Answer

Correct option: (iii) X-rays

X-rays are high-energy electromagnetic radiation used in diagnostic radiography. They penetrate tissues differently and produce image contrast, particularly making bones clearly visible.

Answer

Correct option: (i) Wilhelm Röntgen

Wilhelm Conrad Röntgen discovered X-rays in 1895 while experimenting with cathode-ray tubes. He observed that an unknown penetrating radiation could produce images of internal structures, famously obtaining an image of his wife's hand.

Answer

Correct option: (iii) Gamma rays

Among the listed radiation types, gamma photons are highly penetrating electromagnetic radiation because of their high photon energy and lack of electric charge. Their penetration also depends on photon energy and material thickness/density.

Answer

Correct option: (iv) Sievert

The sievert (Sv) is the SI unit of equivalent dose and effective dose, accounting for radiation type and biological sensitivity. The gray (Gy) measures absorbed dose.

Answer

Correct option: (iii) Gamma rays

Gamma rays are electromagnetic photons and can have very high energies. In radiological physics, photon energy is related to frequency by $E=hf$, so higher-frequency photons have higher energy. The actual energy ranges of X-rays and gamma rays can overlap; the distinction is primarily based on origin rather than an absolute energy boundary.

How X-rays Interact with Living Tissues

X-rays are ionizing photons. When they pass through the body, they may be absorbed, scattered, or transmitted. The relative amount depends on photon energy, tissue composition and thickness.

1. Photoelectric Effect

An X-ray photon transfers most or all of its energy to a bound electron and ejects it from the atom. The resulting vacancy can produce characteristic X-rays or Auger electrons.

The photoelectric effect is more important at lower diagnostic photon energies and in materials with higher atomic number. It contributes strongly to bone contrast.

2. Compton Scattering

The incident photon collides with a loosely bound electron, transfers part of its energy to the electron and changes direction with lower energy.

Compton scattering:

• Produces scattered radiation.
• Reduces image contrast.
• Contributes to radiation dose outside the primary beam.

3. Coherent Scattering

The photon changes direction without significant energy transfer or ionization. Its contribution in diagnostic imaging is comparatively small.

4. Transmission

Some photons pass through the body without interaction and reach the detector. These transmitted photons form a major part of the useful image signal.

Tissue Attenuation

As X-rays pass through tissue, intensity decreases approximately according to:

$$I=I_0e^{-\mu x}$$

where $I_0$ is incident intensity, $\mu$ is the linear attenuation coefficient and $x$ is tissue thickness.

Different tissues have different attenuation coefficients, producing image contrast.

Biological Effects

Ionization can damage cellular molecules, including DNA. Effects may be deterministic/tissue reactions at sufficiently high doses or stochastic risks, such as increased cancer probability, associated with radiation exposure.

Conclusion

X-ray imaging depends on differential attenuation caused mainly by photoelectric absorption and Compton scattering in the diagnostic-energy range. The differences in attenuation between tissues allow internal structures to be visualized.

Rotating-Anode X-ray Tube: Operation

Construction

A rotating-anode X-ray tube contains:

• Cathode: filament and focusing cup.
• Anode/target: rotating disc containing a target material such as tungsten-rhenium.
• Rotor: rotates the anode inside the evacuated tube.
• Stator: external electromagnetic coils drive the rotor.
• Vacuum envelope: allows electrons to travel with minimal gas interaction.

Labelled Schematic

          Rotating Anode / Target
              ↻
        ┌─────────────────┐
        │ Tungsten Target│
        └───────┬─────────┘
                │
Electron beam → ●  X-rays → Window → Patient
                │
             Rotor
                │
Cathode                     Anode
Filament → e− e− e− →→→→→  (+)
   (-)                     Stator outside tube

Working

1. The filament is heated by a current and releases electrons through thermionic emission.
2. A high potential difference accelerates electrons from cathode to anode.
3. The focusing cup directs the electron beam toward the focal track.
4. Electrons strike the tungsten target at high speed.
5. Their rapid deceleration produces bremsstrahlung X-rays; characteristic radiation is also produced.
6. Most electron energy becomes heat, so the anode rotates to distribute heat over a larger focal track.
7. X-rays leave through the tube window and are shaped by collimation for imaging.

Why Rotate the Anode?

A stationary target would concentrate heat at one small area. Rotation spreads heat over a circular focal track, allowing higher tube loading while maintaining a small effective focal spot.

Advantages

• Better heat dissipation.
• Higher tube current/loading capability.
• Longer target life.
• Suitable for CT and high-duty diagnostic systems.

Conclusion

A rotating-anode X-ray tube converts electron kinetic energy into X-rays while distributing the large amount of generated heat over a rotating target track.

Exposure Dose and Radiation Dose Measurement

Exposure Dose

Exposure describes the ionization produced by X-rays or gamma rays in air. It is historically expressed using units such as roentgen (R). Modern radiation protection practice generally uses absorbed, equivalent and effective dose quantities rather than exposure alone.

Important Dose Quantities

1. Absorbed Dose

Energy deposited per unit mass:

$$D=\frac{d\bar{\epsilon}}{dm}$$

SI unit: gray (Gy), where $1\,Gy=1\,J/kg$.

2. Equivalent Dose

Accounts for the radiation type using a radiation weighting factor:

$$H_T=\sum_R w_R D_{T,R}$$

Unit: sievert (Sv).

3. Effective Dose

Accounts for different tissue sensitivities:

$$E=\sum_T w_T H_T$$

Unit: sievert (Sv).

How Radiation Dose is Measured

Different instruments are used according to the application:

• Ionization chambers: accurate measurement of exposure/air kerma and output of X-ray equipment.
• Geiger-Müller counters: radiation detection and survey applications.
• Scintillation detectors: convert radiation into light and electrical pulses.
• Semiconductor detectors: compact, sensitive detectors used in dosimetry and spectroscopy.
• Personal dosimeters: monitor occupational exposure over time.
• Thermoluminescent dosimeters (TLDs): store radiation energy and release light when heated during readout.
• OSL dosimeters: use optically stimulated luminescence for dose readout.

X-ray Equipment Output

In diagnostic radiology, output can be measured using an ionization chamber or solid-state dosimeter. Patient dose may be estimated/measured using quantities such as entrance-surface air kerma or dose-area product, depending on the procedure.

Conclusion

Radiation dose measurement quantifies radiation exposure so that equipment performance can be controlled and patient and occupational doses can be kept appropriately low under the ALARA principle.

Personal Dosimeter: TLD

A Thermoluminescent Dosimeter (TLD) is a personal dosimeter used to monitor occupational exposure to ionizing radiation.

Construction

A typical TLD badge contains:

• One or more thermoluminescent detector elements, such as lithium fluoride (LiF).
• Filters made from materials such as plastic, aluminium or copper to help distinguish radiation energy/type.
• A holder or badge containing the detector.
• Identification and protective casing.

Working Principle

When ionizing radiation strikes the TLD material, electrons are raised to higher-energy states and become trapped at defect sites in the crystal lattice.

During readout:

1. The TLD element is heated in a controlled reader.
2. Trapped electrons return to lower-energy states.
3. Light is emitted during this process.
4. A photodetector measures the emitted light.
5. The light output is related to the absorbed radiation dose after calibration.

Radiation Exposure
       ↓
TLD Crystal
       ↓
Energy stored in traps
       ↓
Controlled Heating
       ↓
Thermoluminescence
       ↓
Photodetector
       ↓
Dose Calculation

Advantages

• Small and lightweight.
• Good sensitivity.
• Passive and requires no battery during exposure.
• Can measure accumulated occupational dose.
• Available in reusable formats after readout.

Limitations

The TLD does not provide an immediate real-time reading during exposure. It requires a reader for dose evaluation and must be properly calibrated.

Applications

TLDs are worn by radiographers, radiologists, nuclear-medicine workers and other personnel who may be occupationally exposed to ionizing radiation.

Conclusion

A TLD records radiation exposure by storing energy in crystal traps and releasing it as light during controlled heating; the emitted light is used to estimate personal dose.