5.4.1 X-Rays

(a) describe the nature of X-rays

Stowmarket Physics X-rays - nature Forms of electromagnetic radiation Short wavelength High frequency Wavelengths m to m Same as gamma rays

(b) describe in simple terms how X-rays are produced

Stowmarket Physics 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

Stowmarket Physics 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

Stowmarket Physics 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)

Stowmarket Physics 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

Stowmarket Physics 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

Stowmarket Physics 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

(d) define intensity as the power per unit cross-sectional area

Stowmarket Physics 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 ) Intensity

(e) select and use the equation I = I 0 e −μx to show how the intensity I of a collimated X ray beam varies with thickness x of medium

Stowmarket Physics Intensity I = I 0 e -µx where I 0 = 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 )

Stowmarket Physics 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 I o = 20 W m -2 x = 4.0 mm = m µ = 600 m -1 I = I o e -µx = 20 x e -(600 x 0.004) = 20 x e -2.4 = 1.8 W m -2

Stowmarket Physics Intensity An X-ray beam transfers 400 J of energy through 5.0 cm 2 each second. Calculate its intensity in W m -2 P = 400 W A = 5.0 cm -2 = m -2 I = P / A = 400 / = 8 x 10 5 W m -2

Stowmarket Physics 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. I o = 50 W m -2 x = 5.0 cm µ = 1.2 cm -1 I = I o e -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)

Stowmarket Physics Assessment Complete questions 1 to 5 on pages 236 and 237 of Physics 2

Stowmarket Physics Assessment – Question 1

Stowmarket Physics Assessment – Question 2

Stowmarket Physics Assessment – Question 3

Stowmarket Physics Assessment – Question 4

Stowmarket Physics Assessment – Question 5

Stowmarket Physics End of Chapter Test – Mark Scheme