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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 Radioactivity Measurement
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.T
Techniques for Measuring 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
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.
Cathode and Anode: The inner wall of the tube serves as the cathode, and a thin wire in the center is the anode.
Window: A thin mica or halogen window at one end allows low-energy beta particles and alpha particles to enter.
High Voltage Supply: Creates a strong electric field inside the tube.
Counting Circuit: Counts electrical pulses generated by ionization events, often providing a visual or auditory indication.
Operation
Ionization: Ionizing radiation enters the tube, ionizing the gas and creating positive ions and free electrons.
Avalanche Effect: High voltage accelerates the electrons toward the anode, causing further ionization in an avalanche effect.
Pulse Generation: The resulting surge in current is detected as an electrical pulse.
Quenching: The quenching agent prevents continuous discharge, allowing the tube to reset.
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 Counting
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
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.
Light Guide: A transparent material that directs light from the scintillator to the photodetector.
Photodetector: Converts emitted light into an electrical signal. Commonly uses a PMT but can also use silicon photomultipliers (SiPMs) or avalanche photodiodes (APDs).
Shielding and Housing: Prevents external light interference and may include lead shielding to block background radiation.
Operation
Ionizing Event: Radiation interacts with the scintillator, depositing energy and causing ionization or excitation.
Light Emission: The scintillator emits light as atoms return to their ground state.
Light Transmission: The light is guided to the photodetector with minimal loss.
Signal Conversion: The PMT converts light into photoelectrons, which are then multiplied to generate an electrical signal.
Signal Processing: Electronics process the signal to extract information about the radiation's energy, timing, and count rate.
3) 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.
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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