24.2.1Medical physics and astronomy (A Level)
Production and Use of X-rays
5 min read 11 parts Lesson 8 of 22 1 quiz
Lesson summary
24.2.1 Production and Use of X-rays — AS PHYSICS 9702 notes: Describe how X-rays are produced, how the tube controls and spectrum work, and how images are improved while limiting dose.X-rays — They are the short-wavelength, high-frequency part of the electromagnetic spectrum, wit…
- Properties
- Accelerating p.d
- Bombardment
- Outer electrons moving down
- Maximum energy
- Minimum wavelength
Section 1 of 11X-rays
X-rays
- They are the short-wavelength, high-frequency part of the electromagnetic spectrum, with wavelengths from about 10−8 m to 10−13 m.
Production
- They are produced when fast-moving electrons decelerate rapidly and transfer their kinetic energy to photons.
Properties
- X-rays travel in straight lines at the speed of light, are unaffected by electric and magnetic fields, blacken film, cause fluorescence and ionise matter.
Photons
- X-rays are uncharged photons, not charged particles.
Section 2 of 11The X-ray tube
Cathode
- A heated filament releases electrons by thermionic emission.
Accelerating p.d.
- A large p.d. of typically 30 kV to 150 kV accelerates the electrons towards the anode.
Vacuum
- The tube is evacuated so electrons do not collide with air molecules.
Target
- A metal anode, commonly tungsten, has a high melting point and high atomic number.
Heat
- Less than 1% of the electron energy becomes X-rays and the rest becomes heat.
- High-power tubes use a rotating anode, cooled by oil or water, to spread the heat.
Section 3 of 11Production mechanism
1Bombardment
- Electrons striking the target lose kinetic energy by transferring it to photons.
2Outer electrons moving down
- Outer-shell electrons in the target atoms drop into vacancies in lower levels and release X-ray photons.
3Maximum energy
- An electron accelerated through V gains Emax = eV, the maximum energy of a photon.
4Minimum wavelength
- Emax = eV = hfmax = hc/λmin.
- fmax = eV/h and λmin = hc/(eV).
Section 4 of 11Controlling the beam
Tube current
- It is set by the filament temperature, and raising it increases the number of electrons hitting the target per second.
- The intensity (photons per second) rises without changing photon energy.
Tube voltage
- It sets the maximum photon energy and hence the penetrating power.
- Higher V gives harder X-rays with greater penetration.
- Lower-energy photons are called soft X-rays.
Hardness and intensity
- Hardness is photon energy and intensity is photons per second, so do not confuse them.
Section 5 of 11The X-ray spectrum
The X-ray spectrum
Continuous spectrum
- Also called bremsstrahlung, it has all photon energies up to eV.
- Electrons decelerate in different ways, some in one event and others in many small steps.
Sharp cut-off
- At λmin, since each electron produces one photon and gives up all its energy in one collision at most.
Characteristic lines
- An incident electron removes an inner-shell electron and an outer electron falls into the vacancy.
- The photon energy equals the difference between the shell levels.
- The lines depend only on the target material and not on the tube voltage, if V is high enough to excite them.
Cut-off wavelength
- It depends on V and not on the target material.
Effect of current
- Raising it increases the spectrum height without moving λmin or the lines.
Section 6 of 11Improving the image
Contrast
- The difference in the degree of blackening between structures.
- Use the correct hardness, with hard X-rays for bones and soft X-rays for tissue.
- Use a contrast medium.
Sharpness
- How well-defined the edges of structures are.
- Use a narrower X-ray beam.
- Reduce scattering with a collimator or lead grid.
- Use a smaller pixel size.
Section 7 of 11Reducing exposure and safety
Hazard
- X-rays are ionising and can damage cells and DNA, potentially causing cancer.
Aim
- Give the lowest dose that produces an adequate image.
Aluminium filter
- It absorbs long-wavelength X-rays that would be absorbed by the body without helping the image.
Collimator and shielding
- Lead collimators limit the area exposed and lead shields protect other parts of the body.
Staff
- Staff stand behind lead screens and wear dosimeters to monitor exposure.
Risk and benefit
- The benefit must outweigh the small dose.
Section 8 of 11Other uses
Imaging
- Dense materials of high proton number, such as bone, absorb more than soft tissue.
Other applications
- Security scanning, inspection of welds and X-ray diffraction of crystals.
Radiotherapy
- High-energy X-rays are directed at tumours to kill cancer cells.
Section 9 of 11More from the specification
More from the specification
Exponential attenuation of X-ray beams
- As an X-ray beam passes through matter, its intensity decreases exponentially according to the linear attenuation model equation I = I0 e-mu * x,
- where mu is the linear absorption (attenuation) coefficient and x is tissue thickness.
Tissue contrast and linear attenuation coefficients
- Different biological tissues have distinct linear absorption coefficients,
- (e.g., dense bone has a much higher mu ~ 12 cm-1 compared to soft muscle tissue mu ~ 0.20 cm-1
- producing high radiographic contrast on film or digital detectors.
Computed tomography (CT scans) vs plain radiographs
- A single plain X-ray radiograph produces a 2D projection with overlapping structures,
- whereas computed tomography takes multiple X-ray projections around the patient to reconstruct 3D slice images.
Radiation dosage and patient safety
- Because CT scans combine multiple beam angles to generate sectional image slices, patient exposure to ionizing radiation is significantly higher than from a single standard planar X-ray examination.
Exponential attenuation of X-ray beams
- As an X-ray beam passes through matter, its intensity decreases exponentially according to the linear attenuation model equation I = I0 e-mu * x,
- where mu is the linear absorption (attenuation) coefficient and x is tissue thickness.
Tissue contrast and linear attenuation coefficients
- Different biological tissues have distinct linear absorption coefficients,
- (e.g., dense bone has a much higher mu ~ 12 cm-1 compared to soft muscle tissue mu ~ 0.20 cm-1
- producing high radiographic contrast on film or digital detectors.
Computed tomography (CT scans) vs plain radiographs
- A single plain X-ray radiograph produces a 2D projection with overlapping structures,
- whereas computed tomography takes multiple X-ray projections around the patient to reconstruct 3D slice images.
Radiation dosage and patient safety
- Because CT scans combine multiple beam angles to generate sectional image slices, patient exposure to ionizing radiation is significantly higher than from a single standard planar X-ray examination.
Computed tomography
- An imaging technique that combines X-ray images of slices taken at many angles into a three-dimensional image.
Attenuation
- The reduction in intensity of radiation as it passes through matter.
Radiation
- Transfer by electromagnetic waves, requiring no medium
Absorption
- The conversion of wave energy to internal energy of the medium.
Linear
- A graph that is a straight line through the origin, so resistance is constant.
Tomography
- Imaging in slices.
Distance
- Total length of path travelled, a scalar.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| X-ray | A high-frequency electromagnetic wave produced when fast electrons are stopped by a metal target. |
| Thermionic emission | The release of electrons from a heated metal surface. |
| Bremsstrahlung | The continuous X-ray spectrum produced when electrons decelerate in the target. |
| Characteristic lines | Sharp X-ray lines at wavelengths determined only by the target material. |
| Contrast | The difference in the degree of blackening between structures in an image. |
| Sharpness | How well-defined the edges of structures are in an image. |
Section 11 of 11Exam technique
1
Saying a higher tube voltage increases the number of electrons — it increases their energy; filament current controls the number.
2
Stating characteristic lines move when the voltage changes — they depend on the target element only.
3
Forgetting that most electron energy becomes heat — less than 1% becomes X-rays.
4
Confusing hardness with intensity — hardness is photon energy and intensity is photons per second.
5
Describing X-rays as charged particles — they are uncharged photons.