decay series "? How many series exist ? 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 series

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