Measurement of radioactivity

• Radiopharmaceuticals contain radioactive isotopes used in nuclear medicine for diagnostic and therapeutic purposes.
• Accurate measurement of their radioactivity is essential for patient safety, regulatory compliance, and effective diagnostic or therapeutic procedures.

Principles of Measurement of radioactivity

• Radioactivity is the release of energy from unstable atomic nuclei in the form of radiation.
• Units of Radioactivity:
  • Becquerel (Bq): One disintegration per second.
  • Curie (Ci): An older unit where 1 Ci ≈ 3.7 × 10¹⁰ Bq.

Techniques of Measurement of radioactivity in Radiopharmaceuticals

1. Geiger-Müller Counter (GM Counter)

• The GM counter is a radiation detector used for measuring ionizing radiation, including alpha particles, beta particles, and gamma rays.
• It operates by ionizing gas within a tube to produce a measurable electrical pulse.
• Key Features: Simple, durable, and capable of detecting low levels of radiation.

Construction

1. GM Tube: A sealed tube filled with a gas mixture, typically argon or neon, at low pressure, with a quenching agent to prevent continuous discharge.
2. Cathode and Anode: The inner wall of the tube serves as the cathode, and a thin wire in the center is the anode.
3. Window: A thin mica or halogen window at one end allows low-energy beta particles and alpha particles to enter.
4. High Voltage Supply: Creates a strong electric field inside the tube.
5. Counting Circuit: Counts electrical pulses generated by ionization events, often providing a visual or auditory indication.

Operation

1. Ionization: Ionizing radiation enters the tube, ionizing the gas and creating positive ions and free electrons.
2. Avalanche Effect: High voltage accelerates the electrons toward the anode, causing further ionization in an avalanche effect.
3. Pulse Generation: The resulting surge in current is detected as an electrical pulse.
4. Quenching: The quenching agent prevents continuous discharge, allowing the tube to reset.
5. Counting: Each pulse corresponds to an ionizing event, enabling measurement of radiation intensity.

Key Features and Limitations

• Sensitivity: Detects various ionizing radiation types but cannot differentiate between them.
• Dead Time: Has a brief period after each event where it cannot detect another, potentially leading to undercounting at high radiation intensities.
• Plateau Region: Must be operated within a specific voltage range for accurate measurements.

2. Scintillation Counter

• Detects ionizing radiation using a scintillator that emits light when struck by radiation.
• The light is converted into an electrical signal by a photomultiplier tube (PMT).

Construction

1. Scintillator: Can be an inorganic crystal (e.g., sodium iodide doped with thallium, NaI(Tl)) or organic material. Inorganic scintillators have higher light output and better energy resolution.
2. Light Guide: A transparent material that directs light from the scintillator to the photodetector.
3. Photodetector: Converts emitted light into an electrical signal. Commonly uses a PMT but can also use silicon photomultipliers (SiPMs) or avalanche photodiodes (APDs).
4. Shielding and Housing: Prevents external light interference and may include lead shielding to block background radiation.

Operation

1. Ionizing Event: Radiation interacts with the scintillator, depositing energy and causing ionization or excitation.
2. Light Emission: The scintillator emits light as atoms return to their ground state.
3. Light Transmission: The light is guided to the photodetector with minimal loss.
4. Signal Conversion: The PMT converts light into photoelectrons, which are then multiplied to generate an electrical signal.
5. Signal Processing: Electronics process the signal to extract information about the radiation’s energy, timing, and count rate.

Semiconductor Detectors

• Use semiconductor materials like silicon or germanium to directly convert ionizing radiation into an electrical signal.
• Advantages: Offer high energy resolution and are commonly used in gamma spectroscopy.

3. Dosimetry Techniques

1. Ionization Chambers

   • Function: Use a gas-filled chamber with electrodes to measure ionization caused by radiation.
   • Application: Used in medical procedures and environmental monitoring for dose rate measurements.

2. Thermoluminescent Dosimeters (TLDs)

   • Function: Contain a material that stores energy when exposed to radiation, releasing it as light upon heating.
   • Application: Commonly used for personal radiation monitoring and environmental measurements.

3. Film Badges

   • Function: Use photographic film to measure radiation exposure by the degree of blackening after development.
   • Current Use: Largely replaced by TLDs but still used in some settings.

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