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Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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What do we know about X-rays?
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced What do we know about X-rays?
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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The discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced The discovery of X-rays 1895 Wilhelm Rontgen
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Working with fluorescent tubes
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced Working with fluorescent tubes The tubes of gas glow when a high voltage is put across them. He was investigating what happened at very low pressures At the point at which the tube went dark he discovered a green colored fluorescent light could be seen coming from a screen sitting about a metre away from the tube.
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Rontgen shielded the tube with heavy black paper and still found the screen fluorescing
He had discovered an invisible ray that was capable of passing through the heavy paper covering the tube. Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced As early as 1896, X-rays were being used in the United States for such things as bone fractures and gun shot wounds.
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1906 – Charles Barkla established that X-rays could be polarised
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced Further discoveries of the nature of X-rays… 1906 – Charles Barkla established that X-rays could be polarised 1912 – Arnold Sommerfeld estimated their wavelength (10-12 – 10-9 m) 1914 – Max von Laue proved that X-rays could be diffracted by crystals
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The production of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced Inside the x-ray machine there is a vacuum which contains a cathode. The cathode is heated which produces a beam of electrons Using very high voltages (thousands or even millions of volts), the electrons are accelerated to very high speeds These accelerated electrons collide with a metal target called the anode. Some of the kinetic energy from the electrons causes the emission of X-rays
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Limitations of X-ray tubes
Only 1% of the electron energy is used to produce the X-rays, the rest heats up the anode. The anode is cooled by oil flowing around it. Operators need to be protected from the radiation. The X-ray tube is surrounded by a lead shielding and the anode is shaped so that the X-rays are directed through a window. Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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The Energy of X-ray Photons
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced The Energy of X-ray Photons The energy of the emitted photons depends on the energy of the incident electrons For an electron accelerated through a potential difference, V E = eV The maximum energy of the X-ray photon is given by Emax = eV
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The spectrum of photon energies from an X-ray tube
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced The spectrum of photon energies from an X-ray tube The positions of these sharp peaks depends on the elements in the anode
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The Wavelength of X-ray Photons
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced The Wavelength of X-ray Photons The energy of a photon is given by E = hf Where h = Planck’s constant For an X-ray tube: fmax = eV / h As λ = c / f we can say: λmin = hc / eV
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced Four men, all members of the Mara Salvatrucha street gang, were caught with cell phones, a charger, and spare chips which they had attempted to smuggle.
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Know some of the history behind the discovery of X-rays
Learning Objectives By the end this lesson you should… Know some of the history behind the discovery of X-rays Be able to describe the nature of X-rays Be able to describe how X-rays are produced The abdomen of a python that had swallowed an electric blanket!!! More odd things to swallow…
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The Photoelectric Effect
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production The Photoelectric Effect When electromagnetic radiation of a high enough energy is incident on a metal surface, electrons are emitted from the surface. Why is this evidence that light is particle?
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hf = Φ + Ek The photoelectric equation:
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production The photoelectric equation: hf = Φ + Ek When dealing with X-ray photons, the large frequencies mean the photon energies are high compared to the work function In practice we can ignore the work function Energy of emitted electron = Photon energy of X-ray
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Photoelectric Absorption
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Photoelectric Absorption
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Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Incident X-ray photon ejects an orbital electron from an atom in the absorbing material An electron from a higher shell may drop down to fill this ‘hole’ giving out energy in the form of a photon
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Compton Scattering - the photoelectric effect - the Compton effect
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Compton Scattering
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Occurs at higher photon energies.
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Occurs at higher photon energies. Incident photon is scattered by an orbital electron in the absorbing material. The electron takes some of the energy of the photon and moves off in the opposite direction to the photon.
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Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Compton discovered that some of the deflected photons had a longer wavelength than the initial wavelength The greater the angle of deflection, the greater the change in wavelength
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- the photoelectric effect
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Derivation of the formula
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Pair Production - the photoelectric effect - the Compton effect
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Pair Production
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Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production At higher photon energies still, the X-ray photons can cause pair production in the absorbing material. An X-ray photon collides with a particle within the material causing the production of an electron-positron pair
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What is the minimum energy at which this effect can occur?
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production What is the minimum energy at which this effect can occur?
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Applications of Absorption Effects
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Applications of Absorption Effects Due to the high energies required, pair production is rare The absorption due to the photoelectric effect depends on Z3. Soft tissue: Z ~ 7 Bone: Z ~ 14 There is a large contrast between the tissue types.
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Applications of Absorption Effects
Learning Objectives By the end of this lesson you should… Be able to describe how X-rays interact with matter using: - the photoelectric effect - the Compton effect - pair production Applications of Absorption Effects Compton scattering depends on density. Bone density ~ 2 x soft tissue density Contrast is much lower In fact, the Compton effect can reduce the detail on X-ray images due to the scattered photons. In practice low photon energies are used to reduce the effect.
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A 27-year-old Chinese man ended up with a pair of 9cm long and 4cm wide scissors stuck in his oesophagus after they slipped while he was using them as a toothpick. More odd things to swallow…
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I = I0 e– x x is the distance travelled through the material
Learning Objectives By the end of this lesson you should… Be able to describe the process of pair production Be able to explain how the X-ray energy determines the type of process occurring in the absorbing material Be able to define the intensity of X-rays Be able to select and use the absorption equation I = I0 e– x x is the distance travelled through the material μ is the attenuation coefficient
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Learning Objectives By the end of this lesson you should… Be able to describe the process of pair production Be able to explain how the X-ray energy determines the type of process occurring in the absorbing material Be able to define the intensity of X-rays Be able to select and use the absorption equation The higher the attenuation coefficient the greater the absorption of the X-rays in the material Typical values: Vacuum = 0 Flesh = 100 m-1 Bone = 300 m-1 Lead = 600 m-1
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Learning Objectives By the end of this lesson you should… Be able to describe the process of pair production Be able to explain how the X-ray energy determines the type of process occurring in the absorbing material Be able to define the intensity of X-rays Be able to select and use the absorption equation Half-value thickness The distance in a medium over which X-ray intensity is attenuated to half its original value
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Calculate the half-value thickness for flesh, bone and lead.
Learning Objectives By the end of this lesson you should… Be able to describe the process of pair production Be able to explain how the X-ray energy determines the type of process occurring in the absorbing material Be able to define the intensity of X-rays Be able to select and use the absorption equation Calculate the half-value thickness for flesh, bone and lead. Typical values of μ: Vacuum = 0 Flesh = 100 m-1 Bone = 300 m-1 Lead = 600 m-1
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Image Intensifiers Converts low intensity X rays into high intensity visible light. Requires lower intensity of X rays –> safer
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Contrast medium Commonly used to improve contrast of blood vessels or GI tract against surrounding tissue.
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Contrast medium Typically barium sulphate /iodine used.
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