Physics of Radiography Electricity X-ray tube and circuitry

2. Physics of Radiography Electricity X-ray tube and circuitry

3. What we know so far… A photon is…………? A negatively charged electron is attracted to what kind of charge (positive or negative)? The Joule and the electron Volt are both measures of what? To calculate energy you can multiply plancks constant (h) by what? Radio waves have lower frequency than x-rays. True or false? What is a vacuum?

4. By the end of the session you should be able to: Draw a detailed labelled diagram of an X-ray tube Demonstrate understanding of where the electron beam originates from Explain how electrons are accelerated within an x-ray tube Understand the importance of the glass envelope Calculate maximum kinetic (movement) energy for a given accelerating voltage Have an awareness of what alternating current is Understand the principles of electromagnetic induction Describe how step up and step down transformers work Know why we need rectifiers and what they do Understand Dose and Dose limitation Understand the inverse square law of attenuation of energy

5. Dosimetry key terms: Radiation absorbed dose (D) Energy absorbed per unit mass of tissue Equivalent dose (H) Dose taking into account a weighting factor due to properties of ionizing radiation Effective dose (E) Dose taking into account a weighting factor due to different sensitivities to radiation of different body parts Collective dose Total effective dose on a population Dose rate Dose per unit time

6. Radiation absorbed dose (D) • Energy absorbed per unit mass of tissue • The amount of ENERGY absorbed from a beam of radiation per unit of mass tissue • Units: Gray (Gy) measured in Joules/kg • mGy (milligray is 1000x smaller) • This used to be measured in rad where: 1 Gray = 100 rads

7. Equivalent dose (H) Dose taking into account a weighting factor due to properties of specific ionizing radiation It is a quantity that expresses the probability that exposure to ionizing radiation will cause biological effects. There are different types of ionizing radiation which ‘lose’ energy in different ways, e.g. alpha particles would only penetrate a few millimetres and be totally absorbed, x-rays are only partially absorbed so the biological effect would be far less severe for x-rays than alpha particles. To work out equivalent dose you multiply the dose by the weighting factor of the radiation Equivalent dose (H) = Radiation absorbed dose (D) x Weighting factor (Wr) Xrays, gamma rays and beta particles; Wr = 1 Alpha particles Wr = 10 It is measured in sieverts(Sv) but used to be rems where 1Sv = 100 rems

8. Effective dose (E) Dose taking into account a weighting factor due to different sensitivities to radiation of different body parts This is what is normally being referred to when the word ‘Dose’ is used. It is calculated by multiplying the sum of the equivalent dose by the tissue weighting factor Effective dose (E) = Sum of Equivalent dose (H) x Wt Units are also Sieverts (Sv) or millisievertsmSv

9. Collective dose Total effective dose on a population from a particular source of radiation. Collective dose = effective dose (E) x population (Units: man-sievert (man-Sv)) Dose rate Dose per unit time e.g. dose per hour often Units measured in microsieverts per hour

10. Sources of radiation

12. Natural sources: • Cosmic radiation from earths atmosphere • Gamma radiation from rocks and soil • Radiation from certain foods e.g. potassium 40 • Radon (gas produced when uranium in granite decays) • Artificial sources: • Nuclear fallout • Radioactive waste • Medical/dental • Occupational 54%

15. Limiting dose to general public e.g. people in a waiting room Consider where x-ray the beam is aimed, e.g. into waiting room/corridors Consider thickness of walls and what they are made of – are they good at attenuating x-rays? Limiting dose to radiation workers Radiation dose sources: Primary beam, scattered radiation, radiation leakage Ways to limit: Stay out of the primary beam! Move further away from the source (recall how intensity decreases by the square of the distance between you and the source)

17. The further you go away from an energy source, the more the energy dissipates. • The energy spreads in all directions and the surface area of the imaginary sphere on which energy acts increases the further away you go. • Energy which is experienced at a distance 2xr away will be one quarter that experienced at r. • Energy which is experienced at a distance 3xr away will be one ninth that experienced at r. • The decrease in energy is said to follow an inverse square law where the energy at a distance d is 1/d2 • (Where d2 is dxd) • Check understanding: • What fraction of initial energy at r be experienced • At a distance of 4xr • What fraction of initial energy at r be experienced • At a distance of 5xr

18. Electric Current & Static Electricity An electric current is the flow of electric charge (carried by electrons) around a circuit. In certain circumstances, there can be a build up of electric charge which is not free to move - this is called static electricity. An excess of electrons causes a negative charge; a shortage of electrons creates a positive charge. Where static electricity continues to build up in one place, it may result in a spark or an electricshock when electrons eventually move. Protons play an important part in electrostatics, as they provide the positive charge in an atom. However, protons are tightly bound within the nucleus, so they are not free to move around.

19. Charge & Conductors When no current is flowing, the free electrons in a conductor such as a metal, drift around in random directions. Press ‘No Current’ and ‘Current’ to see how the electrons behave in a conductor... When the current is switched on, the electrons move along the material and are carried around the circuit. The flow of electrons around the circuit is called the current. The size of the current depends upon the number of electrons passing per second.

20. Charge & Insulators In an insulator such as plastic, electric charge cannot flow because the electrons are all tightly held to the atoms - they are not free to move around. Insulators are useful in electric circuits, because they stop the flow of current, e.g. insulation around wires. However in some situations, insulators can develop a static electrical charge.

21. Charge is a property of certain particles. A particle with charge will experience a force in an electric field (or in a magnetic field if the charge is moving). Charge is either positive or negative. Objects with a similar charge will repel. Objects with opposite charges will attract. Charge is measured in coulombs, C. The amount of charge on an object can be found using a coulomb meter. Current is the rate of flow of charge; it is the amount of charge flowing per second through a conductor. How can you get the Charge to Flow? A conductor is needed for charge to flow through; then you need to attract or repel the charged particles to make them move. The amount of attracting or repelling you do is measured in volts and is called the voltage or the potential difference (p.d. for short). Work is being done on these charged particles to make them move, so the voltage is a measure of the amount of energy that is provided per coulomb of charge. 1 volt = 1 joule per coulomb.

22. Direct current Uses ionic solutions to create flow of charge (electrons). Always involves electrons moving one way round the circuit.

23. Electrical Generators A generator makes electricity from movement - this is known as electromagnetic induction. Generators can work in one of two different ways: Method 1: when the conductor moves and cuts across magnetic field lines, a voltage is induced. The method by which an electric guitar works islargely based upon induced currents and electrical circuits.

24. Electrical Generators A generator makes electricity from movement - this is known as electromagnetic induction. Generators can work in one of two different ways: Method 2: the magnet can move in and out of the coil of wire.The moving magnetic force cuts across wires creating the current.

25. Electrical Generator Today the modern electrical generator (also known as an alternator) is designed in a similar way to the electric motor. Run the animation to see how the direction of magnetism and motion affect the electricity produced. The direction of the induced current can be reversed by: • reversing the direction of movement. • reversing the poles of the magnet. In the generator, the electricity is taken out by the metal split rings and the carbon brushes.

26. Increasing the Induced Voltage and Current http://www.youtube.com/watch?v=Q8t_12NQpZY • The speed of movementThe faster the speed of movement, the greater the induced voltage, and hence current. • The strength of the magnetThe stronger the magnet, the greater the induced voltage, and hence current. • The number of turns in the coil of wireThe greater the number of coils in the wire, the greater the induced voltage, and hence current. • The cross-sectional area of the coilThe larger the cross-sectional area, the greater the induced voltage, and hence current.

27. Increasing the Induced Voltage and Current To summarise we can say that: • a voltage and a current are produced when a conducting wire cuts through magnetic field lines. • the faster the lines are cut, the larger the induced voltage and current. This was proposed by Michael Faraday in his Law of Electromagnetic Induction, which states that: “the size of the induced voltage across the ends of a coilof wire is directly proportional to the rate at whichthe magnetic lines of flux are being cut”

28. The Electrical Generator Run the animation to see the display of the electrical output for the generator... Generators produce an alternating current (AC).

29. Ac/DC Mains electricity is Alternating current, batteries are direct current with a fixed positive end and a fixed negative end. If you want to use alternating current to continually e.g. attract electric charges you have to change the source to allow only positive charges to be used. For this you use a rectifier. Circuit diagram symbols

30. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission

31. Thermionic emission: Thermionic emission is the release of electrons from a heated metal. The electrons in the metal gain kinetic energy from heat. Electrons that gain sufficiently high kinetic energy will be able to escape from the surface of metal.

32. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission - The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling 99% of electrons energy goes into heating rather than x-ray production

34. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission - The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling - A FOCUSSING DEVICE used to direct the electrons to a focal point on the target

35. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission - The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling - A FOCUSSING DEVICE used to direct the electrons to a focal point on the target - GLASS ENVELOPE encases the x-ray tube in a vacuum such that the electrons are not impeded by air molecules and number of electrons and speed of flow can be controlled Glass housing has the purpose of providing an envelope within which a vacuum can be maintained. The vacuum permits independent control of both the number of electrons that constitutes an electron beam and the speed of flow of the electrons. The vacuum eliminates the possibility of collisions between molecules of air and accelerated electrons. In addition, removal of air prevents deterioration of the filament by oxidation.

36. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission - The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling - A FOCUSSING DEVICE used to direct the electrons to a focal point on the target - GLASS ENVELOPE encases the x-ray tube in a vacuum such that the electrons are not impeded by air molecules and number of electrons and speed of flow can be controlled - Between cathode and anode a high-voltage is connected (kV)

37. - The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission - The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling - A FOCUSSING DEVICE used to direct the electrons to a focal point on the target - GLASS ENVELOPE encases the x-ray tube in a vacuum such that the electrons are not impeded by air molecules and number of electrons and speed of flow can be controlled - Between cathode and anode a high-voltage is connected (kV) - A current (mA) determining the amount of electrons being accelerated flows from the cathode to the anode

38. For x-ray production we need: • High filament current using a low voltage supply – this gives more electrons and a higher intensity beam • High tube voltage so the electrons emitted at the filament are accelerated towards the target • Higher electron kinetic energy = greater quality/penetrating power Step down transformer Step up transformer

40. Transformers - Calculations The relationship between the voltages across each of the coils is shown by the following equation: input voltage = number of turns on primary coiloutput voltage number of turns on secondary coil This can be summarised as: VP = NPVS NS

41. Transformers - Calculations Example 1: A transformer has 50 turns on its primary coil and 300 on itssecondary coil. If an alternating voltage of 2 volts is applied across the primary coil, what is the voltage across the secondary coil? VP = NPVS NS 2V = 50VS300 This is a step-up transformer, and the output voltage is 12V. VS = 300 x 2V 50 VS = 12V

42. Transformers - Calculations Example 2: Calculate the output voltage from a transformer when the input voltage is 230V, the number of turns on the primary coil is 500, and the number of turns on the secondary coil is 50. VP = NPVSNS 230V = 500 VS50 This is a step-down transformer, and the output voltage is 23V. VS = 50 x 230V 500 VS = 23V

43. For an input voltage of 230V with 10 turns on the primary, if the secondary coil had 5 turns, what would the output voltage be? For an input voltage of 230V with 100 turns on the primary, if the secondary coil had 10 turns, what would the output voltage be? For an input voltage of 230V with 5 turns on the primary, if the secondary coil had 5 turns, what would the output voltage be? For an input voltage of 230V with 5 turns on the primary, if the secondary coil had 50 turns, what would the output voltage be? For an input voltage of 230V with 5turns on the primary, if the secondary coil had 100 turns, what would the output voltage be?

45. Write 10 good questions that you think you might be asked on your assessment, add answers on the back. Now swap with someone on your table, see how they do! Write an acrostic poem using ‘x-ray tube’