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5.4.1 X-Rays. (a) describe the nature of X-rays. X-rays - nature. Forms of electromagnetic radiation Short wavelength High frequency Wavelengths 10 -8 m to 10 -13 m Same as gamma rays. (b) describe in simple terms how X-rays are produced. X-rays - production.
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X-rays - nature • Forms of electromagnetic radiation • Short wavelength • High frequency • Wavelengths 10-8m to 10-13m • Same as gamma rays
X-rays - production • Produced when fast-moving electrons are rapidly decelerated • As the electrons slow down, their kinetic energy is transformed to photons of electromagnetic radiation • Less energy than gamma rays
X-rays - production • Evacuated tube containing • Cathode – heated filament emits electrons • Anode – rotating – made from tungsten • External power supply – 200kV • Beam of electrons accelerates across the gap between anode and cathode • Electron arrives at 200keV • Electrons lose kinetic energy as X-ray photons
X-rays - production • Shape of the beam controlled by metal tubes (parallel beam = collimated beam) • 1% of kinetic energy converted to X-rays
(c) describe how X-rays interact with matter (limited to photoelectric effect, Compton Effect and pair production)
Absorption mechanisms – photoelectric effect • X-ray photon with energy < 100 keV absorbed by electron of an atom in the target metal • Electron gains enough energy to escape from the atom • See fig. 15.8
Absorption mechanisms – Compton scattering • X-ray photon with energy 0.5 MeV to 5.0 MeV loses energy to electron in the absorbing material • Interaction is inelastic • Scattered photon has less energy – wavelength is greater • See fig. 15.9
Absorption mechanisms – pair production • X-ray photon with energy > 1.02 MeV produces electron-positron pair • Positron is soon annihilated • Not an important process – x-ray energy too low • See fig. 15.10
(d) define intensity as the power per unit cross-sectional area
Intensity The intensity of a beam of radiation indicates the rate at which energy is transferred across unit cross-sectional area. Intensity is defined: Intensity is the power per unit cross-sectional area Intensity I (W m-2) = Power P (W) / Cross-sectional area A (m-2)
(e) select and use the equation I = I0 e−μxto show how the intensity I of a collimated X ray beam varies with thickness x of medium
Intensity I = I0 e-µx where I0 = initial intensity (before absorption) (W m-2) x = thickness of the material (m) µ = attenuation (absorption) coefficient of the material (m -1) I = transmitted intensity (W m-2)
Intensity The attenuation (absorption) coefficient of bone is 600 m-1 for X-rays of energy 20 keV. A beam of such X-rays has an intensity of 20 W m-2. Calculate the intensity of the beam after passing through a 4.0 mm thickness of bone Io = 20 W m-2 x = 4.0 mm = 0.004 m µ = 600 m-1 I = Ioe-µx = 20 x e-(600 x 0.004) = 20 x e-2.4 = 1.8 W m-2
Intensity An X-ray beam transfers 400 J of energy through 5.0 cm2 each second. Calculate its intensity in W m-2 P = 400 W A = 5.0 cm-2 = 0.0005 m-2 I = P / A = 400 / 0.0005 = 8 x 105 W m-2
Intensity An X-ray beam of initial intensity 50 W m-2 is incident on soft tissue of attenuation coefficient 1.2 cm-1. Calculate the intensity of the beam after passing through a 5.0 cm thickness of tissue. Io = 50 W m-2 x = 5.0 cm µ = 1.2 cm-1 I = Ioe-µx = 50 x e-(1.2 x 5.0) = 50 x e-6 = 0.12 W m-2
(f) describe the use of X-rays in imaging internal body structures including the use of image intensifiers and of contrast media (HSW 3, 4c and 6);
(g) explain how soft tissues like the intestines can be imaged using barium meal
(h) describe the operation of a computerised axial tomography (CAT) scanner
(i) describe the advantages of a CAT scan compared with an X-ray image (HSW 4c, 6)
Assessment Complete questions 1 to 5 on pages 236 and 237 of Physics 2
