Physics of Radiography X-ray tube and circuitry

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

By the end of the session you should be able to: 1.Draw a detailed labelled diagram of an X-ray tube 2.Demonstrate understanding of where the electron beam originates from 3.Explain how electrons are accelerated within an x-ray tube 4.Understand the importance of the glass envelope 5.Calculate maximum kinetic (movement) energy for a given accelerating voltage 6.Have an awareness of what alternating current is 7.Understand the principles of electromagnetic induction 8.Describe how step up and step down transformers work 9.Know why we need rectifiers and what they do 10.Understand Dose and Dose limitation 11.Understand the inverse square law

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

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

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

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 millisieverts mSv Organs Tissue weighting factors (Wt) ICRP Gonads0.08 Red Bone Marrow0.12 Colon0.12 Lung0.12 Stomach0.12 Breasts0.12 Bladder0.04 Liver0.04 Oesophagus0.04 Thyroid0.04 Skin0.01 Bone surface0.01 Salivary glands0.01 Brain0.01 Remainder of body0.12 Total1.00

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

Sources of radiation

54% 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

Dose limitation Patients No set dose limits Exams for illness Weigh up pros and cons Dentist makes decision Medical screening Most likely to benefit Must have patients permission Occupational/insurance Should only be requested by medical/dental practitioners though mainly benefits 3 rd party Medical research Approved by ethics Full understanding of risk Radiation workers ICRP sets dose limits Classified workers High level of exposure Compulsory personal monitoring and annual health check Annual dose limit 20mSv Non-classified workers Low level of exposure Monitoring and annual health checks not compulsory (unless particularly high workload) Annual dose limit 6msV General Public Annual dose limit 1msV

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)

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/d 2 (Where d 2 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

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

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.

- 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

- 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

- 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.

- 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)

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.

- 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

- 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 - LEAD CASING absorbs stray x-rays since they are emitted in all directions - 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

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!

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

Step up transformer

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 5 turns on the primary, if the secondary coil had 100 turns, what would the output voltage be?

Review: Fill in blanks on x-ray tube diagram Write an acrostic poem using ‘x-ray tube’