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Nuclear Physics

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Nuclear Physics

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  1. Nuclear Physics • Radioactivity • Properties of α, β and γ radiations • Detectors • Random nature of decay • Natural nuclear deformation • Radiation Hazards • The Nucleus • The Rutherford model of the atom • Mass-energy relationship • Binding energy • Fission and fusion

  2. Decay Series • It is often the case that one radioactive isotope decays to another isotope that is also radioactive. Such successive decays are said to form a decay series.

  3. Properties of ,  and  radiations (1) -particles  -particles  -rays Occurrence Increasing the stability by reducing the size and charge of the nuclei. Altering the Z/N ratio to achieve greater stability. Increasing the stability by emitting  –rays. Nature Helium nuclei Fast-moving electrons Electro-magnetic waves Charge +2 -1 0

  4. Properties of ,  and  radiations (2) Mass (nucleon unit) 4 1/1850 0 Speed Up to 10% speed of light Up to 90% speed of light Speed of light Effect of Fields Very small deflection Large deflection No deflection Ionizing ability Strong Weak (10% of ) Very weak (0.01% of )

  5. Properties of , and  radiations (3) Range in air ~5 cm ~5 m ~500 m Penetrating power Stopped by a sheet of paper Stopped by 5 mm of aluminium Never fully absorbed : reduced to half by 25 mm of lead Energy (MeV) 0.5 – 1.0 0.01 – 10 0.01 – 10

  6. Properties of ,  and  radiations (4) Detectors • Photographic film • Ionization chamber • Cloud chamber • Spark counter • Thin window GM tube • Photographic film • Cloud chamber • GM tube • Photographic film • Cloud chamber • GM tube Radioactive transmutation No transmutation

  7. Radiation Detectors • Photographic Film • To detect ,  and  radiations • Spark counter • To detect -particles • Ionization Chamber • To detect -particles • Cloud Chamber • To detect  and  particles • Geiger-Müller Tube • To detect ,  and  radiations

  8. Photographic Film • The photographic film has been blackened by radioactivity except in the shadow of the key.

  9. Earthed grid To the positive terminal of the EHT supply Spark Counter • The spark counter consists of positively charged wire mounted under an earthed metal grid. • It produces sparks in the presence of ionized particles. • It can only be used to detect α-radiation.

  10. Central electrode Conducting can Radioactive source Insulating cap R 2 kV μA Ionization Chamber (1) • A diagram of the ionization is drawn in the diagram below.

  11. Ionization Chamber (2) • The number of ions produced per second by the source of ionizing radiation can be estimated from the current flowing. This estimate depends on the following conditions: • The ionization chamber must be sufficiently large to enable the radiation to travel its full range. • The electric field must be large enough to ensure that all the ions travel to the electrode before recombining with free electrons. • The ionization chamber can only be used to detect α-particles.

  12. Cloud Chamber (1) • The diagrams below show a diffusion cloud chamber and its structure.

  13. Cloud Chamber (2) • The felt strip round the top of the chamber is soaked with alcohol. • The solid CO2 cools the chamber to a low temperature. • The alcohol vapour condensed on the ions caused by the passage of -particles. • A jet trail is left behind.

  14. Cloud Chamber Tracks (1)

  15. Cloud Chamber Tracks (2)

  16. Cloud Chamber Tracks (3) • Under diffusion cloud chamber, • Alpha source gives thick , straight tracks ; • Beta source produces thin, twisted tracks. They are small in mass and so bounce off from air molecules on collision.

  17. Cloud Chamber Tracks (4) • Gamma source gives scattered, thin tracks. Gamma rays remove electrons from air molecules. These electrons behave like beta particles.

  18. GM Counter • When ionizing radiation enters the GM tube, ions and free electrons are formed. • A flow of charge takes place and causes a pulse of current. • The pulse of current is amplified and counted electronically.

  19. Decay rate (Activity) = Activity of a radioactive isotope (1) • Let N(t) be the number of radioactive nuclei in a sample at time t. The `-’ sign indicates that N(t) decreases with time • The decay rate is directly proportional to N(t). The SI unit of activity is the becquerel (Bq). The constant k is called the decay constant. A large value of k corresponds to rapid decay.

  20. Decay Constant • When the Decay Constant has a low value the curve decreases relatively slowly.

  21. Solving the equation to get • Since this also gives Activity of a radioactive isotope (2) • k can be interpreted as the probability per unit time that any individual nucleus will decay. where No is the number of nuclei present at t = 0.

  22. Throwing Dice(Simulation of Radioactive Decay) • A set of N dice was tossed repeatedly. • Each time record the number that decay and remove them. • The ones removed represent nuclei that decay.

  23. Half-life (1) • The graph shows the number of remaining nuclei N(t) as a function of time.

  24. Half-life (2) • Simulation of radioactive decay of Beryllium-11

  25. Half-life (3) • The half-life t1/2 is the time required for the number of radioactive nuclei to decrease to one-half the original number No. • At t = t1/2, N(t) = No/2, obtaining • Taking logarithms to base e, gives

  26. Uses of Radioactive Isotopes (1) • Medicine • Treating cancer • Brachytherapy • Gamma-therapy • Tracers • Surgical sterilisation • Pacemaker

  27. Uses of Radioactive Isotopes (2) Industry • Smoke detector • Thickness gauge • Sterilisation • Radioactive lightning conductor • Detection of leakage • Flaw detection • Food preservation

  28. Uses of Radioactive Isotopes (3) • Agriculture • Genetic improvement • Pest control • Tracers • Archaeology • Carbon-14 dating • Geological dating • Military affairs • Atomic bomb • Hydrogen bomb

  29. Radiation Dose • The energy transferred by radiation to materials is called radiation dose. • The radiation dose measured in grays (Gy). • 1 gray is equal to one joule of energy transferred to each kg of material. • Equal exposure to different types of radiation do not necessarily produce equal biological effects so we use sieverts (Sv) to measure the radiation effect. • One sievert of radiation produces a constant biological effect regardless of the type of radiation.

  30. Sievert (Sv) • 1 sievert of radiation is the amount of any kind of radiation which would cause the same amount of biological damage in a human being as would result from absorbing 1 gray of X rays.

  31. Radiation Hazards • Background radiation • Natural radiation sources • Man-made radiation sources

  32. Source of Background Radiation

  33. Background Radiation

  34. Dose from background radiation (1)

  35. Dose from background radiation (2)

  36. How much radiation is dangerous? • The diagram gives an indication of the likely effects and implications of a range of radiations and does rates to the whole body.

  37. Sealed and unsealed sources used in schools • Sealed sources • Amercium-241 ( and -emitter) • Cobalt-60 ( and -emitter) • Radium-226 ( and -emitter) • Strontium-90 (-emitter) • Unsealed sources • Uranyl nitrate • Natural thorium

  38. Hazards due to sealed and unsealed sources (1) • Hazards due to sealed sources • α-particles usually do not present any external radiation hazard because they are unable to penetrate to dead layer of skin. But, extremely precautions must be taken to prevent α-emitters from getting into the body. • β-particles never constitute a whole-body external radiation hazard due to their short range in tissue. • γ-rays have very high penetrating power and require greater care to avoid receiving excess dosage.

  39. Hazards due to sealed and unsealed sources (2) • Hazards due to unsealed sources • Unsealed sources usually constitute some kind of internal hazard. This is the absorption and retention of radionuclides into specific organs of the body through intake of the materials present in air and in water. • The radionuclides may be rapidly absorbed by the organs causing damage to these organs.

  40. Handling precautions (1) • The weak sources used at school should always by lifted with forceps. • The sources should never by held near the eyes. • The source should be kept in their boxes when not in use. • The strong sources should be handled by long tongs and transported in thick lead containers.

  41. Handling precautions (2) • Workers should be protected by lead and concrete walls and wear radiation dose badges which keep a check on the amount of radiation they have been exposed to.

  42. Side view Zinc sulphide screen Gold foil -source microscope Evacuated metal box To vacuum pump Alpha-Scattering Experiment(1) • A beam of -particles was directed at a thin sheet of gold-foil and the scattered -particles were detected using a small zinc sulphide screen viewed through a microscope in a vacuum chamber.

  43. Alpha-scattering Experiment (2) • From the experiment it was found that • most of the -particles passed through the foil unaffected, • a few were deflected at very large angles, • some were nearly reflected back in the direction from which they had come.

  44. 10-15 m 10-10m Rutherford’s atomic model • Rutherford’s assumptions: • All the atom’s positive charge is concentrated in a relatively small volume, called the nucleus of the atom • The electrons surround the nucleus at relatively large distance. • Most of the atom’s mass is concentrated in its nucleus.

  45. Difficulties of Rutherford’s model • The Rutherford model was unable to explain why atoms emit line spectra. The main difficulties are: • It predicts that light of a continuous range of frequencies will be emitted; • It predicts atoms are unstable—electrons should quickly spiral into the nucleus.

  46. Mass and Energy • Einstein demonstrated that neither mass nor energy were conserved separately, but that they could be traded one for the other and only the total "mass-energy" was conserved. • The mass-energy relationship • Einstein showed that mass and energy are equivalent. • E = mc2

  47. Unified Atomic Mass Unit • The unified atomic mass unit (u) is defined as one twelfth of the mass of the carbon atom which contains six protons, six neutrons and six electrons. • 1 u = 1.660566 × 10-27 kg • Energy equivalence of mass • 1 u = 931.5 MeV • It is a useful quantity to calculate the energy change in nuclear transformations.

  48. Particle masses in atomic mass units

  49. The atomic mass of a Helium atom In fact, the atomic mass of a helium atom = 4.002603 u

  50. Mass Defect • The difference between the mass of an atom and the mass of its particles taken separately is called the mass defect (Δm). • Δm = Zmp +Nmn- Mnucleus • The mass defect is small compared with the total mass of the atom