5.4.1 X-Rays

(a) describe the nature of X-rays

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sics X-rays - nature

Forms of electromagnetic radiation

Short wavelength

High frequency

Wavelengths 10-8m to 10-13m

Same as gamma rays

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

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sics 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

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sics 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

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sics 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)

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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

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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

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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

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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 = I0 e−μx to show how the intensity I of a collimated X ray beam varies with thickness x of medium

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sics 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)

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sics 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

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sics 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

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sics 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)

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sics Assessment

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

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sics Assessment – Question 1

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sics Assessment – Question 2

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sics Assessment – Question 3

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sics Assessment – Question 4

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sics Assessment – Question 5

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sics End of Chapter Test – Mark Scheme